From 68cb098eadda190783f8f8bdf9fe764df59f7d32 Mon Sep 17 00:00:00 2001 From: "Lorenzo Stoakes (ARM)" Date: Tue, 20 Oct 2026 15:55:30 +0100 Subject: [PATCH] mm/vma: avoid broken uffd merge on mremap Userfaultfd often causes unexpected interactions within mm, and does so at the intersection of uffd, mremap() and VMA merging. When an mremap of a VMA occurs, copy_vma_and_data() calls mremap_userfaultfd_prep() to notify uffd of this. However, mremap_userfaultfd_prep() simply drops the UFFD context for the VMA if the user did not register it for remap, i.e. didn't specify the UFFD_FEATURE_EVENT_REMAP flag. This is OK for the VMA itself, as upon page table move, move_ptes() clears UFFD page table flags, using vma_has_uffd_without_event_remap() to determine whether to do so. However, this doesn't account for merging with adjacent VMAs, whose page tables won't have received this treatment, and thus latent UFFD markers will be left around. Worse, it'll break UFFD for any VMAs it is merged with. This is very surprising behaviour for a user, who will then simply not receive UFFD events for VMAs they ought to. Resolve the issue by explicitly checking for this case via vma_has_uffd_without_event_remap() in vma_merge_copied_range(), the merge function explicitly designated for mremap() operations. Also update the userland VMA tests accordingly. Link: https://lore.kernel.org/20261020-uffd-remap-fix-v1-1-0f34fc181f0a@kernel.org Fixes: 3cfd22be0ad6 ("userfaultfd: clear flag if remap event not enabled") Signed-off-by: Lorenzo Stoakes (ARM) Signed-off-by: Andrew Morton Reported-by: shaos Closes: https://lore.kernel.org/all/20261020110748.959870-1-shawnq1@163.com/ Reviewed-by: Kiryl Shutsemau (Meta) Reviewed-by: Gregory Price (Meta) Reviewed-by: Xueyuan Chen Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Jann Horn Cc: Pedro Falcato Cc: Mike Rapoport Cc: Peter Xu Cc: David Hildenbrand Cc: --- mm/vma.c | 5 +++++ tools/testing/vma/include/stubs.h | 5 +++++ 2 files changed, 10 insertions(+) diff --git a/mm/vma.c b/mm/vma.c index de5ebc57e7a5..1be613044b4a 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -1211,6 +1211,11 @@ static struct vm_area_struct *vma_merge_copied_range(struct vma_merge_struct *vm vmg->copied_from = vmg->middle; vmg->middle = NULL; + + /* If not registered for remap, uffd context will be cleared. */ + if (vma_has_uffd_without_event_remap(vmg->copied_from)) + return NULL; + return vma_merge_new_range(vmg); } diff --git a/tools/testing/vma/include/stubs.h b/tools/testing/vma/include/stubs.h index be8f5161f92a..f786a585caf0 100644 --- a/tools/testing/vma/include/stubs.h +++ b/tools/testing/vma/include/stubs.h @@ -386,6 +386,11 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma, return true; } +static inline bool vma_has_uffd_without_event_remap(struct vm_area_struct *vma) +{ + return false; +} + static inline bool anon_vma_name_eq(struct anon_vma_name *anon_name1, struct anon_vma_name *anon_name2) {