[PATCH 6.18 226/227] mm/vma: fix anon_vma UAF on mremap() faulted, unfaulted merge

Greg Kroah-Hartman
2026-01-28T16:02:49Z

From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

[ upstream commit 61f67c230a5e7c741c352349ea80147fbe65bfae ]

Patch series "mm/vma: fix anon_vma UAF on mremap() faulted, unfaulted
merge", v2.

Commit 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA
merges") introduced the ability to merge previously unavailable VMA merge
scenarios.

However, it is handling merges incorrectly when it comes to mremap() of a
faulted VMA adjacent to an unfaulted VMA. The issues arise in three
cases:

1. Previous VMA unfaulted:

copied -----|
v
|-----------|.............|
| unfaulted |(faulted VMA)|
|-----------|.............|
prev

2. Next VMA unfaulted:

copied -----|
v
|.............|-----------|
|(faulted VMA)| unfaulted |
|.............|-----------|
next

3. Both adjacent VMAs unfaulted:

copied -----|
v
|-----------|.............|-----------|
| unfaulted |(faulted VMA)| unfaulted |
|-----------|.............|-----------|
prev next

This series fixes each of these cases, and introduces self tests to assert
that the issues are corrected.

I also test a further case which was already handled, to assert that my
changes continues to correctly handle it:

4. prev unfaulted, next faulted:

copied -----|
v
|-----------|.............|-----------|
| unfaulted |(faulted VMA)| faulted |
|-----------|.............|-----------|
prev next

This bug was discovered via a syzbot report, linked to in the first patch
in the series, I confirmed that this series fixes the bug.

I also discovered that we are failing to check that the faulted VMA was
not forked when merging a copied VMA in cases 1-3 above, an issue this
series also addresses.

I also added self tests to assert that this is resolved (and confirmed
that the tests failed prior to this).

I also cleaned up vma_expand() as part of this work, renamed
vma_had_uncowed_parents() to vma_is_fork_child() as the previous name was
unduly confusing, and simplified the comments around this function.

This patch (of 4):

Commit 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA
merges") introduced the ability to merge previously unavailable VMA merge
scenarios.

The key piece of logic introduced was the ability to merge a faulted VMA
immediately next to an unfaulted VMA, which relies upon dup_anon_vma() to
correctly handle anon_vma state.

In the case of the merge of an existing VMA (that is changing properties
of a VMA and then merging if those properties are shared by adjacent
VMAs), dup_anon_vma() is invoked correctly.

However in the case of the merge of a new VMA, a corner case peculiar to
mremap() was missed.

The issue is that vma_expand() only performs dup_anon_vma() if the target
(the VMA that will ultimately become the merged VMA): is not the next VMA,
i.e. the one that appears after the range in which the new VMA is to be
established.

A key insight here is that in all other cases other than mremap(), a new
VMA merge either expands an existing VMA, meaning that the target VMA will
be that VMA, or would have anon_vma be NULL.

Specifically:

* __mmap_region() - no anon_vma in place, initial mapping.
* do_brk_flags() - expanding an existing VMA.
* vma_merge_extend() - expanding an existing VMA.
* relocate_vma_down() - no anon_vma in place, initial mapping.

In addition, we are in the unique situation of needing to duplicate
anon_vma state from a VMA that is neither the previous or next VMA being
merged with.

dup_anon_vma() deals exclusively with the target=unfaulted, src=faulted
case. This leaves four possibilities, in each case where the copied VMA
is faulted:

1. Previous VMA unfaulted:

copied -----|
v
|-----------|.............|
| unfaulted |(faulted VMA)|
|-----------|.............|
prev

target = prev, expand prev to cover.

2. Next VMA unfaulted:

copied -----|
v
|.............|-----------|
|(faulted VMA)| unfaulted |
|.............|-----------|
next

target = next, expand next to cover.

