Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com --- Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area; + int ret; int i;
if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) { - __change_memory_common((u64)page_address(area->pages[i]), + ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask); + if (ret) + return ret; } }
On 03/11/25 11:43 AM, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having
A small nit:
Commit description needs to follow after the SHA ID ^^^^^^^^^^
a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well")
Does is really need a Fixes: ? There is no problem which is being fixed.
Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area;
- int ret; int i;
if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
__change_memory_common((u64)page_address(area->pages[i]),
ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask);if (ret)return ret;
Although the change does make sense.
On 03/11/25 1:18 pm, Anshuman Khandual wrote:
On 03/11/25 11:43 AM, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having
A small nit:
Commit description needs to follow after the SHA ID ^^^^^^^^^^
Didn't do that for brevity's sake, it is there in the fixes tag.
a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well")
Does is really need a Fixes: ? There is no problem which is being fixed.
If an error happens in the linear map alias permission change, it will be suppressed due to the return value not being checked.
Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area;
- int ret; int i;
if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
__change_memory_common((u64)page_address(area->pages[i]),
ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask);if (ret)return ret;Although the change does make sense.
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area;
- int ret; int i;
if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
__change_memory_common((u64)page_address(area->pages[i]),
ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask);if (ret)return ret;
Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
Will
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area;
- int ret; int i;
if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
__change_memory_common((u64)page_address(area->pages[i]),
ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask);if (ret)return ret;Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Thanks, Yang
Will
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area; + int ret; int i; if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
- __change_memory_common((u64)page_address(area->pages[i]),
+ ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask); + if (ret) + return ret;
Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Thanks, Yang
Will
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area; + int ret; int i; if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
- __change_memory_common((u64)page_address(area->pages[i]),
+ ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask); + if (ret) + return ret;
Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Will
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area; + int ret; int i; if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
- __change_memory_common((u64)page_address(area->pages[i]),
+ ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask); + if (ret) + return ret;
Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
I think we had concluded in the discussion of the linear map series that those calls will always succeed - I'll refresh my memory on that and get back to you later!
Will
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area; + int ret; int i; if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
- __change_memory_common((u64)page_address(area->pages[i]),
+ ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask); + if (ret) + return ret;
Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
Will
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full"), __change_memory_common has a real chance of failing due to split failure. Before that commit, this line was introduced in c55191e96caa, still having a chance of failing if it needs to allocate pagetable memory in apply_to_page_range, although that has never been observed to be true. In general, we should always propagate the return value to the caller.
Cc: stable@vger.kernel.org Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM areas to its linear alias as well") Signed-off-by: Dev Jain dev.jain@arm.com
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c index 5135f2d66958..b4ea86cd3a71 100644 --- a/arch/arm64/mm/pageattr.c +++ b/arch/arm64/mm/pageattr.c @@ -148,6 +148,7 @@ static int change_memory_common(unsigned long addr, int numpages, unsigned long size = PAGE_SIZE * numpages; unsigned long end = start + size; struct vm_struct *area; + int ret; int i; if (!PAGE_ALIGNED(addr)) { @@ -185,8 +186,10 @@ static int change_memory_common(unsigned long addr, int numpages, if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || pgprot_val(clear_mask) == PTE_RDONLY)) { for (i = 0; i < area->nr_pages; i++) {
- __change_memory_common((u64)page_address(area->pages[i]),
+ ret = __change_memory_common((u64)page_address(area->pages[i]), PAGE_SIZE, set_mask, clear_mask); + if (ret) + return ret;
Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing.... and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
Will
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: > Post a166563e7ec3 ("arm64: mm: support large block mapping when > rodata=full"), > __change_memory_common has a real chance of failing due to split > failure. > Before that commit, this line was introduced in c55191e96caa, > still having > a chance of failing if it needs to allocate pagetable memory in > apply_to_page_range, although that has never been observed to be > true. > In general, we should always propagate the return value to the > caller. > > Cc: stable@vger.kernel.org > Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM > areas to its linear alias as well") > Signed-off-by: Dev Jain dev.jain@arm.com > --- > Based on Linux 6.18-rc4. > > arch/arm64/mm/pageattr.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c > index 5135f2d66958..b4ea86cd3a71 100644 > --- a/arch/arm64/mm/pageattr.c > +++ b/arch/arm64/mm/pageattr.c > @@ -148,6 +148,7 @@ static int change_memory_common(unsigned > long addr, int numpages, > unsigned long size = PAGE_SIZE * numpages; > unsigned long end = start + size; > struct vm_struct *area; > + int ret; > int i; > if (!PAGE_ALIGNED(addr)) { > @@ -185,8 +186,10 @@ static int change_memory_common(unsigned > long addr, int numpages, > if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || > pgprot_val(clear_mask) == PTE_RDONLY)) { > for (i = 0; i < area->nr_pages; i++) { > - __change_memory_common((u64)page_address(area->pages[i]), > + ret = > __change_memory_common((u64)page_address(area->pages[i]), > PAGE_SIZE, set_mask, clear_mask); > + if (ret) > + return ret; Hmm, this means we can return failure half-way through the operation. Is that something callers are expecting to handle? If so, how can they tell how far we got?
IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing....
and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
1. set_memory_rox succeed to split the whole range, then set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Hopefully I don't miss anything.
Thanks, Yang
Will
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote: > On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >> Post a166563e7ec3 ("arm64: mm: support large block mapping when >> rodata=full"), >> __change_memory_common has a real chance of failing due to split >> failure. >> Before that commit, this line was introduced in c55191e96caa, >> still having >> a chance of failing if it needs to allocate pagetable memory in >> apply_to_page_range, although that has never been observed to >> be true. >> In general, we should always propagate the return value to the >> caller. >> >> Cc: stable@vger.kernel.org >> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >> areas to its linear alias as well") >> Signed-off-by: Dev Jain dev.jain@arm.com >> --- >> Based on Linux 6.18-rc4. >> >> arch/arm64/mm/pageattr.c | 5 ++++- >> 1 file changed, 4 insertions(+), 1 deletion(-) >> >> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c >> index 5135f2d66958..b4ea86cd3a71 100644 >> --- a/arch/arm64/mm/pageattr.c >> +++ b/arch/arm64/mm/pageattr.c >> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >> long addr, int numpages, >> unsigned long size = PAGE_SIZE * numpages; >> unsigned long end = start + size; >> struct vm_struct *area; >> + int ret; >> int i; >> if (!PAGE_ALIGNED(addr)) { >> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >> long addr, int numpages, >> if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || >> pgprot_val(clear_mask) == PTE_RDONLY)) { >> for (i = 0; i < area->nr_pages; i++) { >> - __change_memory_common((u64)page_address(area->pages[i]), >> + ret = >> __change_memory_common((u64)page_address(area->pages[i]), >> PAGE_SIZE, set_mask, clear_mask); >> + if (ret) >> + return ret; > Hmm, this means we can return failure half-way through the > operation. Is > that something callers are expecting to handle? If so, how can > they tell > how far we got? IIUC the callers don't have to know whether it is half-way or not because the callers will change the permission back (e.g. to RW) for the whole range when freeing memory.
Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing....
and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
Hopefully I don't miss anything.
