Re: [PATCH] mm: correctly reference merged VMA

[Lorenzo Stoakes]

Andrew - sorry, I forgot to put a subject-prefix in here - we need this as
a (fairly urgent) hotfix for 6.13 please.

Thanks!

On Fri, Dec 06, 2024 at 09:52:29PM +0000, Lorenzo Stoakes wrote:
> On second merge attempt on mmap() we incorrectly discard the possibly
> merged VMA, resulting in a possible use-after-free (and most certainly a
> reference to the wrong VMA) in this instance in the subsequent
> __mmap_complete() invocation.
>
> Correct this mistake by reassigning vma correctly if a merge succeeds in
> this case.
>
> Suggested-by: Jann Horn <jannh@xxxxxxxxxx>
> Reported-by: syzbot+91cf8da9401355f946c3@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/67536a25.050a0220.a30f1.0149.GAE@xxxxxxxxxx/
> Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()")
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
>  mm/vma.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index 8a454a7bbc80..1ec349141f5e 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2460,10 +2460,13 @@ unsigned long __mmap_region(struct file *file, unsigned long addr,
>
>  	/* If flags changed, we might be able to merge, so try again. */
>  	if (map.retry_merge) {
> +		struct vm_area_struct *merged;
>  		VMG_MMAP_STATE(vmg, &map, vma);
>
>  		vma_iter_config(map.vmi, map.addr, map.end);
> -		vma_merge_existing_range(&vmg);
> +		merged = vma_merge_existing_range(&vmg);
> +		if (merged)
> +			vma = merged;
>  	}
>
>  	__mmap_complete(&map, vma);
> --
> 2.47.1