mm/rmap: add anon_vma lifetime debug check

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/[e-mail]
[2] https://lore.kernel.org/r/[e-mail]

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jann Horn <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Acked-by: Harry Yoo <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/rmap.h

···
 	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);
+	}
 }
 
 /*