Thanks, Yang
Will
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote: > On 11/3/25 7:16 AM, Will Deacon wrote: >> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>> Post a166563e7ec3 ("arm64: mm: support large block mapping when >>> rodata=full"), >>> __change_memory_common has a real chance of failing due to split >>> failure. >>> Before that commit, this line was introduced in c55191e96caa, >>> still having >>> a chance of failing if it needs to allocate pagetable memory in >>> apply_to_page_range, although that has never been observed to >>> be true. >>> In general, we should always propagate the return value to the >>> caller. >>> >>> Cc: stable@vger.kernel.org >>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>> areas to its linear alias as well") >>> Signed-off-by: Dev Jain dev.jain@arm.com >>> --- >>> Based on Linux 6.18-rc4. >>> >>> arch/arm64/mm/pageattr.c | 5 ++++- >>> 1 file changed, 4 insertions(+), 1 deletion(-) >>> >>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c >>> index 5135f2d66958..b4ea86cd3a71 100644 >>> --- a/arch/arm64/mm/pageattr.c >>> +++ b/arch/arm64/mm/pageattr.c >>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>> long addr, int numpages, >>> unsigned long size = PAGE_SIZE * numpages; >>> unsigned long end = start + size; >>> struct vm_struct *area; >>> + int ret; >>> int i; >>> if (!PAGE_ALIGNED(addr)) { >>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>> long addr, int numpages, >>> if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || >>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>> for (i = 0; i < area->nr_pages; i++) { >>> - __change_memory_common((u64)page_address(area->pages[i]), >>> + ret = >>> __change_memory_common((u64)page_address(area->pages[i]), >>> PAGE_SIZE, set_mask, clear_mask); >>> + if (ret) >>> + return ret; >> Hmm, this means we can return failure half-way through the >> operation. Is >> that something callers are expecting to handle? If so, how can >> they tell >> how far we got? > IIUC the callers don't have to know whether it is half-way or not > because the callers will change the permission back (e.g. to RW) > for the > whole range when freeing memory. Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing....
and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Thanks, Yang
Hopefully I don't miss anything.
Thanks, Yang
Will
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: > On 04/11/25 12:15 am, Yang Shi wrote: >> On 11/3/25 7:16 AM, Will Deacon wrote: >>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>> Post a166563e7ec3 ("arm64: mm: support large block mapping when >>>> rodata=full"), >>>> __change_memory_common has a real chance of failing due to split >>>> failure. >>>> Before that commit, this line was introduced in c55191e96caa, >>>> still having >>>> a chance of failing if it needs to allocate pagetable memory in >>>> apply_to_page_range, although that has never been observed to >>>> be true. >>>> In general, we should always propagate the return value to >>>> the caller. >>>> >>>> Cc: stable@vger.kernel.org >>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>> areas to its linear alias as well") >>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>> --- >>>> Based on Linux 6.18-rc4. >>>> >>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c >>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>> --- a/arch/arm64/mm/pageattr.c >>>> +++ b/arch/arm64/mm/pageattr.c >>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>> long addr, int numpages, >>>> unsigned long size = PAGE_SIZE * numpages; >>>> unsigned long end = start + size; >>>> struct vm_struct *area; >>>> + int ret; >>>> int i; >>>> if (!PAGE_ALIGNED(addr)) { >>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>>> long addr, int numpages, >>>> if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || >>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>> for (i = 0; i < area->nr_pages; i++) { >>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>> + ret = >>>> __change_memory_common((u64)page_address(area->pages[i]), >>>> PAGE_SIZE, set_mask, clear_mask); >>>> + if (ret) >>>> + return ret; >>> Hmm, this means we can return failure half-way through the >>> operation. Is >>> that something callers are expecting to handle? If so, how can >>> they tell >>> how far we got? >> IIUC the callers don't have to know whether it is half-way or not >> because the callers will change the permission back (e.g. to >> RW) for the >> whole range when freeing memory. > Yes, it is the caller's responsibility to set > VM_FLUSH_RESET_PERMS flag. > Upon vfree(), it will change the direct map permissions back to RW. Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing....
and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Thanks, Yang
Hopefully I don't miss anything.
Thanks, Yang
Will
On 11/10/25 8:55 PM, Dev Jain wrote:
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote: > On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: >> On 04/11/25 12:15 am, Yang Shi wrote: >>> On 11/3/25 7:16 AM, Will Deacon wrote: >>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>>> Post a166563e7ec3 ("arm64: mm: support large block mapping when >>>>> rodata=full"), >>>>> __change_memory_common has a real chance of failing due to >>>>> split >>>>> failure. >>>>> Before that commit, this line was introduced in c55191e96caa, >>>>> still having >>>>> a chance of failing if it needs to allocate pagetable memory in >>>>> apply_to_page_range, although that has never been observed >>>>> to be true. >>>>> In general, we should always propagate the return value to >>>>> the caller. >>>>> >>>>> Cc: stable@vger.kernel.org >>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>>> areas to its linear alias as well") >>>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>>> --- >>>>> Based on Linux 6.18-rc4. >>>>> >>>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>> >>>>> diff --git a/arch/arm64/mm/pageattr.c >>>>> b/arch/arm64/mm/pageattr.c >>>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>>> --- a/arch/arm64/mm/pageattr.c >>>>> +++ b/arch/arm64/mm/pageattr.c >>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>>> long addr, int numpages, >>>>> unsigned long size = PAGE_SIZE * numpages; >>>>> unsigned long end = start + size; >>>>> struct vm_struct *area; >>>>> + int ret; >>>>> int i; >>>>> if (!PAGE_ALIGNED(addr)) { >>>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>>>> long addr, int numpages, >>>>> if (rodata_full && (pgprot_val(set_mask) == >>>>> PTE_RDONLY || >>>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>>> for (i = 0; i < area->nr_pages; i++) { >>>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>>> + ret = >>>>> __change_memory_common((u64)page_address(area->pages[i]), >>>>> PAGE_SIZE, set_mask, clear_mask); >>>>> + if (ret) >>>>> + return ret; >>>> Hmm, this means we can return failure half-way through the >>>> operation. Is >>>> that something callers are expecting to handle? If so, how >>>> can they tell >>>> how far we got? >>> IIUC the callers don't have to know whether it is half-way or not >>> because the callers will change the permission back (e.g. to >>> RW) for the >>> whole range when freeing memory. >> Yes, it is the caller's responsibility to set >> VM_FLUSH_RESET_PERMS flag. >> Upon vfree(), it will change the direct map permissions back to >> RW. > Ok, but vfree() ends up using update_range_prot() to do that and > if we > need to worry about that failing (as per your commit message), then > we're in trouble because the calls to set_area_direct_map() are > unchecked. > > In other words, this patch is either not necessary or it is > incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing....
and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
This is what I meant "kernel just calls them on the whole area".
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
Sorry I don't follow you. What "erroneous conclusion" do you mean? You can call set_memory_* on a random point, but set_direct_map_invalid_noflush() should be called on the random point too. The current code of set_area_direct_map() doesn't consider this case because there is no such call. Is this what you meant?
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Proper comment to explain the subtle behavior is definitely welcome.
Thanks, Yang
Thanks, Yang
Hopefully I don't miss anything.
Thanks, Yang
> > Will
On 11/11/25 10:38 am, Yang Shi wrote:
On 11/10/25 8:55 PM, Dev Jain wrote:
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote: > > On 04/11/25 6:26 pm, Will Deacon wrote: >> On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: >>> On 04/11/25 12:15 am, Yang Shi wrote: >>>> On 11/3/25 7:16 AM, Will Deacon wrote: >>>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>>>> Post a166563e7ec3 ("arm64: mm: support large block mapping >>>>>> when >>>>>> rodata=full"), >>>>>> __change_memory_common has a real chance of failing due to >>>>>> split >>>>>> failure. >>>>>> Before that commit, this line was introduced in c55191e96caa, >>>>>> still having >>>>>> a chance of failing if it needs to allocate pagetable >>>>>> memory in >>>>>> apply_to_page_range, although that has never been observed >>>>>> to be true. >>>>>> In general, we should always propagate the return value to >>>>>> the caller. >>>>>> >>>>>> Cc: stable@vger.kernel.org >>>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>>>> areas to its linear alias as well") >>>>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>>>> --- >>>>>> Based on Linux 6.18-rc4. >>>>>> >>>>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/arch/arm64/mm/pageattr.c >>>>>> b/arch/arm64/mm/pageattr.c >>>>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>>>> --- a/arch/arm64/mm/pageattr.c >>>>>> +++ b/arch/arm64/mm/pageattr.c >>>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>>>> long addr, int numpages, >>>>>> unsigned long size = PAGE_SIZE * numpages; >>>>>> unsigned long end = start + size; >>>>>> struct vm_struct *area; >>>>>> + int ret; >>>>>> int i; >>>>>> if (!PAGE_ALIGNED(addr)) { >>>>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>>>>> long addr, int numpages, >>>>>> if (rodata_full && (pgprot_val(set_mask) == >>>>>> PTE_RDONLY || >>>>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>>>> for (i = 0; i < area->nr_pages; i++) { >>>>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>>>> + ret = >>>>>> __change_memory_common((u64)page_address(area->pages[i]), >>>>>> PAGE_SIZE, set_mask, >>>>>> clear_mask); >>>>>> + if (ret) >>>>>> + return ret; >>>>> Hmm, this means we can return failure half-way through the >>>>> operation. Is >>>>> that something callers are expecting to handle? If so, how >>>>> can they tell >>>>> how far we got? >>>> IIUC the callers don't have to know whether it is half-way or >>>> not >>>> because the callers will change the permission back (e.g. to >>>> RW) for the >>>> whole range when freeing memory. >>> Yes, it is the caller's responsibility to set >>> VM_FLUSH_RESET_PERMS flag. >>> Upon vfree(), it will change the direct map permissions back >>> to RW. >> Ok, but vfree() ends up using update_range_prot() to do that >> and if we >> need to worry about that failing (as per your commit message), >> then >> we're in trouble because the calls to set_area_direct_map() are >> unchecked. >> >> In other words, this patch is either not necessary or it is >> incomplete. > > Here is the relevant email, in the discussion between Ryan and > Yang: > > https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco... > > > We had concluded that all callers of set_memory_ro() or > set_memory_rox() (which require the > linear map perm change back to default, upon vfree() ) will call > it for the entire region (vm_struct). > So, when we do the set_direct_map_invalid_noflush, it is > guaranteed that the region has already > been split. So this call cannot fail. > > https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco... > > > This email notes that there is some code doing set_memory_rw() > and unnecessarily setting the VM_FLUSH_RESET_PERMS > flag, but in that case we don't care about the > set_direct_map_invalid_noflush call failing because the protections > are already RW. > > Although we had also observed that all of this is fragile and > depends on the caller doing the > correct thing. The real solution should be somehow getting rid > of the BBM style invalidation. > Ryan had proposed some methods in that email thread. > > One solution which I had thought of, is that, observe that we > are doing an overkill by > setting the linear map to invalid and then default, for the > *entire* region. What we > can do is iterate over the linear map alias of the vm_struct > *area and only change permission > back to RW for the pages which are *not* RW. And, those relevant > mappings are guaranteed to > be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing....
and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
This is what I meant "kernel just calls them on the whole area".
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
Sorry I don't follow you. What "erroneous conclusion" do you mean? You can call set_memory_* on a random point, but set_direct_map_invalid_noflush() should be called on the random point too. The current code of set_area_direct_map() doesn't consider this case because there is no such call. Is this what you meant?
I was referring to the discussion in the linear map work - I think we had concluded that we don't need to worry about the BBM style invalidation failing, *because* no one does a partial set_memory_*.
What I am saying - we don't care whether caller does a partial or a full set_memory_*, we are still safe, because the linear map alias change on both sides (set_memory_* -> __change_memory_common, and vm_reset_perms -> set_area_direct_map() )
operate on the entire region.
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Proper comment to explain the subtle behavior is definitely welcome.
Thanks, Yang
Thanks, Yang
Hopefully I don't miss anything.
Thanks, Yang
> >> >> Will >
On 11/11/2025 05:12, Dev Jain wrote:
On 11/11/25 10:38 am, Yang Shi wrote:
On 11/10/25 8:55 PM, Dev Jain wrote:
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote: > > On 05/11/25 9:27 am, Dev Jain wrote: >> >> On 04/11/25 6:26 pm, Will Deacon wrote: >>> On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: >>>> On 04/11/25 12:15 am, Yang Shi wrote: >>>>> On 11/3/25 7:16 AM, Will Deacon wrote: >>>>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>>>>> Post a166563e7ec3 ("arm64: mm: support large block mapping when >>>>>>> rodata=full"), >>>>>>> __change_memory_common has a real chance of failing due to split >>>>>>> failure. >>>>>>> Before that commit, this line was introduced in c55191e96caa, >>>>>>> still having >>>>>>> a chance of failing if it needs to allocate pagetable memory in >>>>>>> apply_to_page_range, although that has never been observed to be true. >>>>>>> In general, we should always propagate the return value to the caller. >>>>>>> >>>>>>> Cc: stable@vger.kernel.org >>>>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>>>>> areas to its linear alias as well") >>>>>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>>>>> --- >>>>>>> Based on Linux 6.18-rc4. >>>>>>> >>>>>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>>>> >>>>>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c >>>>>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>>>>> --- a/arch/arm64/mm/pageattr.c >>>>>>> +++ b/arch/arm64/mm/pageattr.c >>>>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>>>>> long addr, int numpages, >>>>>>> unsigned long size = PAGE_SIZE * numpages; >>>>>>> unsigned long end = start + size; >>>>>>> struct vm_struct *area; >>>>>>> + int ret; >>>>>>> int i; >>>>>>> if (!PAGE_ALIGNED(addr)) { >>>>>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>>>>>> long addr, int numpages, >>>>>>> if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || >>>>>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>>>>> for (i = 0; i < area->nr_pages; i++) { >>>>>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>>>>> + ret = >>>>>>> __change_memory_common((u64)page_address(area->pages[i]), >>>>>>> PAGE_SIZE, set_mask, clear_mask); >>>>>>> + if (ret) >>>>>>> + return ret; >>>>>> Hmm, this means we can return failure half-way through the >>>>>> operation. Is >>>>>> that something callers are expecting to handle? If so, how can they >>>>>> tell >>>>>> how far we got? >>>>> IIUC the callers don't have to know whether it is half-way or not >>>>> because the callers will change the permission back (e.g. to RW) for the >>>>> whole range when freeing memory. >>>> Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. >>>> Upon vfree(), it will change the direct map permissions back to RW. >>> Ok, but vfree() ends up using update_range_prot() to do that and if we >>> need to worry about that failing (as per your commit message), then >>> we're in trouble because the calls to set_area_direct_map() are unchecked. >>> >>> In other words, this patch is either not necessary or it is incomplete. >> >> Here is the relevant email, in the discussion between Ryan and Yang: >> >> https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8- >> c3f9caf64119@os.amperecomputing.com/ >> >> We had concluded that all callers of set_memory_ro() or set_memory_rox() >> (which require the >> linear map perm change back to default, upon vfree() ) will call it for >> the entire region (vm_struct). >> So, when we do the set_direct_map_invalid_noflush, it is guaranteed that >> the region has already >> been split. So this call cannot fail. >> >> https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae- >> b60bcf67658c@os.amperecomputing.com/ >> >> This email notes that there is some code doing set_memory_rw() and >> unnecessarily setting the VM_FLUSH_RESET_PERMS >> flag, but in that case we don't care about the >> set_direct_map_invalid_noflush call failing because the protections >> are already RW. >> >> Although we had also observed that all of this is fragile and depends on >> the caller doing the >> correct thing. The real solution should be somehow getting rid of the >> BBM style invalidation. >> Ryan had proposed some methods in that email thread. >> >> One solution which I had thought of, is that, observe that we are doing >> an overkill by >> setting the linear map to invalid and then default, for the *entire* >> region. What we >> can do is iterate over the linear map alias of the vm_struct *area and >> only change permission >> back to RW for the pages which are *not* RW. And, those relevant >> mappings are guaranteed to >> be split because they were changed from RW to not RW. > > @Yang and Ryan, > > I saw Yang's patch here: > https://lore.kernel.org/all/20251023204428.477531-1- > yang@os.amperecomputing.com/ > and realized that currently we are splitting away the linear map alias of > the *entire* region. > > Shouldn't this then imply that set_direct_map_invalid_noflush will never > fail, since even > > a set_memory_rox() call on a single page will split the linear map for > the entire region, > > and thus there is no fragility here which we were discussing about? I may > be forgetting > > something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
This is what I meant "kernel just calls them on the whole area".
