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

[Suren Baghdasaryan]

On Wed, Jul 23, 2025 at 10:50 AM Jann Horn <jannh@xxxxxxxxxx> 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.

Thanks for analyzing this Jann. Yeah, I missed the fact that
vma_refcount_put() uses vma->vm_mm.

> >
> > > 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...

This is tricky. Let me look into it some more before suggesting any fixes.