Re: [BUG] hard-to-hit mm_struct UAF due to insufficiently careful vma_refcount_put() wrt SLAB_TYPESAFE_BY_RCU

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jul 23, 2025 at 11:19 AM Jann Horn <jannh@google.com> wrote:
>
> On Wed, Jul 23, 2025 at 8:10 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> > On 7/23/25 19:49, Jann Horn wrote:
> > > On Wed, Jul 23, 2025 at 7:32 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> > >> On 7/23/25 18:26, Jann Horn wrote:
> > >> > There's a racy UAF in `vma_refcount_put()` when called on the
> > >> > `lock_vma_under_rcu()` path because `SLAB_TYPESAFE_BY_RCU` is used
> > >> > without sufficient protection against concurrent object reuse:
> > >>
> > >> Oof.
> > >>
> > >> > I'm not sure what the right fix is; I guess one approach would be to
> > >> > have a special version of vma_refcount_put() for cases where the VMA
> > >> > has been recycled by another MM that grabs an extra reference to the
> > >> > MM? But then dropping a reference to the MM afterwards might be a bit
> > >> > annoying and might require something like mmdrop_async()...
> > >>
> > >> Would we need mmdrop_async()? Isn't this the case for mmget_not_zero() and
> > >> mmput_async()?
> > >
> > > Now I'm not sure anymore if either of those approaches would work,
> > > because they rely on the task that's removing the VMA to wait until we
> > > do __refcount_dec_and_test() before deleting the MM... but I don't
> > > think we have any such guarantee...
> >
> > I think it would be waiting in exit_mmap->vma_mark_detached(), but then
> > AFAIU you're right and we'd really need to work with mmgrab/mmdrop because
> > at that point the  mmget_not_zero() would already be failing...
>
> Ah, I see! vma_mark_detached() drops its reference, then does
> __vma_enter_locked() to bump the refcount by VMA_LOCK_OFFSET again
> (after which the reader path can't acquire it anymore), then waits
> until the refcount drops to VMA_LOCK_OFFSET, and then decrements it
> down to 0 from there. Makes sense.

Yes, that's what I was checking to understand the race. In your explanation:

A1 found the vma
A2 detached it
A3 attached it to another mm
A1 refcounts the vma
A1 realizes it's from another mm and calls vma_end_read() which tries
to wake up another mm's waiter.

Vlastimil is right that if A1 was able to successfully elevate vma's
refcount then:
1. vma must be attached to some valid mm. This is true because if the
vma is detached, vma_start_read() would not be able to elevate its
refcount. Once vma_start_read() elevates the refcount, vma will not
detach from under us because vma_mark_detached() will block until no
readers are using the vma.
2. vma->mm can't be destroyed from under us because of that
exit_mmap()->vma_mark_detached() which again will ensure no readers
are holding a reference to the vmas of that mm.

So, a special version of vma_refcount_put() that takes mm as a
parameter and does mmgrab/mmdrop before using that mm might work. I'll
do some more digging and maybe test this solution with your reproducer
to see if that works as I would expect.
Thanks,
Suren.