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
Sorry I don't follow you. What "erroneous conclusion" do you mean? You can call set_memory_* on a random point, but set_direct_map_invalid_noflush() should be called on the random point too. The current code of set_area_direct_map() doesn't consider this case because there is no such call. Is this what you meant?
I was referring to the discussion in the linear map work - I think we had concluded that we don't need to worry about the BBM style invalidation failing, *because* no one does a partial set_memory_*.
What I am saying - we don't care whether caller does a partial or a full set_memory_*, we are still safe, because the linear map alias change on both sides (set_memory_* -> __change_memory_common, and vm_reset_perms -> set_area_direct_map() )
operate on the entire region.
I'm thoughoughly confused again. I thought we had concluded this was all safe when discussing in the context of the "block mapping the linear map" series. But now I'm a bit unclear on whether we have a bug. I think I'm hearing that we don't need this patch and Dev will submit an alternative which just adds some comments to explain why this is safe?
Thanks, Ryan
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Proper comment to explain the subtle behavior is definitely welcome.
Thanks, Yang
Thanks, Yang
Hopefully I don't miss anything.
Thanks, Yang
> > >> >>> >>> Will >>
On 11/11/25 9:52 AM, Ryan Roberts wrote:
On 11/11/2025 05:12, Dev Jain wrote:
On 11/11/25 10:38 am, Yang Shi wrote:
On 11/10/25 8:55 PM, Dev Jain wrote:
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote: > > On 11/10/25 7:39 PM, Dev Jain wrote: >> On 05/11/25 9:27 am, Dev Jain wrote: >>> On 04/11/25 6:26 pm, Will Deacon wrote: >>>> On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: >>>>> On 04/11/25 12:15 am, Yang Shi wrote: >>>>>> On 11/3/25 7:16 AM, Will Deacon wrote: >>>>>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>>>>>> Post a166563e7ec3 ("arm64: mm: support large block mapping when >>>>>>>> rodata=full"), >>>>>>>> __change_memory_common has a real chance of failing due to split >>>>>>>> failure. >>>>>>>> Before that commit, this line was introduced in c55191e96caa, >>>>>>>> still having >>>>>>>> a chance of failing if it needs to allocate pagetable memory in >>>>>>>> apply_to_page_range, although that has never been observed to be true. >>>>>>>> In general, we should always propagate the return value to the caller. >>>>>>>> >>>>>>>> Cc: stable@vger.kernel.org >>>>>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>>>>>> areas to its linear alias as well") >>>>>>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>>>>>> --- >>>>>>>> Based on Linux 6.18-rc4. >>>>>>>> >>>>>>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>>>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>>>>> >>>>>>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c >>>>>>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>>>>>> --- a/arch/arm64/mm/pageattr.c >>>>>>>> +++ b/arch/arm64/mm/pageattr.c >>>>>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>>>>>> long addr, int numpages, >>>>>>>> unsigned long size = PAGE_SIZE * numpages; >>>>>>>> unsigned long end = start + size; >>>>>>>> struct vm_struct *area; >>>>>>>> + int ret; >>>>>>>> int i; >>>>>>>> if (!PAGE_ALIGNED(addr)) { >>>>>>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>>>>>>> long addr, int numpages, >>>>>>>> if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || >>>>>>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>>>>>> for (i = 0; i < area->nr_pages; i++) { >>>>>>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>>>>>> + ret = >>>>>>>> __change_memory_common((u64)page_address(area->pages[i]), >>>>>>>> PAGE_SIZE, set_mask, clear_mask); >>>>>>>> + if (ret) >>>>>>>> + return ret; >>>>>>> Hmm, this means we can return failure half-way through the >>>>>>> operation. Is >>>>>>> that something callers are expecting to handle? If so, how can they >>>>>>> tell >>>>>>> how far we got? >>>>>> IIUC the callers don't have to know whether it is half-way or not >>>>>> because the callers will change the permission back (e.g. to RW) for the >>>>>> whole range when freeing memory. >>>>> Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. >>>>> Upon vfree(), it will change the direct map permissions back to RW. >>>> Ok, but vfree() ends up using update_range_prot() to do that and if we >>>> need to worry about that failing (as per your commit message), then >>>> we're in trouble because the calls to set_area_direct_map() are unchecked. >>>> >>>> In other words, this patch is either not necessary or it is incomplete. >>> Here is the relevant email, in the discussion between Ryan and Yang: >>> >>> https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8- >>> c3f9caf64119@os.amperecomputing.com/ >>> >>> We had concluded that all callers of set_memory_ro() or set_memory_rox() >>> (which require the >>> linear map perm change back to default, upon vfree() ) will call it for >>> the entire region (vm_struct). >>> So, when we do the set_direct_map_invalid_noflush, it is guaranteed that >>> the region has already >>> been split. So this call cannot fail. >>> >>> https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae- >>> b60bcf67658c@os.amperecomputing.com/ >>> >>> This email notes that there is some code doing set_memory_rw() and >>> unnecessarily setting the VM_FLUSH_RESET_PERMS >>> flag, but in that case we don't care about the >>> set_direct_map_invalid_noflush call failing because the protections >>> are already RW. >>> >>> Although we had also observed that all of this is fragile and depends on >>> the caller doing the >>> correct thing. The real solution should be somehow getting rid of the >>> BBM style invalidation. >>> Ryan had proposed some methods in that email thread. >>> >>> One solution which I had thought of, is that, observe that we are doing >>> an overkill by >>> setting the linear map to invalid and then default, for the *entire* >>> region. What we >>> can do is iterate over the linear map alias of the vm_struct *area and >>> only change permission >>> back to RW for the pages which are *not* RW. And, those relevant >>> mappings are guaranteed to >>> be split because they were changed from RW to not RW. >> @Yang and Ryan, >> >> I saw Yang's patch here: >> https://lore.kernel.org/all/20251023204428.477531-1- >> yang@os.amperecomputing.com/ >> and realized that currently we are splitting away the linear map alias of >> the *entire* region. >> >> Shouldn't this then imply that set_direct_map_invalid_noflush will never >> fail, since even >> >> a set_memory_rox() call on a single page will split the linear map for >> the entire region, >> >> and thus there is no fragility here which we were discussing about? I may >> be forgetting >> >> something, this linear map stuff is confusing enough already. > It still may fail due to page table allocation failure when doing split. > But it is still fine. We may run into 3 cases: > > 1. set_memory_rox succeed to split the whole range, then > set_direct_map_invalid_noflush() will succeed too > 2. set_memory_rox fails to split, for example, just change partial range > permission due to page table allocation failure, then > set_direct_map_invalid_noflush() may > a. successfully change the permission back to default till where > set_memory_rox fails at since that range has been successfully split. It > is ok since the remaining range is actually not changed to ro by > set_memory_rox at all > b. successfully change the permission back to default for the whole > range (for example, memory pressure is mitigated when > set_direct_map_invalid_noflush() is called). It is definitely fine as well Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
This is what I meant "kernel just calls them on the whole area".
