Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...).
So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1].
This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify() and mk_pte().
Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection.
This fixes two known cases:
(a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access. (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access.
Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional.
Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general.
Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory (iow, less special) in that regard, even though, mixing mprotect with uffd-wp is controversial.
[1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com
Reported-by: Ives van Hoorne ives@codesandbox.io Debugged-by: Peter Xu peterx@redhat.com Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Cc: stable@vger.kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Hugh Dickins hugh@veritas.com Cc: Alistair Popple apopple@nvidia.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: Andrea Arcangeli aarcange@redhat.com Signed-off-by: David Hildenbrand david@redhat.com ---
As discussed in [2], this is supposed to replace the fix by Peter: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte
This survives vm/selftests and my reproducers: * migrating pages that are uffd-wp'ed using mbind() on a machine with 2 NUMA nodes * Using a PROT_WRITE mapping with uffd-wp * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and mprotect()'ing it PROT_WRITE * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and temporarily mprotect()'ing it PROT_READ
uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers fail.
It would be good to get some more testing feedback and review.
[2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com
--- fs/userfaultfd.c | 28 ++++++++++++++++++++++------ mm/mmap.c | 4 ++++ 2 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 98ac37e34e3d..fb0733f2e623 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) return ctx->features & UFFD_FEATURE_INITIALIZED; }
+static void userfaultfd_set_vm_flags(struct vm_area_struct *vma, + vm_flags_t flags) +{ + const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP); + + vma->vm_flags = flags; + /* + * For shared mappings, we want to enable writenotify while + * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply + * recalculate vma->vm_page_prot whenever userfaultfd-wp is involved. + */ + if ((vma->vm_flags & VM_SHARED) && uffd_wp) + vma_set_page_prot(vma); +} + static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, int wake_flags, void *key) { @@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx, for_each_vma(vmi, vma) { if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; - vma->vm_flags &= ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, + vma->vm_flags & ~__VM_UFFD_FLAGS); } } mmap_write_unlock(mm); @@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs) octx = vma->vm_userfaultfd_ctx.ctx; if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; - vma->vm_flags &= ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); return 0; }
@@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma, } else { /* Drop uffd context if remap feature not enabled */ vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; - vma->vm_flags &= ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); } }
@@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file) prev = vma; }
- vma->vm_flags = new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; } mmap_write_unlock(mm); @@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */ - vma->vm_flags = new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx.ctx = ctx;
if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma)) @@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */ - vma->vm_flags = new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
skip: diff --git a/mm/mmap.c b/mm/mmap.c index a5eb2f175da0..6033d20198b0 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot) if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma)) return 1;
+ /* Do we need write faults for uffd-wp tracking? */ + if (userfaultfd_wp(vma)) + return 1; + /* Specialty mapping? */ if (vm_flags & VM_PFNMAP) return 0;
base-commit: 8ed710da2873c2aeb3bb805864a699affaf1d03b
On 08.12.22 12:41, David Hildenbrand wrote:
Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...).
So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1].
This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify() and mk_pte().
Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection.
This fixes two known cases:
(a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access. (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access.
Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional.
Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general.
Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory (iow, less special) in that regard, even though, mixing mprotect with uffd-wp is controversial.
[1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com
Reported-by: Ives van Hoorne ives@codesandbox.io Debugged-by: Peter Xu peterx@redhat.com Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Cc: stable@vger.kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Hugh Dickins hugh@veritas.com
No idea how a wrong mail address from Hugh sneaked in 2 (I assume, copy-paste issue from de1ccfb64824). Let's properly cc him and keep the full patch.
Cc: Alistair Popple apopple@nvidia.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: Andrea Arcangeli aarcange@redhat.com Signed-off-by: David Hildenbrand david@redhat.com
As discussed in [2], this is supposed to replace the fix by Peter: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte
This survives vm/selftests and my reproducers:
- migrating pages that are uffd-wp'ed using mbind() on a machine with 2 NUMA nodes
- Using a PROT_WRITE mapping with uffd-wp
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and mprotect()'ing it PROT_WRITE
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and temporarily mprotect()'ing it PROT_READ
uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers fail.
It would be good to get some more testing feedback and review.
[2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com
fs/userfaultfd.c | 28 ++++++++++++++++++++++------ mm/mmap.c | 4 ++++ 2 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 98ac37e34e3d..fb0733f2e623 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) return ctx->features & UFFD_FEATURE_INITIALIZED; } +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
vm_flags_t flags)+{
- const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP);
- vma->vm_flags = flags;
- /*
* For shared mappings, we want to enable writenotify while* userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply* recalculate vma->vm_page_prot whenever userfaultfd-wp is involved.*/- if ((vma->vm_flags & VM_SHARED) && uffd_wp)
vma_set_page_prot(vma);+}
- static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, int wake_flags, void *key) {
@@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx, for_each_vma(vmi, vma) { if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
vma->vm_flags &= ~__VM_UFFD_FLAGS;
userfaultfd_set_vm_flags(vma,vma->vm_flags & ~__VM_UFFD_FLAGS); }@@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs) octx = vma->vm_userfaultfd_ctx.ctx; if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
vma->vm_flags &= ~__VM_UFFD_FLAGS;
userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);@@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma, } else { /* Drop uffd context if remap feature not enabled */ vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
vma->vm_flags &= ~__VM_UFFD_FLAGS;
userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);@@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file) prev = vma; }
vma->vm_flags = new_flags;
userfaultfd_set_vm_flags(vma, new_flags);@@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */
vma->vm_flags = new_flags;
userfaultfd_set_vm_flags(vma, new_flags);if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma)) @@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */
vma->vm_flags = new_flags;
userfaultfd_set_vm_flags(vma, new_flags);skip: diff --git a/mm/mmap.c b/mm/mmap.c index a5eb2f175da0..6033d20198b0 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot) if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma)) return 1;
- /* Do we need write faults for uffd-wp tracking? */
- if (userfaultfd_wp(vma))
return 1;- /* Specialty mapping? */ if (vm_flags & VM_PFNMAP) return 0;
base-commit: 8ed710da2873c2aeb3bb805864a699affaf1d03b
On 08.12.22 12:45, David Hildenbrand wrote:
On 08.12.22 12:41, David Hildenbrand wrote:
Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...).
So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1].
This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify() and mk_pte().
Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection.
This fixes two known cases:
(a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access. (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access.
Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional.
Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general.
Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory (iow, less special) in that regard, even though, mixing mprotect with uffd-wp is controversial.
[1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com
Reported-by: Ives van Hoorne ives@codesandbox.io Debugged-by: Peter Xu peterx@redhat.com Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Cc: stable@vger.kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Hugh Dickins hugh@veritas.com
No idea how a wrong mail address from Hugh sneaked in 2 (I assume, copy-paste issue from de1ccfb64824). Let's properly cc him and keep the full patch.
This time really ;)
Cc: Alistair Popple apopple@nvidia.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: Andrea Arcangeli aarcange@redhat.com Signed-off-by: David Hildenbrand david@redhat.com
As discussed in [2], this is supposed to replace the fix by Peter: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte
This survives vm/selftests and my reproducers:
- migrating pages that are uffd-wp'ed using mbind() on a machine with 2 NUMA nodes
- Using a PROT_WRITE mapping with uffd-wp
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and mprotect()'ing it PROT_WRITE
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and temporarily mprotect()'ing it PROT_READ
uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers fail.
It would be good to get some more testing feedback and review.
