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 >>