Re: [PATCH] mm/rmap: Add anon_vma lifetime debug check

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Jul 24, 2025 at 09:13:50PM +0200, Jann Horn wrote:
> If an anon page is mapped into userspace, its anon_vma must be alive,
> otherwise rmap walks can hit UAF.

This is extremely weird. I guess it's a UAF of vma->anon_vma

Because we:

put_anon_vma()
->__put_anon_vma()
->anon_vma_free() (also the root anon_vma... interestingly!)

But none of this obviously updates the vma to set vma->anon_vma to NULL.

So yeah god almighty. To get this, we must be dereffing vma->anon_vma
somewhere unexpected.

>
> There have been syzkaller reports a few months ago[1][2] of UAF in rmap

Will try to take a look when I get a chance.

> 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.

Fun.

Please hook me in (I mean you're going to anyway right :P) on this stuff,
as I'm looking to rework the anon_vma stuff so am naturally interested in
any and all rmap anon stuff.

For my sins ;)

Maybe I"ll dig into these syzkallers.

>
> Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous pages to
> hopefully catch such issues more reliably.

Good idea.

>
> 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.

David already addressed.

>
> [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@google.com>

Nit below, and pending David's requests, but LGTM so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.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)) {
> +		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);

Maybe stupid question, but wouldn't we pick this up with KASAN? Or would we
pick it up far too late I guess?

We're sort of relying on this

a. being a UAF

b. the thing we're UAF-ing not either corrupting this field or (if that
    memory is actually reused as an anon_vma - I'm not familiar with slab
    caches - so maybe it's quite likely - getting its refcount incremented.

Which is fine - because hey this is the only way we can get a hint that
this is happening, but I think we should describe what we're doing.

> +	}
>  }

David has made all the points about the code, yes I agree with him re:
folio helpers etc.

>
>  /*
>
> ---
> base-commit: 01a412d06bc5786eb4e44a6c8f0f4659bd4c9864
> change-id: 20250724-anonvma-uaf-debug-a9db0eb4177b
>
> --
> Jann Horn <jannh@google.com>
>