[2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com
fs/userfaultfd.c | 28 ++++++++++++++++++++++------ mm/mmap.c | 4 ++++ 2 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 98ac37e34e3d..fb0733f2e623 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) return ctx->features & UFFD_FEATURE_INITIALIZED; } +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
vm_flags_t flags)+{
- const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP);
- vma->vm_flags = flags;
- /*
* For shared mappings, we want to enable writenotify while* userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply* recalculate vma->vm_page_prot whenever userfaultfd-wp is involved.*/- if ((vma->vm_flags & VM_SHARED) && uffd_wp)
vma_set_page_prot(vma);+}
- static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, int wake_flags, void *key) {
@@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx, for_each_vma(vmi, vma) { if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
vma->vm_flags &= ~__VM_UFFD_FLAGS;
userfaultfd_set_vm_flags(vma,vma->vm_flags & ~__VM_UFFD_FLAGS); } } mmap_write_unlock(mm);@@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs) octx = vma->vm_userfaultfd_ctx.ctx; if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
vma->vm_flags &= ~__VM_UFFD_FLAGS;
userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); return 0;@@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma, } else { /* Drop uffd context if remap feature not enabled */ vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
vma->vm_flags &= ~__VM_UFFD_FLAGS;
userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);@@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file) prev = vma; }
vma->vm_flags = new_flags;
userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;@@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */
vma->vm_flags = new_flags;
userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx.ctx = ctx; if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))@@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */
vma->vm_flags = new_flags;
userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;diff --git a/mm/mmap.c b/mm/mmap.c index a5eb2f175da0..6033d20198b0 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot) if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma)) return 1;
- /* Do we need write faults for uffd-wp tracking? */
- if (userfaultfd_wp(vma))
return 1;- /* Specialty mapping? */ if (vm_flags & VM_PFNMAP) return 0;
base-commit: 8ed710da2873c2aeb3bb805864a699affaf1d03b
On Thu, Dec 08, 2022 at 12:41:37PM +0100, David Hildenbrand wrote:
Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...).
So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1].
This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify() and mk_pte().
Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection.
This fixes two known cases:
(a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access. (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access.
Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional.
Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general.
Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory (iow, less special) in that regard, even though, mixing mprotect with uffd-wp is controversial.
[1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com
Reported-by: Ives van Hoorne ives@codesandbox.io Debugged-by: Peter Xu peterx@redhat.com Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Cc: stable@vger.kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Hugh Dickins hugh@veritas.com Cc: Alistair Popple apopple@nvidia.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: Andrea Arcangeli aarcange@redhat.com Signed-off-by: David Hildenbrand david@redhat.com
Acked-by: Peter Xu peterx@redhat.com
One trivial nit.
As discussed in [2], this is supposed to replace the fix by Peter: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte
This survives vm/selftests and my reproducers:
- migrating pages that are uffd-wp'ed using mbind() on a machine with 2 NUMA nodes
- Using a PROT_WRITE mapping with uffd-wp
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and mprotect()'ing it PROT_WRITE
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and temporarily mprotect()'ing it PROT_READ
uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers fail.
It would be good to get some more testing feedback and review.
[2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com
fs/userfaultfd.c | 28 ++++++++++++++++++++++------ mm/mmap.c | 4 ++++ 2 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 98ac37e34e3d..fb0733f2e623 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) return ctx->features & UFFD_FEATURE_INITIALIZED; } +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
vm_flags_t flags)+{
- const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP);
IIUC this can be "uffd_wp_changed" then switch "|" to "^". But not a hot path at all, so shouldn't matter a lot.
Thanks,
- vma->vm_flags = flags;
- /*
* For shared mappings, we want to enable writenotify while* userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply* recalculate vma->vm_page_prot whenever userfaultfd-wp is involved.*/- if ((vma->vm_flags & VM_SHARED) && uffd_wp)
vma_set_page_prot(vma);+}
On 08.12.22 17:29, Peter Xu wrote:
On Thu, Dec 08, 2022 at 12:41:37PM +0100, David Hildenbrand wrote:
Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...).
So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1].
This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify() and mk_pte().
Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection.
This fixes two known cases:
(a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access. (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access.
Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional.
Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general.
Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory (iow, less special) in that regard, even though, mixing mprotect with uffd-wp is controversial.
[1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com
Reported-by: Ives van Hoorne ives@codesandbox.io Debugged-by: Peter Xu peterx@redhat.com Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Cc: stable@vger.kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Hugh Dickins hugh@veritas.com Cc: Alistair Popple apopple@nvidia.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: Andrea Arcangeli aarcange@redhat.com Signed-off-by: David Hildenbrand david@redhat.com
Acked-by: Peter Xu peterx@redhat.com
One trivial nit.
