On 1/20/23 17:26, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
>
> Inline the work of __vma_adjust() into vma_merge(). This reduces code
> size and has the added benefits of the comments for the cases being
> located with the code.
>
> Change the comments referencing vma_adjust() accordingly.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
...
> @@ -1054,32 +945,85 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> vm_userfaultfd_ctx, anon_name)) {
> merge_next = true;
> }
> +
> + remove = remove2 = adjust = NULL;
> /* Can we merge both the predecessor and the successor? */
> if (merge_prev && merge_next &&
> - is_mergeable_anon_vma(prev->anon_vma,
> - next->anon_vma, NULL)) { /* cases 1, 6 */
> - err = __vma_adjust(vmi, prev, prev->vm_start,
> - next->vm_end, prev->vm_pgoff, prev);
> - res = prev;
> - } else if (merge_prev) { /* cases 2, 5, 7 */
> - err = __vma_adjust(vmi, prev, prev->vm_start,
> - end, prev->vm_pgoff, prev);
> - res = prev;
> + is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
> + remove = mid; /* case 1 */
> + vma_end = next->vm_end;
> + err = dup_anon_vma(res, remove);
> + if (mid != next) { /* case 6 */
> + remove2 = next;
> + if (!remove->anon_vma)
> + err = dup_anon_vma(res, remove2);
> + }
> + } else if (merge_prev) {
> + err = 0; /* case 2 */
> + if (mid && end > mid->vm_start) {
> + err = dup_anon_vma(res, mid);
> + if (end == mid->vm_end) { /* case 7 */
> + remove = mid;
> + } else { /* case 5 */
> + adjust = mid;
> + adj_next = (end - mid->vm_start);
> + }
> + }
> } else if (merge_next) {
> - if (prev && addr < prev->vm_end) /* case 4 */
> - err = __vma_adjust(vmi, prev, prev->vm_start,
> - addr, prev->vm_pgoff, next);
> - else /* cases 3, 8 */
> - err = __vma_adjust(vmi, mid, addr, next->vm_end,
> - next->vm_pgoff - pglen, next);
> res = next;
> + if (prev && addr < prev->vm_end) { /* case 4 */
> + vma_end = addr;
> + adjust = mid;
> + adj_next = -(vma->vm_end - addr);
> + err = dup_anon_vma(res, adjust);
I think this one is wrong, and should be fixed as below. I'm not
exactly sure about user visible effects, but shouldn't matter if
we fix before rc1? I guess what can happen is we end up with pages
becoming part of 'prev' that have anon_vma originally from 'mid'
which is not connected to 'prev', so eventually some rmap operation
will fail to do the right thing etc. Or 'mid' is unmapped, its
anon_vma freed and we have a use-after free. Probably rare to happen,
but nasty enough.
----8<----
>From 854f4cef0fecde9a0a89ff1a5beb0a1e2115363f Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@xxxxxxx>
Date: Wed, 22 Feb 2023 16:51:46 +0100
Subject: [PATCH urgent for 6.3-rc1] mm/mremap: fix dup_anon_vma() in vma_merge() case 4
In case 4, we are shrinking 'prev' (PPPP in the comment) and expanding
'mid' (NNNN). So we need to make sure 'mid' clones the anon_vma from
'prev', if it doesn't have any. After commit 0503ea8f5ba7 ("mm/mmap:
remove __vma_adjust()") we can fail to do that due to wrong parameters
for dup_anon_vma(). The call is a no-op because res == next, adjust ==
mid and mid == next. Fix it.
Fixes: 0503ea8f5ba7 ("mm/mmap: remove __vma_adjust()")
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
mm/mmap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index 20f21f0949dd..740b54be3ed4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -973,7 +973,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
vma_end = addr;
adjust = mid;
adj_next = -(vma->vm_end - addr);
- err = dup_anon_vma(res, adjust);
+ err = dup_anon_vma(adjust, prev);
} else {
vma = next; /* case 3 */
vma_start = addr;
--
2.39.2
