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

[Jann Horn <jannh@google.com>]

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...