FAILED: patch "[PATCH] mm/vma: avoid broken uffd merge on mremap" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 68cb098eadda190783f8f8bdf9fe764df59f7d32
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 68cb098eadda190783f8f8bdf9fe764df59f7d32 Mon Sep 17 00:00:00 2001
From: "Lorenzo Stoakes (ARM)" <[email protected]>
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.

---
 mm/vma.c                          | 5 +++++
 tools/testing/vma/include/stubs.h | 5 +++++
 2 files changed, 10 insertions(+)

index de5ebc57e7a58..1be613044b4a5 100644
@@ -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);
 }
 

index be8f5161f92a2..f786a585caf05 100644
@@ -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)
 {

2.53.0

From: "Lorenzo Stoakes (ARM)" <[email protected]>

[ 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.

---
 mm/mmap.c | 3 +++
 1 file changed, 3 insertions(+)

index 17ec4d40edc11..471dbff3e11de 100644
@@ -3414,6 +3414,9 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 	if (can_self_merge && keep_source &&
 	    (addr == vma->vm_end || vma->vm_start == addr + len))
 		new_vma = NULL;
+	else if (vma_has_uffd_without_event_remap(vma))
+		/* If not registered for remap, uffd context will be cleared. */
+		new_vma = NULL;
 	else
 		new_vma = vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
 				    vma->anon_vma, vma->vm_file, pgoff,

2.53.0