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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 24 Jul 2025 21:13:50 +0200 Jann Horn <jannh@google.com> 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.
> 
> --- 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);
> +	}
>  }

PAGE_MAPPING_ANON is now FOLIO_MAPPING_ANON.

The subtraction to clear a bitflag works, but my brain would prefer &=
FOLIO_MAPPING_ANON.  Oh well.

Plus gratuitous 80-col fix:

--- a/include/linux/rmap.h~mm-rmap-add-anon_vma-lifetime-debug-check-fix
+++ a/include/linux/rmap.h
@@ -458,8 +458,9 @@ static inline void __folio_rmap_sanity_c
 	 */
 	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);
+		struct anon_vma *anon_vma;
 
+		anon_vma = (void *)(mapping - FOLIO_MAPPING_ANON);
 		VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0, folio);
 	}
 }
_