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

[Jann Horn]

On Fri, Jul 25, 2025 at 5:07 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
> On Fri, Jul 25, 2025 at 04:48:09PM +0200, Jann Horn wrote:
> > On Fri, Jul 25, 2025 at 3:49 PM Lorenzo Stoakes
> > <lorenzo.stoakes@xxxxxxxxxx> wrote:
> > > On Fri, Jul 25, 2025 at 02:00:18PM +0200, Jann Horn wrote:
> > > > > We're sort of relying on this
> > > > >
> > > > > a. being a UAF
> > > > >
> > > > > b. the thing we're UAF-ing not either corrupting this field or (if that
> > > > >     memory is actually reused as an anon_vma - I'm not familiar with slab
> > > > >     caches - so maybe it's quite likely - getting its refcount incremented.
> > > >
> > > > KASAN sees the memory read I'm doing with this atomic_read(), so in
> > > > KASAN builds, if this is a UAF, it should trigger a KASAN splat
> > > > (modulo KASAN limitations around when UAF can be detected). Basically,
> > > > in KASAN builds, the actual explicit check I'm doing here is only
> > > > relevant if the object has not yet been freed. That's why I wrote the
> > > > comment "Part of the purpose of the atomic_read() is to make KASAN
> > > > check that the anon_vma is still alive.".
> > >
> > > Hm, I'm confused, how can you detect a UAF if the object cannot yet be
> > > freed? :P
> > >
> > > or would that be the case were it not an atomic_read()?
> > >
> > > I guess this permits this to be detected in a timely manner.
> >
> > If the anon_vma hasn't yet been freed, but its refcount is 0, then
> > that's still a bug because we rely on the anon_vma to have a nonzero
> > refcount as long as there are folios with a nonzero mapcount that are
> > tied to it, and it is likely to allow UAF at a later point.
>
> But how is this happening?
>
> The only places where we might explicitly manipulate anon_vma->refcount
> are:
>
> - anon_vma_ctor() -> set to 0 on construction used by slab.
> - folio_lock_anon_vma_read() / put_anon_vma() - both cases call
>   __put_anon_vma() when 0 to free the anon_vma.
>
> So how could we get to a refcount of 0 but the anon_vma still be hanging
> around, except if it's freshly allocated?

Due to SLAB_TYPESAFE_BY_RCU, the anon_vma is guaranteed to still be
accessible (possibly post-recycling) for an RCU grace period after its
refcount drops to zero. Under CONFIG_SLUB_RCU_DEBUG (which you need
for KASAN to catch UAF in such slabs semi-reliably), from KASAN's
perspective, the anon_vma is effectively freed after an RCU grace
period.

Basically CONFIG_SLUB_RCU_DEBUG turns kmem_cache_free() on
SLAB_TYPESAFE_BY_RCU slabs into something like kfree_rcu(), and this
allows KASAN to catch UAF access.

> It's surely only UAF?

I mean, "UAF" is kind of vague when talking about SLAB_TYPESAFE_BY_RCU
slabs. I am only using the term "UAF" when talking about a situation
where accessing the anon_vma object is entirely forbidden because an
RCU grace period has passed after it was "freed" with
kmem_cache_free().