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
Sorry I don't follow you. What "erroneous conclusion" do you mean? You can call set_memory_* on a random point, but set_direct_map_invalid_noflush() should be called on the random point too. The current code of set_area_direct_map() doesn't consider this case because there is no such call. Is this what you meant?
I was referring to the discussion in the linear map work - I think we had concluded that we don't need to worry about the BBM style invalidation failing, *because* no one does a partial set_memory_*.
What I am saying - we don't care whether caller does a partial or a full set_memory_*, we are still safe, because the linear map alias change on both sides (set_memory_* -> __change_memory_common, and vm_reset_perms -> set_area_direct_map() )
operate on the entire region.
I'm thoughoughly confused again. I thought we had concluded this was all safe when discussing in the context of the "block mapping the linear map" series. But now I'm a bit unclear on whether we have a bug. I think I'm hearing that we don't need this patch and Dev will submit an alternative which just adds some comments to explain why this is safe?
IIUC, it is still all safe. I think Dev's patch is right. We should check the return value of __change_memory_common() because page table split may fail. I had the return value check in my old patches which called page table split function outside __change_memory_common().
W/o this patch, the callers of set_memory_*() may continue to run with some memory in wrong permissions, for example, RW memory but they are supposed to be RO, instead of jumping to error handling path.
Thanks, Yang
Thanks, Ryan
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Proper comment to explain the subtle behavior is definitely welcome.
Thanks, Yang
Thanks, Yang
> Hopefully I don't miss anything. > > Thanks, > Yang > > >> >>>> Will
On 11/11/25 11:22 pm, Ryan Roberts wrote:
On 11/11/2025 05:12, Dev Jain wrote:
On 11/11/25 10:38 am, Yang Shi wrote:
On 11/10/25 8:55 PM, Dev Jain wrote:
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote: > > On 11/10/25 7:39 PM, Dev Jain wrote: >> On 05/11/25 9:27 am, Dev Jain wrote: >>> On 04/11/25 6:26 pm, Will Deacon wrote: >>>> On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: >>>>> On 04/11/25 12:15 am, Yang Shi wrote: >>>>>> On 11/3/25 7:16 AM, Will Deacon wrote: >>>>>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>>>>>> Post a166563e7ec3 ("arm64: mm: support large block mapping when >>>>>>>> rodata=full"), >>>>>>>> __change_memory_common has a real chance of failing due to split >>>>>>>> failure. >>>>>>>> Before that commit, this line was introduced in c55191e96caa, >>>>>>>> still having >>>>>>>> a chance of failing if it needs to allocate pagetable memory in >>>>>>>> apply_to_page_range, although that has never been observed to be true. >>>>>>>> In general, we should always propagate the return value to the caller. >>>>>>>> >>>>>>>> Cc: stable@vger.kernel.org >>>>>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>>>>>> areas to its linear alias as well") >>>>>>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>>>>>> --- >>>>>>>> Based on Linux 6.18-rc4. >>>>>>>> >>>>>>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>>>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>>>>> >>>>>>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c >>>>>>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>>>>>> --- a/arch/arm64/mm/pageattr.c >>>>>>>> +++ b/arch/arm64/mm/pageattr.c >>>>>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>>>>>> long addr, int numpages, >>>>>>>> unsigned long size = PAGE_SIZE * numpages; >>>>>>>> unsigned long end = start + size; >>>>>>>> struct vm_struct *area; >>>>>>>> + int ret; >>>>>>>> int i; >>>>>>>> if (!PAGE_ALIGNED(addr)) { >>>>>>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>>>>>>> long addr, int numpages, >>>>>>>> if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || >>>>>>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>>>>>> for (i = 0; i < area->nr_pages; i++) { >>>>>>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>>>>>> + ret = >>>>>>>> __change_memory_common((u64)page_address(area->pages[i]), >>>>>>>> PAGE_SIZE, set_mask, clear_mask); >>>>>>>> + if (ret) >>>>>>>> + return ret; >>>>>>> Hmm, this means we can return failure half-way through the >>>>>>> operation. Is >>>>>>> that something callers are expecting to handle? If so, how can they >>>>>>> tell >>>>>>> how far we got? >>>>>> IIUC the callers don't have to know whether it is half-way or not >>>>>> because the callers will change the permission back (e.g. to RW) for the >>>>>> whole range when freeing memory. >>>>> Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. >>>>> Upon vfree(), it will change the direct map permissions back to RW. >>>> Ok, but vfree() ends up using update_range_prot() to do that and if we >>>> need to worry about that failing (as per your commit message), then >>>> we're in trouble because the calls to set_area_direct_map() are unchecked. >>>> >>>> In other words, this patch is either not necessary or it is incomplete. >>> Here is the relevant email, in the discussion between Ryan and Yang: >>> >>> https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8- >>> c3f9caf64119@os.amperecomputing.com/ >>> >>> We had concluded that all callers of set_memory_ro() or set_memory_rox() >>> (which require the >>> linear map perm change back to default, upon vfree() ) will call it for >>> the entire region (vm_struct). >>> So, when we do the set_direct_map_invalid_noflush, it is guaranteed that >>> the region has already >>> been split. So this call cannot fail. >>> >>> https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae- >>> b60bcf67658c@os.amperecomputing.com/ >>> >>> This email notes that there is some code doing set_memory_rw() and >>> unnecessarily setting the VM_FLUSH_RESET_PERMS >>> flag, but in that case we don't care about the >>> set_direct_map_invalid_noflush call failing because the protections >>> are already RW. >>> >>> Although we had also observed that all of this is fragile and depends on >>> the caller doing the >>> correct thing. The real solution should be somehow getting rid of the >>> BBM style invalidation. >>> Ryan had proposed some methods in that email thread. >>> >>> One solution which I had thought of, is that, observe that we are doing >>> an overkill by >>> setting the linear map to invalid and then default, for the *entire* >>> region. What we >>> can do is iterate over the linear map alias of the vm_struct *area and >>> only change permission >>> back to RW for the pages which are *not* RW. And, those relevant >>> mappings are guaranteed to >>> be split because they were changed from RW to not RW. >> @Yang and Ryan, >> >> I saw Yang's patch here: >> https://lore.kernel.org/all/20251023204428.477531-1- >> yang@os.amperecomputing.com/ >> and realized that currently we are splitting away the linear map alias of >> the *entire* region. >> >> Shouldn't this then imply that set_direct_map_invalid_noflush will never >> fail, since even >> >> a set_memory_rox() call on a single page will split the linear map for >> the entire region, >> >> and thus there is no fragility here which we were discussing about? I may >> be forgetting >> >> something, this linear map stuff is confusing enough already. > It still may fail due to page table allocation failure when doing split. > But it is still fine. We may run into 3 cases: > > 1. set_memory_rox succeed to split the whole range, then > set_direct_map_invalid_noflush() will succeed too > 2. set_memory_rox fails to split, for example, just change partial range > permission due to page table allocation failure, then > set_direct_map_invalid_noflush() may > a. successfully change the permission back to default till where > set_memory_rox fails at since that range has been successfully split. It > is ok since the remaining range is actually not changed to ro by > set_memory_rox at all > b. successfully change the permission back to default for the whole > range (for example, memory pressure is mitigated when > set_direct_map_invalid_noflush() is called). It is definitely fine as well Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
This is what I meant "kernel just calls them on the whole area".
