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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 --- 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