kernel/linux.git/mm/userfaultfd.c, branch v7.2-rc1

userfaultfd: gate must_wait writability check on pte_present()

2026-06-09T01:21:29+00:00

userfaultfd_must_wait() and userfaultfd_huge_must_wait() read the PTE without taking the page table lock and then apply pte_write() / huge_pte_write() to it. Those accessors decode bits from the present encoding only; on a swap or migration entry they read the offset bits that happen to share the same position and return an undefined result. The intent of the check is "is this fault still WP-blocked?". A non-marker swap entry means the page is in transit -- the userfault context the original fault delivered against is no longer the same, and the swap-in or migration completion path will re-deliver a fresh fault if userspace still needs to handle it. Worst case under the current code the garbage write bit says "wait", and the thread stays asleep until a UFFDIO_WAKE that may never arrive. Gate the writability check on pte_present() so the lockless re-check only inspects present-PTE bits when the entry is actually present. The non-present, non-marker case returns "don't wait" and lets the fault path retry. Link: https://lore.kernel.org/20260529172331.356655-6-kas@kernel.org Fixes: 369cd2121be4 ("userfaultfd: hugetlbfs: userfaultfd_huge_must_wait for hugepmd ranges") Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl") Signed-off-by: Kiryl Shutsemau Reported-by: Sashiko AI review Reviewed-by: Lorenzo Stoakes Cc: David Hildenbrand Cc: Michal Hocko Cc: Mike Rapoport Cc: Peter Xu Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Balbir Singh Cc: Signed-off-by: Andrew Morton

userfaultfd: make functions that are not used outside uffd static

2026-06-04T21:45:03+00:00

After merging fs/userfaultfd.c into mm/userfaultfd.c, several functions that were previously shared between the two files are now only used within mm/userfaultfd.c. Make them static and remove their declarations from include/linux/userfaultfd_k.h. Link: https://lore.kernel.org/20260523173759.3964908-3-rppt@kernel.org Assisted-by: Copilot:claude-opus-4-6 Signed-off-by: Mike Rapoport (Microsoft) Cc: Al Viro Cc: Christian Brauner Cc: David Hildenbrand Cc: Jan Kara Cc: "Kirill A. Shutemov" Cc: Peter Xu Signed-off-by: Andrew Morton

userfaultfd: merge fs/userfaultfd.c into mm/userfaultfd.c

2026-06-04T21:45:03+00:00

Patch series "userfaultfd: merge fs/userfaultfd.c into mm/userfaultfd.c", v3. These patches merge fs/userfaultfd.c into mm/userfaultfd.c and make functions used only inside mm/userfaultfd.c static. This patch (of 2): Historically userfaultfd implementation has been split between fs/userfaultfd.c and mm/userfaultfd.c. The mm/ part implemented memory management operations, while the fs/ part implemented file descriptor handling and called into the mm/ part for the actual memory management work. This separation is quite artificial and fs/userfaultfd.c does not seem to belong to fs/ because it's only a user if vfs APIs and like for other users, for example, memfd and secretmem, the file descriptor handling could live in mm/ as well. "Append" fs/userfaultfd.c to mm/userfaultfd and update fs/Makefile and MAINTAINERS accordingly. No intended functional changes. Link: https://lore.kernel.org/20260523173759.3964908-1-rppt@kernel.org Link: https://lore.kernel.org/20260523173759.3964908-2-rppt@kernel.org Assisted-by: Copilot:claude-opus-4-6 Signed-off-by: Mike Rapoport (Microsoft) Reviewed-by: Christian Brauner (Amutable) Cc: Al Viro Cc: David Hildenbrand Cc: Jan Kara Cc: "Kirill A. Shutemov" Cc: Peter Xu Signed-off-by: Andrew Morton

userfaultfd: remove redundant check in vm_uffd_ops()