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
Sorry I don't follow you. What "erroneous conclusion" do you mean? You can call set_memory_* on a random point, but set_direct_map_invalid_noflush() should be called on the random point too. The current code of set_area_direct_map() doesn't consider this case because there is no such call. Is this what you meant?
I was referring to the discussion in the linear map work - I think we had concluded that we don't need to worry about the BBM style invalidation failing, *because* no one does a partial set_memory_*.
What I am saying - we don't care whether caller does a partial or a full set_memory_*, we are still safe, because the linear map alias change on both sides (set_memory_* -> __change_memory_common, and vm_reset_perms -> set_area_direct_map() )
operate on the entire region.
I'm thoughoughly confused again. I thought we had concluded this was all safe when discussing in the context of the "block mapping the linear map" series. But now I'm a bit unclear on whether we have a bug. I think I'm hearing that we don't need this patch and Dev will submit an alternative which just adds some comments to explain why this is safe?
We need this patch because currently we will suppress a linear map alias change -ENOMEM, and the caller will see that set_memory_* succeeded. So basically now we have done set_memory_* without having the corresponding security measure succeed.
The comment refers to the linear map series - we should document why we don't care about -ENOMEM failure of set_direct_map_invalid_noflush().
Thanks, Ryan
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Proper comment to explain the subtle behavior is definitely welcome.
Thanks, Yang
Thanks, Yang
> Hopefully I don't miss anything. > > Thanks, > Yang > > >> >>>> Will
On 11/10/25 9:12 PM, Dev Jain wrote:
On 11/11/25 10:38 am, Yang Shi wrote:
On 11/10/25 8:55 PM, Dev Jain wrote:
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote: > > On 05/11/25 9:27 am, Dev Jain wrote: >> >> On 04/11/25 6:26 pm, Will Deacon wrote: >>> On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: >>>> On 04/11/25 12:15 am, Yang Shi wrote: >>>>> On 11/3/25 7:16 AM, Will Deacon wrote: >>>>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>>>>> Post a166563e7ec3 ("arm64: mm: support large block mapping >>>>>>> when >>>>>>> rodata=full"), >>>>>>> __change_memory_common has a real chance of failing due to >>>>>>> split >>>>>>> failure. >>>>>>> Before that commit, this line was introduced in c55191e96caa, >>>>>>> still having >>>>>>> a chance of failing if it needs to allocate pagetable >>>>>>> memory in >>>>>>> apply_to_page_range, although that has never been observed >>>>>>> to be true. >>>>>>> In general, we should always propagate the return value to >>>>>>> the caller. >>>>>>> >>>>>>> Cc: stable@vger.kernel.org >>>>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>>>>> areas to its linear alias as well") >>>>>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>>>>> --- >>>>>>> Based on Linux 6.18-rc4. >>>>>>> >>>>>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>>>> >>>>>>> diff --git a/arch/arm64/mm/pageattr.c >>>>>>> b/arch/arm64/mm/pageattr.c >>>>>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>>>>> --- a/arch/arm64/mm/pageattr.c >>>>>>> +++ b/arch/arm64/mm/pageattr.c >>>>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>>>>> long addr, int numpages, >>>>>>> unsigned long size = PAGE_SIZE * numpages; >>>>>>> unsigned long end = start + size; >>>>>>> struct vm_struct *area; >>>>>>> + int ret; >>>>>>> int i; >>>>>>> if (!PAGE_ALIGNED(addr)) { >>>>>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>>>>>> long addr, int numpages, >>>>>>> if (rodata_full && (pgprot_val(set_mask) == >>>>>>> PTE_RDONLY || >>>>>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>>>>> for (i = 0; i < area->nr_pages; i++) { >>>>>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>>>>> + ret = >>>>>>> __change_memory_common((u64)page_address(area->pages[i]), >>>>>>> PAGE_SIZE, set_mask, >>>>>>> clear_mask); >>>>>>> + if (ret) >>>>>>> + return ret; >>>>>> Hmm, this means we can return failure half-way through the >>>>>> operation. Is >>>>>> that something callers are expecting to handle? If so, how >>>>>> can they tell >>>>>> how far we got? >>>>> IIUC the callers don't have to know whether it is half-way >>>>> or not >>>>> because the callers will change the permission back (e.g. to >>>>> RW) for the >>>>> whole range when freeing memory. >>>> Yes, it is the caller's responsibility to set >>>> VM_FLUSH_RESET_PERMS flag. >>>> Upon vfree(), it will change the direct map permissions back >>>> to RW. >>> Ok, but vfree() ends up using update_range_prot() to do that >>> and if we >>> need to worry about that failing (as per your commit message), >>> then >>> we're in trouble because the calls to set_area_direct_map() >>> are unchecked. >>> >>> In other words, this patch is either not necessary or it is >>> incomplete. >> >> Here is the relevant email, in the discussion between Ryan and >> Yang: >> >> https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco... >> >> >> We had concluded that all callers of set_memory_ro() or >> set_memory_rox() (which require the >> linear map perm change back to default, upon vfree() ) will >> call it for the entire region (vm_struct). >> So, when we do the set_direct_map_invalid_noflush, it is >> guaranteed that the region has already >> been split. So this call cannot fail. >> >> https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco... >> >> >> This email notes that there is some code doing set_memory_rw() >> and unnecessarily setting the VM_FLUSH_RESET_PERMS >> flag, but in that case we don't care about the >> set_direct_map_invalid_noflush call failing because the >> protections >> are already RW. >> >> Although we had also observed that all of this is fragile and >> depends on the caller doing the >> correct thing. The real solution should be somehow getting rid >> of the BBM style invalidation. >> Ryan had proposed some methods in that email thread. >> >> One solution which I had thought of, is that, observe that we >> are doing an overkill by >> setting the linear map to invalid and then default, for the >> *entire* region. What we >> can do is iterate over the linear map alias of the vm_struct >> *area and only change permission >> back to RW for the pages which are *not* RW. And, those >> relevant mappings are guaranteed to >> be split because they were changed from RW to not RW. > > @Yang and Ryan, > > I saw Yang's patch here: > https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing.... > > and realized that currently we are splitting away the linear map > alias of the *entire* region. > > Shouldn't this then imply that set_direct_map_invalid_noflush > will never fail, since even > > a set_memory_rox() call on a single page will split the linear > map for the entire region, > > and thus there is no fragility here which we were discussing > about? I may be forgetting > > something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
This is what I meant "kernel just calls them on the whole area".
