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

[Jann Horn <jannh@google.com>]

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@google.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);
+	}
 }
 
 /*

---
base-commit: 01a412d06bc5786eb4e44a6c8f0f4659bd4c9864
change-id: 20250724-anonvma-uaf-debug-a9db0eb4177b

-- 
Jann Horn <jannh@google.com>