From 292411fda25bdcb8cd0cb513fa5583a36dd36354 Mon Sep 17 00:00:00 2001 From: David Carlier Date: Fri, 24 Apr 2026 19:36:38 +0100 Subject: mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry() 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 --- mm/userfaultfd.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 885da1e56466..180bad42fc79 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -443,8 +443,10 @@ static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr) return ret; } -static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio) +static int mfill_copy_folio_retry(struct mfill_state *state, + struct folio *folio) { + const struct vm_uffd_ops *orig_ops = vma_uffd_ops(state->vma); unsigned long src_addr = state->src_addr; void *kaddr; int err; @@ -465,6 +467,14 @@ static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio if (err) return err; + /* + * The VMA type may have changed while the lock was dropped + * (e.g. replaced with a hugetlb mapping), making the caller's + * ops pointer stale. + */ + if (vma_uffd_ops(state->vma) != orig_ops) + return -EAGAIN; + err = mfill_establish_pmd(state); if (err) return err; -- cgit v1.2.3