2026-06-01T04:50:26+00:00

Lorenzo says: static const struct vm_uffd_ops *vma_uffd_ops(struct vm_area_struct *vma) { if (vma_is_anonymous(vma)) return &anon_uffd_ops; return vma->vm_ops ? vma->vm_ops->uffd_ops : NULL; } This is doing a redundant check _and_ making life confusing, as if !vma->vm_ops is a condition that can be reached there, it can't, as vma_is_anonymous() is literally a !vma->vm_ops check :) Remove the redundant check. Link: https://lore.kernel.org/20260527184751.4147364-4-rppt@kernel.org Fixes: 0f48947c4232 ("userfaultfd: introduce vm_uffd_ops") Signed-off-by: Mike Rapoport (Microsoft) Suggested-by: Lorenzo Stoakes Reviewed-by: Lorenzo Stoakes Cc: David Hildenbrand Cc: Liam R. Howlett Cc: Peter Xu Cc: David Carlier Cc: Michael Bommarito Signed-off-by: Andrew Morton

userfaultfd: refuse to __mfill_atomic_pte() for unsupported VMAs

2026-06-01T04:50:25+00:00

__mfill_atomic_pte() unconditionally dereferences ops because there is an assumption that VMAs that can undergo mfill_* operations are vetted on registration and must have valid vm_uffd_ops. Add a guard against potential bugs and make sure __mfill_atomic_pte() bails out if ops is NULL. Link: https://lore.kernel.org/20260527184751.4147364-3-rppt@kernel.org Fixes: ad9ac3081332 ("userfaultfd: introduce vm_uffd_ops->alloc_folio()") Signed-off-by: Mike Rapoport (Microsoft) Suggested-by: Lorenzo Stoakes Reviewed-by: Lorenzo Stoakes Reviewed-by: David CARLIER Cc: David Hildenbrand Cc: Liam R. Howlett Cc: Michael Bommarito Cc: Peter Xu Signed-off-by: Andrew Morton

userfaultfd: verify VMA state across UFFDIO_COPY retry

2026-06-01T04:50:25+00:00

Patch series "userfaultfd: verify VMA state across UFFDIO_COPY retry", v2. ... and two more small fixes. This patch (of 3): mfill_copy_folio_retry() drops the VMA lock for copy_from_user() and reacquires it afterwards. The destination VMA can be replaced during that window. The existing check compares vma_uffd_ops() before and after the retry, but if a shmem VMA with MAP_SHARED is replaced with a shmem VMA with MAP_PRIVATE (or vice versa) the replacement goes undetected. The change from MAP_PRIVATE to MAP_SHARED will treat the folio allocated with shmem_alloc_folio() as anonymous and this will cause BUG() when mfill_atomic_install_pte() will try to folio_add_new_anon_rmap(). The change from MAP_SHARED to MAP_PRIVATE allows injection of folios into the page cache of the original VMA. There is no need to change for hugetlb because it never uses mfill_copy_folio_retry(). Introduce helpers for more comprehensive comparison of VMA state: - mfill_retry_state_save() to save the relevant VMA state into a struct mfill_retry_state (original uffd_ops, relevant VMA flags, vm_file and pgoff) before dropping the lock - mfill_retry_state_changed() to compare the saved state with the state of the VMA acquired after retaking the locks - mfill_retry_state_put() to release vm_file pinning. Use DEFINE_FREE() cleanup to wrap mfill_retry_state_put() to avoid complicating error handling paths in mfill_copy_folio_retry(). Link: https://lore.kernel.org/20260527184751.4147364-1-rppt@kernel.org Link: https://lore.kernel.org/20260527184751.4147364-2-rppt@kernel.org Fixes: 292411fda25b ("mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry()") Fixes: 6ab703034f14 ("userfaultfd: mfill_atomic(): remove retry logic") Co-developed-by: Michael Bommarito Signed-off-by: Michael Bommarito Signed-off-by: Mike Rapoport (Microsoft) Suggested-by: Peter Xu Co-developed-by: David Carlier Signed-off-by: David Carlier Reviewed-by: Lorenzo Stoakes Cc: David Hildenbrand Cc: Liam R. Howlett Signed-off-by: Andrew Morton

mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry()

2026-04-27T12:54:27+00:00

mfill_copy_folio_retry() drops mmap_lock for the copy_from_user() call. During this window, the VMA can be replaced with a different type (e.g. hugetlb), making the caller's ops pointer stale. Subsequent use of the stale ops would dispatch into the wrong per-vma handlers. Capture the VMA's ops via vma_uffd_ops() before dropping the lock and compare against the current vma_uffd_ops() after re-acquiring it. Return -EAGAIN if they differ so the operation can be retried. This avoids comparing against the caller's ops which may have been overridden to anon_uffd_ops for MAP_PRIVATE file-backed mappings. Link: https://lore.kernel.org/20260424183638.196227-1-devnexen@gmail.com Fixes: 6ab703034f14 ("userfaultfd: mfill_atomic(): remove retry logic") Reported-by: Usama Arif Closes: https://lore.kernel.org/all/20260410114809.3592720-1-usama.arif@linux.dev/ Signed-off-by: David Carlier Acked-by: Mike Rapoport (Microsoft) Cc: Jann Horn Cc: Liam Howlett Cc: Lorenzo Stoakes Cc: Peter Xu Signed-off-by: Andrew Morton

