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

[Jann Horn <jannh@google.com>]

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

-- 
Jann Horn <jannh@google.com>