Re: [PATCH v2] mm/rmap: Add anon_vma lifetime debug check

[Vlastimil Babka <vbabka@suse.cz>]

On 7/25/25 14:16, Jann Horn wrote:
> If an anon folio 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 folios to
> hopefully catch such issues more reliably.
> 
> [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
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> Changes in v2:
> - applied akpm's fixup (use FOLIO_MAPPING_ANON, ...)
> - remove CONFIG_DEBUG_VM check and use folio_test_* helpers (David)
> - more verbose comment (Lorenzo)
> - replaced "page" mentions with "folio" in commit message
> - Link to v1: https://lore.kernel.org/r/20250724-anonvma-uaf-debug-v1-1-29989ddc4e2a@google.com
> ---
>  include/linux/rmap.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 20803fcb49a7..6cd020eea37a 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -449,6 +449,28 @@ 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.
> +	 * Note that the atomic_read() mainly does two things:
> +	 *
> +	 * 1. In KASAN builds with CONFIG_SLUB_RCU_DEBUG, it causes KASAN to
> +	 *    check that the associated anon_vma has not yet been freed (subject

I think more precisely it checks that the slab folio hosting the anon_vma
could not have been yet freed, IIUC? If the anon_vma itself has been freed
then this will not trigger.

> +	 *    to KASAN's usual limitations). This check will pass if the
> +	 *    anon_vma's refcount has already dropped to 0 but an RCU grace
> +	 *    period hasn't passed since then.

AFAIU this says it more accurately and matches my interpretation above?

> +	 * 2. If the anon_vma has not yet been freed, it checks that the
> +	 *    anon_vma still has a nonzero refcount (as opposed to being in the
> +	 *    middle of an RCU delay for getting freed).

Again the RCU delay would apply to the slab page, unless you talk about the
CONFIG_SLUB_RCU_DEBUG specific path (IIRC).

That said, I wonder if here in __folio_rmap_sanity_checks() we are even in a
situation where we rely on SLAB_TYPESAFE_BY_RCU in order to not touch
something that's not anon_vma anymore? I think we expect it to exist? Can we
thus invent a CONFIG_SLUB_RCU_DEBUG-specific assert that assert the anon_vma
itself has not been freed yet (i.e. even if within a grace period?).

I was wondering what actually does rely on SLAB_TYPESAFE_BY_RCU, thanks
Lorenzo for pointing me to folio_get_anon_vma(). But that's only used
elsewhere than here, no?

> +	 */
> +	if (folio_test_anon(folio) && !folio_test_ksm(folio)) {

So you verified this compiles out completely without DEBUG_VM?

> +		unsigned long mapping = (unsigned long)folio->mapping;
> +		struct anon_vma *anon_vma;
> +
> +		anon_vma = (void *)(mapping - FOLIO_MAPPING_ANON);
> +		VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0, folio);
> +	}
>  }
>  
>  /*
> 
> ---
> base-commit: 1d1c610e32ab2489c49fccb7472a6bef136a0a8b
> change-id: 20250724-anonvma-uaf-debug-a9db0eb4177b
>