userfaultfd: mfill_atomic(): remove retry logic

2026-04-18T07:10:54+00:00

Since __mfill_atomic_pte() handles the retry for both anonymous and shmem, there is no need to retry copying the date from the userspace in the loop in mfill_atomic(). Drop the retry logic from mfill_atomic(). [rppt@kernel.org: remove safety measure of not returning ENOENT from _copy] Link: https://lore.kernel.org/ac5zcDUY8CFHr6Lw@kernel.org Link: https://lore.kernel.org/20260402041156.1377214-12-rppt@kernel.org Signed-off-by: Mike Rapoport (Microsoft) Cc: Andrea Arcangeli Cc: Andrei Vagin Cc: Axel Rasmussen Cc: Baolin Wang Cc: David Hildenbrand (Arm) Cc: Harry Yoo Cc: Harry Yoo (Oracle) Cc: Hugh Dickins Cc: James Houghton Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Muchun Song Cc: Nikita Kalyazin Cc: Oscar Salvador Cc: Paolo Bonzini Cc: Peter Xu Cc: Sean Christopherson Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: David Carlier Signed-off-by: Andrew Morton

shmem, userfaultfd: implement shmem uffd operations using vm_uffd_ops

2026-04-18T07:10:54+00:00

Add filemap_add() and filemap_remove() methods to vm_uffd_ops and use them in __mfill_atomic_pte() to add shmem folios to page cache and remove them in case of error. Implement these methods in shmem along with vm_uffd_ops->alloc_folio() and drop shmem_mfill_atomic_pte(). Since userfaultfd now does not reference any functions from shmem, drop include if linux/shmem_fs.h from mm/userfaultfd.c mfill_atomic_install_pte() is not used anywhere outside of mm/userfaultfd, make it static. Link: https://lore.kernel.org/20260402041156.1377214-11-rppt@kernel.org Signed-off-by: Mike Rapoport (Microsoft) Reviewed-by: James Houghton Cc: Andrea Arcangeli Cc: Andrei Vagin Cc: Axel Rasmussen Cc: Baolin Wang Cc: David Hildenbrand (Arm) Cc: Harry Yoo Cc: Harry Yoo (Oracle) Cc: Hugh Dickins Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Muchun Song Cc: Nikita Kalyazin Cc: Oscar Salvador Cc: Paolo Bonzini Cc: Peter Xu Cc: Sean Christopherson Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: David Carlier Signed-off-by: Andrew Morton

userfaultfd: introduce vm_uffd_ops->alloc_folio()

2026-04-18T07:10:54+00:00

and use it to refactor mfill_atomic_pte_zeroed_folio() and mfill_atomic_pte_copy(). mfill_atomic_pte_zeroed_folio() and mfill_atomic_pte_copy() perform almost identical actions: * allocate a folio * update folio contents (either copy from userspace of fill with zeros) * update page tables with the new folio Split a __mfill_atomic_pte() helper that handles both cases and uses newly introduced vm_uffd_ops->alloc_folio() to allocate the folio. Pass the ops structure from the callers to __mfill_atomic_pte() to later allow using anon_uffd_ops for MAP_PRIVATE mappings of file-backed VMAs. Note, that the new ops method is called alloc_folio() rather than folio_alloc() to avoid clash with alloc_tag macro folio_alloc(). Link: https://lore.kernel.org/20260402041156.1377214-10-rppt@kernel.org Signed-off-by: Mike Rapoport (Microsoft) Reviewed-by: James Houghton Cc: Andrea Arcangeli Cc: Andrei Vagin Cc: Axel Rasmussen Cc: Baolin Wang Cc: David Hildenbrand (Arm) Cc: Harry Yoo Cc: Harry Yoo (Oracle) Cc: Hugh Dickins Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Muchun Song Cc: Nikita Kalyazin Cc: Oscar Salvador Cc: Paolo Bonzini Cc: Peter Xu Cc: Sean Christopherson Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: David Carlier Signed-off-by: Andrew Morton