[PATCH 6.12.y] mm/vma: avoid broken uffd merge on mremap

[Sasha Levin <sashal@kernel.org>]

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>

[ Upstream commit 68cb098eadda190783f8f8bdf9fe764df59f7d32 ]

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) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: shaos <shawnq1@163.com>
Closes: https://lore.kernel.org/all/20261020110748.959870-1-shawnq1@163.com/
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: <stable@vger.kernel.org>
[ sashal: moved the merge guard into copy_vma() because vma_merge_copied_range() is absent ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/vma.c                         | 5 ++++-
 tools/testing/vma/vma_internal.h | 2 ++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/vma.c b/mm/vma.c
index 66b0472b7d8a6..85660d67b7942 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -1694,7 +1694,10 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 		faulted_in_anon_vma = true;
 	}
 
-	new_vma = vma_merge_new_range(&vmg);
+	/* If not registered for remap, uffd context will be cleared. */
+	new_vma = NULL;
+	if (!vma_has_uffd_without_event_remap(vma))
+		new_vma = vma_merge_new_range(&vmg);
 
 	if (new_vma) {
 		/*
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index 0a95dbb0346fb..213e40afa8ee3 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -605,6 +605,8 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
 	return true;
 }
 
+#define vma_has_uffd_without_event_remap(vma) false
+
 static inline bool anon_vma_name_eq(struct anon_vma_name *anon_name1,
 				    struct anon_vma_name *anon_name2)
 {
-- 
2.53.0