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

[David Hildenbrand]

On 25.07.25 13:08, Jann Horn wrote:
> On Thu, Jul 24, 2025 at 11:56 PM David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxx
> > > [2] https://lore.kernel.org/r/67a76f33.050a0220.3d72c.0028.GAE@xxxxxxxxxx
> > >
> > > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > > ---
> > >    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.

That makes sense, the volatile read will not get optimized out.

> > 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.

Well, ksm is a subtype of anon.

folio_test_anon_not_ksm()

let's rather ... not :)

--
Cheers,

David / dhildenb