3. Both adjacent VMAs unfaulted:

copied -----|
v
|-----------|.............|-----------|
| unfaulted |(faulted VMA)| unfaulted |
|-----------|.............|-----------|
prev next

target = prev, expand prev to cover.

4. prev unfaulted, next faulted:

copied -----|
v
|-----------|.............|-----------|
| unfaulted |(faulted VMA)| faulted |
|-----------|.............|-----------|
prev next

target = prev, expand prev to cover. Essentially equivalent to 3, but
with additional requirement that next's anon_vma is the same as the copied
VMA's. This is covered by the existing logic.

To account for this very explicitly, we introduce
vma_merge_copied_range(), which sets a newly introduced vmg->copied_from
field, then invokes vma_merge_new_range() which handles the rest of the
logic.

We then update the key vma_expand() function to clean up the logic and
make what's going on clearer, making the 'remove next' case less special,
before invoking dup_anon_vma() unconditionally should we be copying from a
VMA.

Note that in case 3, the if (remove_next) ... branch will be a no-op, as
next=src in this instance and src is unfaulted.

In case 4, it won't be, but since in this instance next=src and it is
faulted, this will have required tgt=faulted, src=faulted to be
compatible, meaning that next->anon_vma == vmg->copied_from->anon_vma, and
thus a single dup_anon_vma() of next suffices to copy anon_vma state for
the copied-from VMA also.

If we are copying from a VMA in a successful merge we must _always_
propagate anon_vma state.

This issue can be observed most directly by invoked mremap() to move
around a VMA and cause this kind of merge with the MREMAP_DONTUNMAP flag
specified.

This will result in unlink_anon_vmas() being called after failing to
duplicate anon_vma state to the target VMA, which results in the anon_vma
itself being freed with folios still possessing dangling pointers to the
anon_vma and thus a use-after-free bug.

This bug was discovered via a syzbot report, which this patch resolves.

We further make a change to update the mergeable anon_vma check to assert
the copied-from anon_vma did not have CoW parents, as otherwise
dup_anon_vma() might incorrectly propagate CoW ancestors from the next VMA
in case 4 despite the anon_vma's being identical for both VMAs.

Link: https://lkml.kernel.org/r/cover.1767638272.git.lorenzo.stoakes@oracle.com
Link: https://lkml.kernel.org/r/b7930ad2b1503a657e29fe928eb33061d7eadf5b.1767638272.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Fixes: 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges")
Reported-by: syzbot+b165fc2e11771c66d8ba@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/694a2745.050a0220.19928e.0017.GAE@google.com/
Reported-by: syzbot+5272541ccbbb14e2ec30@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/694e3dc6.050a0220.35954c.0066.GAE@google.com/
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Jeongjun Park <aha310510@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Yeoreum Yun <yeoreum.yun@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ updated to account for lack of sticky VMA flags + built, tested confirmed working ]
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/vma.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++----------------
mm/vma.h | 3 ++
2 files changed, 56 insertions(+), 18 deletions(-)

--- a/mm/vma.c
+++ b/mm/vma.c
@@ -835,6 +835,8 @@ static __must_check struct vm_area_struc
VM_WARN_ON_VMG(middle &&
!(vma_iter_addr(vmg->vmi) >= middle->vm_start &&
vma_iter_addr(vmg->vmi) < middle->vm_end), vmg);
+ /* An existing merge can never be used by the mremap() logic. */
+ VM_WARN_ON_VMG(vmg->copied_from, vmg);

vmg->state = VMA_MERGE_NOMERGE;

