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

[Vlastimil Babka]

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...