linux-kernel - Re: [PATCH v4 48/49] mm/mmap: Remove __vma

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Message-ID: <ad91d62b-37eb-4b73-707a-3c45c9e16256@suse.cz>
Date:   Wed, 22 Feb 2023 17:17:10 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        maple-tree@...ts.infradead.org
Subject: Re: [PATCH v4 48/49] mm/mmap: Remove __vma_adjust()

On 1/20/23 17:26, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> 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@...cle.com>

...

> @@ -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@...e.cz>
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@...e.cz>
---
 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