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* [PATCH] mm/vma: fix anon_vma UAF on mremap() faulted, unfaulted merge
@ 2026-01-02 20:55 Lorenzo Stoakes
  2026-01-02 21:00 ` Lorenzo Stoakes
  0 siblings, 1 reply; 2+ messages in thread
From: Lorenzo Stoakes @ 2026-01-02 20:55 UTC (permalink / raw)
  To: Andrew Morton
  Cc: Liam R . Howlett, Vlastimil Babka, Jann Horn, Pedro Falcato,
	Yeoreum Yun, linux-mm, linux-kernel, David Hildenbrand,
	Jeongjun Park, Rik van Riel, Harry Yoo

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.

To account for this, introduce a new field in struct vma_merge_struct
specifically for the mremap() case, and update vma_expand() to explicitly
check for this case and invoke dup_anon_vma() to ensure anon_vma state is
correctly propagated.

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.

The following program reproduces the issue (and is fixed by this patch):

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>

#define RESERVED_PGS	(100)
#define VMA_A_PGS	(10)
#define VMA_B_PGS	(10)
#define NUM_ITERS	(1000)

static void trigger_bug(void)
{
	unsigned long page_size = sysconf(_SC_PAGE_SIZE);
	char *reserved, *ptr_a, *ptr_b;

	/*
	 * The goal here is to achieve:
	 *
	 * mremap() with MREMAP_DONTUNMAP such that A and B merge:
	 *
	 *      |-------------------------|
	 *      |                         |
	 *      |    |-----------|   |---------|
	 *      v    | unfaulted |   | faulted |
	 *           |-----------|   |---------|
	 *                 B              A
	 *
	 * Then unmap VMA A to trigger the bug.
	 */

	/* Reserve a region of memory to operate in. */
	reserved = mmap(NULL, RESERVED_PGS * page_size, PROT_NONE,
			MAP_PRIVATE | MAP_ANON, -1, 0);
	if (reserved == MAP_FAILED) {
		perror("mmap reserved");
		exit(EXIT_FAILURE);
	}

	/* Map VMA A into place. */
	ptr_a = mmap(&reserved[page_size], VMA_A_PGS * page_size,
		     PROT_READ | PROT_WRITE,
		     MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
	if (ptr_a == MAP_FAILED) {
		perror("mmap VMA A");
		exit(EXIT_FAILURE);
	}
	/* Fault it in. */
	ptr_a[0] = 'x';

	/*
	 * Now move it out of the way so we can place VMA B in position,
	 * unfaulted.
	 */
	ptr_a = mremap(ptr_a, VMA_A_PGS * page_size, VMA_A_PGS * page_size,
		       MREMAP_FIXED | MREMAP_MAYMOVE, &reserved[50 * page_size]);
	if (ptr_a == MAP_FAILED) {
		perror("mremap VMA A out of the way");
		exit(EXIT_FAILURE);
	}

	/* Map VMA B into place. */
	ptr_b = mmap(&reserved[page_size + VMA_A_PGS * page_size],
		     VMA_B_PGS * page_size, PROT_READ | PROT_WRITE,
		     MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
	if (ptr_b == MAP_FAILED) {
		perror("mmap VMA B");
		exit(EXIT_FAILURE);
	}

	/* Now move VMA A into position w/MREMAP_DONTUNMAP + free anon_vma. */
	ptr_a = mremap(ptr_a, VMA_A_PGS * page_size, VMA_A_PGS * page_size,
		       MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
		       &reserved[page_size]);
	if (ptr_a == MAP_FAILED) {
		perror("mremap VMA A with MREMAP_DONTUNMAP");
		exit(EXIT_FAILURE);
	}

	/* Finally, unmap VMA A which should trigger the bug. */
	munmap(ptr_a, VMA_A_PGS * page_size);

	/* Cleanup in case bug didn't trigger sufficiently visibly... */
	munmap(reserved, RESERVED_PGS * page_size);
}

int main(void)
{
	int i;

	for (i = 0; i < NUM_ITERS; i++)
		trigger_bug();

	return EXIT_SUCCESS;
}

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/
Cc: stable@kernel.org
---
 mm/vma.c | 58 ++++++++++++++++++++++++++++++++++++++++++--------------
 mm/vma.h |  3 +++
 2 files changed, 47 insertions(+), 14 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index 6377aa290a27..2268f518a89b 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -1130,26 +1130,50 @@ int vma_expand(struct vma_merge_struct *vmg)
 	mmap_assert_write_locked(vmg->mm);

 	vma_start_write(target);
-	if (next && (target != next) && (vmg->end == next->vm_end)) {
+	if (next && vmg->end == next->vm_end) {
+		struct vm_area_struct *copied_from = vmg->copied_from;
 		int ret;

-		sticky_flags |= next->vm_flags & VM_STICKY;
-		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;
+		if (target != next) {
+			sticky_flags |= next->vm_flags & VM_STICKY;
+			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;
+		} else if (copied_from) {
+			vma_start_write(next);
+
+			/*
+			 * We are copying from a VMA (i.e. mremap()'ing) to
+			 * next, and thus must ensure that either anon_vma's are
+			 * already compatible (in which case this call is a nop)
+			 * or all anon_vma state is propagated to next
+			 */
+			ret = dup_anon_vma(next, copied_from, &anon_dup);
+			if (ret)
+				return ret;
+		} else {
+			/* In no other case may the anon_vma differ. */
+			VM_WARN_ON_VMG(target->anon_vma != next->anon_vma, 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);
+	/*
+	 * We should only see a copy with next as the target on a new merge
+	 * which sets the end to the next of next.
+	 */
+	VM_WARN_ON_VMG(target == next && vmg->copied_from &&
+		       vmg->end != next->vm_end, vmg);
 	/* Only handles expanding */
 	VM_WARN_ON_VMG(target->vm_start < vmg->start ||
 		       target->vm_end > vmg->end, vmg);
@@ -1807,6 +1831,13 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 	VMA_ITERATOR(vmi, mm, addr);
 	VMG_VMA_STATE(vmg, &vmi, NULL, vma, addr, addr + len);

+	/*
+	 * VMG_VMA_STATE() installs vma in middle, but this is a new VMA, inform
+	 * merging logic correctly.
+	 */
+	vmg.copied_from = vma;
+	vmg.middle = NULL;
+
 	/*
 	 * If anonymous vma has not yet been faulted, update new pgoff
 	 * to match new location, to increase its chance of merging.
@@ -1828,7 +1859,6 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 	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);
diff --git a/mm/vma.h b/mm/vma.h
index e4c7bd79de5f..50f0bdb0eb79 100644
--- 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 we are copying a VMA, which VMA are we copying from? */
+	struct vm_area_struct *copied_from;
+
 	/* Flags which callers can use to modify merge behaviour: */

 	/*
--
2.52.0


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