@@ -1102,6 +1104,33 @@ struct vm_area_struct *vma_merge_new_ran
}

/*
+ * vma_merge_copied_range - Attempt to merge a VMA that is being copied by
+ * mremap()
+ *
+ * @vmg: Describes the VMA we are adding, in the copied-to range @vmg->start to
+ * @vmg->end (exclusive), which we try to merge with any adjacent VMAs if
+ * possible.
+ *
+ * vmg->prev, next, start, end, pgoff should all be relative to the COPIED TO
+ * range, i.e. the target range for the VMA.
+ *
+ * Returns: In instances where no merge was possible, NULL. Otherwise, a pointer
+ * to the VMA we expanded.
+ *
+ * ASSUMPTIONS: Same as vma_merge_new_range(), except vmg->middle must contain
+ * the copied-from VMA.
+ */
+static struct vm_area_struct *vma_merge_copied_range(struct vma_merge_struct *vmg)
+{
+ /* We must have a copied-from VMA. */
+ VM_WARN_ON_VMG(!vmg->middle, vmg);
+
+ vmg->copied_from = vmg->middle;
+ vmg->middle = NULL;
+ return vma_merge_new_range(vmg);
+}
+
+/*
* vma_expand - Expand an existing VMA
*
* @vmg: Describes a VMA expansion operation.
@@ -1123,38 +1152,45 @@ int vma_expand(struct vma_merge_struct *
bool remove_next = false;
struct vm_area_struct *target = vmg->target;
struct vm_area_struct *next = vmg->next;
+ int ret = 0;

VM_WARN_ON_VMG(!target, vmg);

mmap_assert_write_locked(vmg->mm);
-
vma_start_write(target);
- if (next && (target != next) && (vmg->end == next->vm_end)) {
- int ret;

+ if (next && target != next && vmg->end == next->vm_end)
remove_next = true;
- /* This should already have been checked by this point. */
- VM_WARN_ON_VMG(!can_merge_remove_vma(next), vmg);
- vma_start_write(next);
- /*
- * In this case we don't report OOM, so vmg->give_up_on_mm is
- * safe.
- */
- ret = dup_anon_vma(target, next, &anon_dup);
- if (ret)
- return ret;
- }

+ /* We must have a target. */
+ VM_WARN_ON_VMG(!target, vmg);
+ /* This should have already been checked by this point. */
+ VM_WARN_ON_VMG(remove_next && !can_merge_remove_vma(next), vmg);
/* Not merging but overwriting any part of next is not handled. */
VM_WARN_ON_VMG(next && !remove_next &&
next != target && vmg->end > next->vm_start, vmg);
- /* Only handles expanding */
+ /* Only handles expanding. */
VM_WARN_ON_VMG(target->vm_start < vmg->start ||
target->vm_end > vmg->end, vmg);

+ /*
+ * If we are removing the next VMA or copying from a VMA
+ * (e.g. mremap()'ing), we must propagate anon_vma state.
+ *
+ * Note that, by convention, callers ignore OOM for this case, so
+ * we don't need to account for vmg->give_up_on_mm here.
+ */
if (remove_next)
- vmg->__remove_next = true;
+ ret = dup_anon_vma(target, next, &anon_dup);
+ if (!ret && vmg->copied_from)
+ ret = dup_anon_vma(target, vmg->copied_from, &anon_dup);
+ if (ret)
+ return ret;

+ if (remove_next) {
+ vma_start_write(next);
+ vmg->__remove_next = true;
+ }
if (commit_merge(vmg))
goto nomem;

@@ -1837,10 +1873,9 @@ struct vm_area_struct *copy_vma(struct v
if (new_vma && new_vma->vm_start < addr + len)
return NULL; /* should never get here */

- vmg.middle = NULL; /* New VMA range. */
vmg.pgoff = pgoff;
vmg.next = vma_iter_next_rewind(&vmi, NULL);
- new_vma = vma_merge_new_range(&vmg);
+ new_vma = vma_merge_copied_range(&vmg);

if (new_vma) {
/*
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -106,6 +106,9 @@ struct vma_merge_struct {
struct anon_vma_name *anon_name;
enum vma_merge_state state;

+ /* If copied from (i.e. mremap()'d) the VMA from which we are copying. */
+ struct vm_area_struct *copied_from;
+
/* Flags which callers can use to modify merge behaviour: */

/*