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
Sorry I don't follow you. What "erroneous conclusion" do you mean? You can call set_memory_* on a random point, but set_direct_map_invalid_noflush() should be called on the random point too. The current code of set_area_direct_map() doesn't consider this case because there is no such call. Is this what you meant?
I was referring to the discussion in the linear map work - I think we had concluded that we don't need to worry about the BBM style invalidation failing, *because* no one does a partial set_memory_*.
Yes, we don't have to worry about it.
What I am saying - we don't care whether caller does a partial or a full set_memory_*, we are still safe, because the linear map alias change on both sides (set_memory_* -> __change_memory_common, and vm_reset_perms -> set_area_direct_map() )
operate on the entire region.
Yes, this is the current behavior. My patch changes change_memory_common() to just do permission update for the requested range from the callers instead of assuming change the entire region, although there is no one calls set_memory_* on a partial range. Shall set_area_direct_map() be aware of potential partial range change from set_memory_*()? Maybe. But it is just called from vfree() which just free the entire region.
What happened if someone does something crazy, for example, call set_memory_* on a partial range, then call vfree? IIUC, it is fine as well. It is still covered by the 3 cases that I mentioned in the previous email if I don't miss anything, right?
Thanks, Yang
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Proper comment to explain the subtle behavior is definitely welcome.
Thanks, Yang
Thanks, Yang
Hopefully I don't miss anything.
Thanks, Yang
> > >> >>> >>> Will >>
On 12/11/25 5:15 am, Yang Shi wrote:
On 11/10/25 9:12 PM, Dev Jain wrote:
On 11/11/25 10:38 am, Yang Shi wrote:
On 11/10/25 8:55 PM, Dev Jain wrote:
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote: > > > On 11/10/25 7:39 PM, Dev Jain wrote: >> >> On 05/11/25 9:27 am, Dev Jain wrote: >>> >>> On 04/11/25 6:26 pm, Will Deacon wrote: >>>> On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote: >>>>> On 04/11/25 12:15 am, Yang Shi wrote: >>>>>> On 11/3/25 7:16 AM, Will Deacon wrote: >>>>>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>>>>>>> Post a166563e7ec3 ("arm64: mm: support large block >>>>>>>> mapping when >>>>>>>> rodata=full"), >>>>>>>> __change_memory_common has a real chance of failing due >>>>>>>> to split >>>>>>>> failure. >>>>>>>> Before that commit, this line was introduced in >>>>>>>> c55191e96caa, >>>>>>>> still having >>>>>>>> a chance of failing if it needs to allocate pagetable >>>>>>>> memory in >>>>>>>> apply_to_page_range, although that has never been >>>>>>>> observed to be true. >>>>>>>> In general, we should always propagate the return value >>>>>>>> to the caller. >>>>>>>> >>>>>>>> Cc: stable@vger.kernel.org >>>>>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>>>>>>> areas to its linear alias as well") >>>>>>>> Signed-off-by: Dev Jain dev.jain@arm.com >>>>>>>> --- >>>>>>>> Based on Linux 6.18-rc4. >>>>>>>> >>>>>>>> arch/arm64/mm/pageattr.c | 5 ++++- >>>>>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>>>>> >>>>>>>> diff --git a/arch/arm64/mm/pageattr.c >>>>>>>> b/arch/arm64/mm/pageattr.c >>>>>>>> index 5135f2d66958..b4ea86cd3a71 100644 >>>>>>>> --- a/arch/arm64/mm/pageattr.c >>>>>>>> +++ b/arch/arm64/mm/pageattr.c >>>>>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>>>>>>> long addr, int numpages, >>>>>>>> unsigned long size = PAGE_SIZE * numpages; >>>>>>>> unsigned long end = start + size; >>>>>>>> struct vm_struct *area; >>>>>>>> + int ret; >>>>>>>> int i; >>>>>>>> if (!PAGE_ALIGNED(addr)) { >>>>>>>> @@ -185,8 +186,10 @@ static int >>>>>>>> change_memory_common(unsigned >>>>>>>> long addr, int numpages, >>>>>>>> if (rodata_full && (pgprot_val(set_mask) == >>>>>>>> PTE_RDONLY || >>>>>>>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>>>>>>> for (i = 0; i < area->nr_pages; i++) { >>>>>>>> - __change_memory_common((u64)page_address(area->pages[i]), >>>>>>>> + ret = >>>>>>>> __change_memory_common((u64)page_address(area->pages[i]), >>>>>>>> PAGE_SIZE, set_mask, >>>>>>>> clear_mask); >>>>>>>> + if (ret) >>>>>>>> + return ret; >>>>>>> Hmm, this means we can return failure half-way through the >>>>>>> operation. Is >>>>>>> that something callers are expecting to handle? If so, how >>>>>>> can they tell >>>>>>> how far we got? >>>>>> IIUC the callers don't have to know whether it is half-way >>>>>> or not >>>>>> because the callers will change the permission back (e.g. >>>>>> to RW) for the >>>>>> whole range when freeing memory. >>>>> Yes, it is the caller's responsibility to set >>>>> VM_FLUSH_RESET_PERMS flag. >>>>> Upon vfree(), it will change the direct map permissions back >>>>> to RW. >>>> Ok, but vfree() ends up using update_range_prot() to do that >>>> and if we >>>> need to worry about that failing (as per your commit >>>> message), then >>>> we're in trouble because the calls to set_area_direct_map() >>>> are unchecked. >>>> >>>> In other words, this patch is either not necessary or it is >>>> incomplete. >>> >>> Here is the relevant email, in the discussion between Ryan and >>> Yang: >>> >>> https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco... >>> >>> >>> We had concluded that all callers of set_memory_ro() or >>> set_memory_rox() (which require the >>> linear map perm change back to default, upon vfree() ) will >>> call it for the entire region (vm_struct). >>> So, when we do the set_direct_map_invalid_noflush, it is >>> guaranteed that the region has already >>> been split. So this call cannot fail. >>> >>> https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco... >>> >>> >>> This email notes that there is some code doing set_memory_rw() >>> and unnecessarily setting the VM_FLUSH_RESET_PERMS >>> flag, but in that case we don't care about the >>> set_direct_map_invalid_noflush call failing because the >>> protections >>> are already RW. >>> >>> Although we had also observed that all of this is fragile and >>> depends on the caller doing the >>> correct thing. The real solution should be somehow getting rid >>> of the BBM style invalidation. >>> Ryan had proposed some methods in that email thread. >>> >>> One solution which I had thought of, is that, observe that we >>> are doing an overkill by >>> setting the linear map to invalid and then default, for the >>> *entire* region. What we >>> can do is iterate over the linear map alias of the vm_struct >>> *area and only change permission >>> back to RW for the pages which are *not* RW. And, those >>> relevant mappings are guaranteed to >>> be split because they were changed from RW to not RW. >> >> @Yang and Ryan, >> >> I saw Yang's patch here: >> https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing.... >> >> and realized that currently we are splitting away the linear >> map alias of the *entire* region. >> >> Shouldn't this then imply that set_direct_map_invalid_noflush >> will never fail, since even >> >> a set_memory_rox() call on a single page will split the linear >> map for the entire region, >> >> and thus there is no fragility here which we were discussing >> about? I may be forgetting >> >> something, this linear map stuff is confusing enough already. > > It still may fail due to page table allocation failure when > doing split. But it is still fine. We may run into 3 cases: > > 1. set_memory_rox succeed to split the whole range, then > set_direct_map_invalid_noflush() will succeed too > 2. set_memory_rox fails to split, for example, just change > partial range permission due to page table allocation failure, > then set_direct_map_invalid_noflush() may > a. successfully change the permission back to default till > where set_memory_rox fails at since that range has been > successfully split. It is ok since the remaining range is > actually not changed to ro by set_memory_rox at all > b. successfully change the permission back to default for the > whole range (for example, memory pressure is mitigated when > set_direct_map_invalid_noflush() is called). It is definitely > fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they call set_memory_rox and set_direct_map_invalid_noflush on the same range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the changed protections, on the *entire* alias of the vm_struct region, protects us from the fragility we were talking about earlier.
