linux-kernel - Re: [PATCH] mm/rmap: Add anon

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Message-ID: <85c4a092-14df-4478-811c-f3789610e4b8@redhat.com>
Date: Thu, 24 Jul 2025 23:56:03 +0200
From: David Hildenbrand <david@...hat.com>
To: Jann Horn <jannh@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Rik van Riel
 <riel@...riel.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Harry Yoo <harry.yoo@...cle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: Add anon_vma lifetime debug check

On 24.07.25 21:13, Jann Horn wrote:
> If an anon page is mapped into userspace, its anon_vma must be alive,
> otherwise rmap walks can hit UAF.
> 
> There have been syzkaller reports a few months ago[1][2] of UAF in rmap
> walks that seems to indicate that there can be pages with elevated mapcount
> whose anon_vma has already been freed, but I think we never figured out
> what the cause is; and syzkaller only hit these UAFs when memory pressure
> randomly caused reclaim to rmap-walk the affected pages, so it of course
> didn't manage to create a reproducer.
> 
> Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous pages to
> hopefully catch such issues more reliably.
> 
> Implementation note: I'm checking IS_ENABLED(CONFIG_DEBUG_VM) because,
> unlike the checks above, this one would otherwise be hard to write such
> that it completely compiles away in non-debug builds by itself, without
> looking extremely ugly.
> 
> [1] https://lore.kernel.org/r/67abaeaf.050a0220.110943.0041.GAE@google.com
> [2] https://lore.kernel.org/r/67a76f33.050a0220.3d72c.0028.GAE@google.com
> 
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
>   include/linux/rmap.h | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index c4f4903b1088..ba694c436f59 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -449,6 +449,19 @@ static inline void __folio_rmap_sanity_checks(const struct folio *folio,
>   	default:
>   		VM_WARN_ON_ONCE(true);
>   	}
> +
> +	/*
> +	 * Anon folios must have an associated live anon_vma as long as they're
> +	 * mapped into userspace.
> +	 * Part of the purpose of the atomic_read() is to make KASAN check that
> +	 * the anon_vma is still alive.
> +	 */
> +	if (IS_ENABLED(CONFIG_DEBUG_VM) && PageAnonNotKsm(page)) {

1) You probably don't need the CONFIG_DEBUG_VM check: the 
VM_WARN_ON_FOLIO should make everything get optimized out ... right?

2) We have a folio here, so ... better

if (folio_test_anon(folio) && !folio_test_ksm(folio)) {
	...
}

> +		unsigned long mapping = (unsigned long)folio->mapping;
> +		struct anon_vma *anon_vma = (void *)(mapping - PAGE_MAPPING_ANON);
> +
> +		VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0, folio);
> +	}

In general,

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb