[PATCH] mm/vma: avoid broken uffd merge on mremap

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

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.

Reported-by: shaos <shawnq1@163.com>
Closes: https://lore.kernel.org/all/20261020110748.959870-1-shawnq1@163.com/
Fixes: 3cfd22be0ad6 ("userfaultfd: clear flag if remap event not enabled")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 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)
 {

---
base-commit: 3eb3e848d72cde2156447cc960ca321bb2ac41a9
change-id: 20261020-uffd-remap-fix-97626873b5c7

Cheers,
-- 
Lorenzo Stoakes (ARM) <ljs@kernel.org>