As discussed in [2], this is supposed to replace the fix by Peter: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte
This survives vm/selftests and my reproducers:
- migrating pages that are uffd-wp'ed using mbind() on a machine with 2 NUMA nodes
- Using a PROT_WRITE mapping with uffd-wp
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and mprotect()'ing it PROT_WRITE
- Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and temporarily mprotect()'ing it PROT_READ
uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers fail.
It would be good to get some more testing feedback and review.
[2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com
fs/userfaultfd.c | 28 ++++++++++++++++++++++------ mm/mmap.c | 4 ++++ 2 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 98ac37e34e3d..fb0733f2e623 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) return ctx->features & UFFD_FEATURE_INITIALIZED; } +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
vm_flags_t flags)+{
- const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP);
IIUC this can be "uffd_wp_changed" then switch "|" to "^". But not a hot path at all, so shouldn't matter a lot.
Yes, let's do that (we can also remove the !! here):
This hunk will be:
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 98ac37e34e3d..a988485ada05 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) return ctx->features & UFFD_FEATURE_INITIALIZED; }
+static void userfaultfd_set_vm_flags(struct vm_area_struct *vma, + vm_flags_t flags) +{ + const bool uffd_wp_changed = (vma->vm_flags ^ flags) & VM_UFFD_WP; + + vma->vm_flags = flags; + /* + * For shared mappings, we want to enable writenotify while + * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply + * recalculate vma->vm_page_prot whenever userfaultfd-wp changes. + */ + if ((vma->vm_flags & VM_SHARED) && uffd_wp_changed) + vma_set_page_prot(vma); +} +
I'll wait for some more (+retest) before I resend tomorrow.
Thanks!
On Thu, Dec 08, 2022 at 05:44:35PM +0100, David Hildenbrand wrote:
I'll wait for some more (+retest) before I resend tomorrow.
One more thing just to double check:
It's 6a56ccbcf6c6 ("mm/autonuma: use can_change_(pte|pmd)_writable() to replace savedwrite", 2022-11-30) that just started to break uffd-wp on numa, am I right?
With the old code, pte_modify() will persist uffd-wp bit, afaict, and we used to do savedwrite for numa hints. That all look correct to me until the savedwrite removal patchset with/without vm_page_prot changes.
If that's the case, we'd better also mention that in the commit message and has another Fixes: for that one to be clear.
On Thu, Dec 08, 2022 at 03:06:06PM -0500, Peter Xu wrote:
On Thu, Dec 08, 2022 at 05:44:35PM +0100, David Hildenbrand wrote:
I'll wait for some more (+retest) before I resend tomorrow.
One more thing just to double check:
It's 6a56ccbcf6c6 ("mm/autonuma: use can_change_(pte|pmd)_writable() to replace savedwrite", 2022-11-30) that just started to break uffd-wp on numa, am I right?
With the old code, pte_modify() will persist uffd-wp bit, afaict, and we used to do savedwrite for numa hints. That all look correct to me until the savedwrite removal patchset with/without vm_page_prot changes.
If that's the case, we'd better also mention that in the commit message and has another Fixes: for that one to be clear.
Nah, never mind. I think the savedwrite will not guarantee pte write protected just like the migration path. The commit message is correct.
On 08.12.22 21:21, Peter Xu wrote:
On Thu, Dec 08, 2022 at 03:06:06PM -0500, Peter Xu wrote:
On Thu, Dec 08, 2022 at 05:44:35PM +0100, David Hildenbrand wrote:
I'll wait for some more (+retest) before I resend tomorrow.
One more thing just to double check:
It's 6a56ccbcf6c6 ("mm/autonuma: use can_change_(pte|pmd)_writable() to replace savedwrite", 2022-11-30) that just started to break uffd-wp on numa, am I right?
With the old code, pte_modify() will persist uffd-wp bit, afaict, and we used to do savedwrite for numa hints. That all look correct to me until the savedwrite removal patchset with/without vm_page_prot changes.
If that's the case, we'd better also mention that in the commit message and has another Fixes: for that one to be clear.
Nah, never mind. I think the savedwrite will not guarantee pte write protected just like the migration path. The commit message is correct.
Right, the problem is not the uffd-wp bit getting lost, but the write bit getting set, which is independent of 6a56ccbcf6c6. Thanks for double-checking 6a56ccbcf6c6.
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