This is what I meant "kernel just calls them on the whole area".
Suppose you have a range from 0 till size - 1, and you call set_memory_* on a random point (page) p. The argument we discussed above is independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller does a partial set_memory_*.
Sorry I don't follow you. What "erroneous conclusion" do you mean? You can call set_memory_* on a random point, but set_direct_map_invalid_noflush() should be called on the random point too. The current code of set_area_direct_map() doesn't consider this case because there is no such call. Is this what you meant?
I was referring to the discussion in the linear map work - I think we had concluded that we don't need to worry about the BBM style invalidation failing, *because* no one does a partial set_memory_*.
Yes, we don't have to worry about it.
What I am saying - we don't care whether caller does a partial or a full set_memory_*, we are still safe, because the linear map alias change on both sides (set_memory_* -> __change_memory_common, and vm_reset_perms -> set_area_direct_map() )
operate on the entire region.
Yes, this is the current behavior. My patch changes change_memory_common() to just do permission update for the requested range from the callers instead of assuming change the entire region, although there is no one calls set_memory_* on a partial range. Shall set_area_direct_map() be aware of potential partial range change from set_memory_*()? Maybe. But it is just called from vfree() which just free the entire region.
What happened if someone does something crazy, for example, call set_memory_* on a partial range, then call vfree? IIUC, it is fine as well. It is still covered by the 3 cases that I mentioned in the previous email if I don't miss anything, right?
Assuming the caller also does set_vm_flush_reset_perms(), the 3 cases will work out.
Thanks, Yang
I would like to send a patch clearly documenting this behaviour, assuming no one else finds a hole in this reasoning.
Proper comment to explain the subtle behavior is definitely welcome.
Thanks, Yang
Thanks, Yang
> > Hopefully I don't miss anything. > > Thanks, > Yang > > >> >> >>> >>>> >>>> Will >>> >
On 11/11/25 10:07 am, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote: > On 11/3/25 7:16 AM, Will Deacon wrote: >> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote: >>> Post a166563e7ec3 ("arm64: mm: support large block mapping when >>> rodata=full"), >>> __change_memory_common has a real chance of failing due to split >>> failure. >>> Before that commit, this line was introduced in c55191e96caa, >>> still having >>> a chance of failing if it needs to allocate pagetable memory in >>> apply_to_page_range, although that has never been observed to >>> be true. >>> In general, we should always propagate the return value to the >>> caller. >>> >>> Cc: stable@vger.kernel.org >>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM >>> areas to its linear alias as well") >>> Signed-off-by: Dev Jain dev.jain@arm.com >>> --- >>> Based on Linux 6.18-rc4. >>> >>> arch/arm64/mm/pageattr.c | 5 ++++- >>> 1 file changed, 4 insertions(+), 1 deletion(-) >>> >>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c >>> index 5135f2d66958..b4ea86cd3a71 100644 >>> --- a/arch/arm64/mm/pageattr.c >>> +++ b/arch/arm64/mm/pageattr.c >>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned >>> long addr, int numpages, >>> unsigned long size = PAGE_SIZE * numpages; >>> unsigned long end = start + size; >>> struct vm_struct *area; >>> + int ret; >>> int i; >>> if (!PAGE_ALIGNED(addr)) { >>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned >>> long addr, int numpages, >>> if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY || >>> pgprot_val(clear_mask) == PTE_RDONLY)) { >>> for (i = 0; i < area->nr_pages; i++) { >>> - __change_memory_common((u64)page_address(area->pages[i]), >>> + ret = >>> __change_memory_common((u64)page_address(area->pages[i]), >>> PAGE_SIZE, set_mask, clear_mask); >>> + if (ret) >>> + return ret; >> Hmm, this means we can return failure half-way through the >> operation. Is >> that something callers are expecting to handle? If so, how can >> they tell >> how far we got? > IIUC the callers don't have to know whether it is half-way or not > because the callers will change the permission back (e.g. to RW) > for the > whole range when freeing memory. Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag. Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and if we need to worry about that failing (as per your commit message), then we're in trouble because the calls to set_area_direct_map() are unchecked.
In other words, this patch is either not necessary or it is incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@os.ampereco...
We had concluded that all callers of set_memory_ro() or set_memory_rox() (which require the linear map perm change back to default, upon vfree() ) will call it for the entire region (vm_struct). So, when we do the set_direct_map_invalid_noflush, it is guaranteed that the region has already been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@os.ampereco...
This email notes that there is some code doing set_memory_rw() and unnecessarily setting the VM_FLUSH_RESET_PERMS flag, but in that case we don't care about the set_direct_map_invalid_noflush call failing because the protections are already RW.
Although we had also observed that all of this is fragile and depends on the caller doing the correct thing. The real solution should be somehow getting rid of the BBM style invalidation. Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are doing an overkill by setting the linear map to invalid and then default, for the *entire* region. What we can do is iterate over the linear map alias of the vm_struct *area and only change permission back to RW for the pages which are *not* RW. And, those relevant mappings are guaranteed to be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here: https://lore.kernel.org/all/20251023204428.477531-1-yang@os.amperecomputing....
and realized that currently we are splitting away the linear map alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will never fail, since even
a set_memory_rox() call on a single page will split the linear map for the entire region,
and thus there is no fragility here which we were discussing about? I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing split. But it is still fine. We may run into 3 cases:
- set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too 2. set_memory_rox fails to split, for example, just change partial range permission due to page table allocation failure, then set_direct_map_invalid_noflush() may a. successfully change the permission back to default till where set_memory_rox fails at since that range has been successfully split. It is ok since the remaining range is actually not changed to ro by set_memory_rox at all b. successfully change the permission back to default for the whole range (for example, memory pressure is mitigated when set_direct_map_invalid_noflush() is called). It is definitely fine as well
Correct, what I mean to imply here is that, your patch will break this? If set_memory_* is applied on x till y, your patch changes the linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we must reset back [0, x) to default?
Okay I realize this is wrong, please read the last bit as "and this is okay because [0, x) is default already", and therefore
your patch is correct.
Let me send a patch documenting this so we don't get confused in the future, it is easy to forget all of this.
Hopefully I don't miss anything.
Thanks, Yang
Will
linux-stable-mirror@lists.linaro.org
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Anshuman Khandual -
Dev Jain -
Ryan Roberts -
Will Deacon -
Yang Shi