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

[Jann Horn]

On Wed, Jul 23, 2025 at 8:10 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 7/23/25 19:49, Jann Horn wrote:
> > On Wed, Jul 23, 2025 at 7:32 PM Vlastimil Babka <vbabka@xxxxxxx> 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.