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

[Jann Horn <jannh@google.com>]

On Thu, Jul 24, 2025 at 11:56 PM David Hildenbrand <david@redhat.com> wrote:
> On 24.07.25 21:13, Jann Horn wrote:
> > 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)) {
>
> 1) You probably don't need the CONFIG_DEBUG_VM check: the
> VM_WARN_ON_FOLIO should make everything get optimized out ... right?

The PageAnonNotKsm() check is outside the VM_WARN_ON_FOLIO(), and it
uses page_folio(), which uses _compound_head(), which does
READ_ONCE(page->compound_head); and READ_ONCE() unfortunately doesn't
just mean "I want a read without tearing", it also (intentionally)
prevents the compiler from removing the read when it sees that it's
not being used for anything.

> 2) We have a folio here, so ... better
>
> if (folio_test_anon(folio) && !folio_test_ksm(folio)) {
>         ...
> }

Hrm, okay. It kind of irks me to write it as two checks when really I
want to ask "is it this one specific type", but yeah, will change it.

These helpers don't use READ_ONCE(), so the compiler should then also
be able to remove the check...

> > +             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);
> > +     }
>
> In general,
>
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks!