[PATCH] mm/mremap: reset unfaulted VMA page offset for MREMAP_DONTUNMAP

[Lorenzo Stoakes (ARM)]

Uniquely an mremap() invocation using the MREMAP_DONTUNMAP flag can reset
a faulted VMA into an unfaulted one.

It does so after the page tables have been moved to the copied VMA with
MREMAP_DONTUNMAP leaving the old VMA in place which is naturally unfaulted
as the page tables it had are no longer present.

However, in doing so, it violates the invariant that the anonymous page
offset of an unfaulted VMA is vma->vm_start >> PAGE_SHIFT.

This is because a VMA may have been faulted in, mremap()'d (causing a delta
between its page offset and vma->vm_start >> PAGE_SHIFT), and then
mremap()'d again with MREMAP_DONTUNMAP resulting in the unfaulting.

This condition is a violation of a fundamental assumption in mm, but now
also triggers an assert in assert_sane_pgoff() which explicitly checks for
this condition.

Correct it by resetting the VMA's page offset at the point of completing
the MREMAP_DONTUNMAP operation.

Reported-by: syzbot+f12658786a4153df5113@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/6a87853b.ae6ddae5.3da009.0023.GAE@xxxxxxxxxx/
Fixes: 1583aa278f5f ("mm: mremap: unlink anon_vmas when mremap with MREMAP_DONTUNMAP success")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 mm/mremap.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index e8df5cdb0ac9..2b4b523a86b8 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1331,18 +1331,30 @@ static void dontunmap_complete(struct vma_remap_struct *vrm,
 {
 	unsigned long start = vrm->addr;
 	unsigned long end = vrm->addr + vrm->old_len;
-	unsigned long old_start = vrm->vma->vm_start;
-	unsigned long old_end = vrm->vma->vm_end;
+	struct vm_area_struct *vma = vrm->vma;
+	unsigned long old_start = vma->vm_start;
+	unsigned long old_end = vma->vm_end;
 
 	/* We always clear VMA_LOCKED[ONFAULT]_BIT on the old VMA. */
-	vma_clear_flags_mask(vrm->vma, VMA_LOCKED_MASK);
+	vma_clear_flags_mask(vma, VMA_LOCKED_MASK);
 
 	/*
 	 * anon_vma links of the old vma is no longer needed after its page
 	 * table has been moved.
 	 */
-	if (new_vma != vrm->vma && start == old_start && end == old_end)
-		unlink_anon_vmas(vrm->vma);
+	if (new_vma != vma && start == old_start && end == old_end) {
+		const pgoff_t pgoff_unfaulted = vma->vm_start >> PAGE_SHIFT;
+
+		unlink_anon_vmas(vma);
+		/*
+		 * The VMA is now unfaulted and it is an invariant that
+		 * unfaulted anonymous VMAs have page offset equal to
+		 * vma->vm_start >> PAGE_SHIFT.
+		 */
+		vma_set_anon_pgoff(vma, pgoff_unfaulted);
+		if (vma_is_anonymous(vma) && !vma->vm_file)
+			vma_set_pgoff(vma, pgoff_unfaulted);
+	}
 
 	/* Because we won't unmap we don't need to touch locked_vm. */
 }

---
base-commit: efecab401cb15fd3bb9bc05990609acb6b267ff2
change-id: 20260824-fix-mremap-dontunmap-pgoff-a687134e995e

Best regards,
-- 
Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>