Re: [PATCH v6 6/6] nbd: fix uaf in nbd_handle_reply()

[yukuai (C)]

On 2021/09/15 16:20, Christoph Hellwig wrote:
> On Wed, Sep 15, 2021 at 04:15:37PM +0800, Yu Kuai wrote:
> > +++ b/block/blk-core.c
> > @@ -489,6 +489,7 @@ void blk_queue_exit(struct request_queue *q)
> >   {
> >   	percpu_ref_put(&q->q_usage_counter);
> >   }
> > +EXPORT_SYMBOL(blk_queue_exit);
>
> These needs to be an EXPORT_SYMBOL_GPL.  But more importantly it
> needs to be a separate properly documented patch, and this function
> needs to grow a kerneldoc comment as well.
>
> > +		/*
> > +		 * Get q_usage_counter can prevent accessing freed request
> > +		 * through blk_mq_tag_to_rq() in nbd_handle_reply(). If
> > +		 * q_usage_counter is zero, then no request is inflight, which
> > +		 * means something is wrong since we expect to find a request to
> > +		 * complete here.
> > +		 */
> > +		if (!percpu_ref_tryget(&q->q_usage_counter)) {
> > +			dev_err(disk_to_dev(nbd->disk), "%s: no io inflight\n",
> > +				__func__);
> > +			break;
> > +		}
>
> And this needs a properly documented wrapper as well.
>
> > +
> >   		cmd = nbd_handle_reply(nbd, args->index, &reply);
> > -		if (IS_ERR(cmd))
> > +		if (IS_ERR(cmd)) {
> > +			blk_queue_exit(q);
> >   			break;
> > +		}
> >   
> >   		rq = blk_mq_rq_from_pdu(cmd);
> >   		if (likely(!blk_should_fake_timeout(rq->q)))
> >   			blk_mq_complete_request(rq);
> > +		blk_queue_exit(q);
>
> That being said I can't say I like how this exposed block layer
> internals.  We don't really need a reference to the queue here
> anywhere, you just use it as a dumb debug check.  If we really want to
> reuse (abuse?) q_usage_counter a helper to just grab a reference and
> immediately drop it might be a better fit.
> .

Hi,

The uaf is because blk_mq_sched_free_requests() can concurrent with
nbd_read_stat(), and hold the ref during nbd_read_stat() can break the
concurrency, thus the ref can't be dropped immediately.

I'll use percpu_ref_put() directly as Ming suggestted.

Thanks,
Kuai