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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/67abaeaf.050a0220.110943.0041.GAE@google.com [2] https://lore.kernel.org/r/67a76f33.050a0220.3d72c.0028.GAE@google.com Link: https://lkml.kernel.org/r/20250725-anonvma-uaf-debug-v2-1-bc3c7e5ba5b1@google.com Signed-off-by: Jann Horn <jannh@google.com> Acked-by: David Hildenbrand <david@redhat.com> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> Acked-by: Vlastimil Babka <vbabka@suse.cz> Acked-by: Harry Yoo <harry.yoo@oracle.com> Cc: David Hildenbrand <david@redhat.com> Cc: Jann Horn <jannh@google.com> Cc: Liam Howlett <liam.howlett@oracle.com> Cc: Rik van Riel <riel@surriel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -449,6 +449,28 @@ static inline void __folio_rmap_sanity_checks(const struct folio *folio,
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default:
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VM_WARN_ON_ONCE(true);
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}
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/*
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* Anon folios must have an associated live anon_vma as long as they're
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* mapped into userspace.
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* Note that the atomic_read() mainly does two things:
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*
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* 1. In KASAN builds with CONFIG_SLUB_RCU_DEBUG, it causes KASAN to
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* check that the associated anon_vma has not yet been freed (subject
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* to KASAN's usual limitations). This check will pass if the
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* anon_vma's refcount has already dropped to 0 but an RCU grace
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* period hasn't passed since then.
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* 2. If the anon_vma has not yet been freed, it checks that the
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* anon_vma still has a nonzero refcount (as opposed to being in the
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* middle of an RCU delay for getting freed).
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*/
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if (folio_test_anon(folio) && !folio_test_ksm(folio)) {
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unsigned long mapping = (unsigned long)folio->mapping;
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struct anon_vma *anon_vma;
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anon_vma = (void *)(mapping - FOLIO_MAPPING_ANON);
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VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0, folio);
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}
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}
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/*
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