linux-kernel - Re: [PATCH] mm: correctly reference merged VMA

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Message-ID: <qvsozunpi4yvooov4usji4xu3v2dt52f3mxcwkwtdcywu6xpgb@oald6acxjrui>
Date: Fri, 6 Dec 2024 17:40:39 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: correctly reference merged VMA

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [241206 16:52]:
> 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@...gle.com>
> Reported-by: syzbot+91cf8da9401355f946c3@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/67536a25.050a0220.a30f1.0149.GAE@google.com/
> Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()")
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

My cries to delete this optimisation continue.

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  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