Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req()

[Ming Lei <ming.lei@redhat.com>]

On Tue, Sep 14, 2021 at 05:19:31PM +0800, yukuai (C) wrote:
> On 在 2021/09/14 15:46, Ming Lei wrote:
> 
> > If the above can happen, blk_mq_find_and_get_req() may not fix it too, just
> > wondering why not take the following simpler way for avoiding the UAF?
> > 
> > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> > index 5170a630778d..dfa5cce71f66 100644
> > --- a/drivers/block/nbd.c
> > +++ b/drivers/block/nbd.c
> > @@ -795,9 +795,13 @@ static void recv_work(struct work_struct *work)
> >   						     work);
> >   	struct nbd_device *nbd = args->nbd;
> >   	struct nbd_config *config = nbd->config;
> > +	struct request_queue *q = nbd->disk->queue;
> >   	struct nbd_cmd *cmd;
> >   	struct request *rq;
> > +	if (!percpu_ref_tryget(&q->q_usage_counter))
> > +                return;
> > +
> >   	while (1) {
> >   		cmd = nbd_read_stat(nbd, args->index);
> >   		if (IS_ERR(cmd)) {
> > @@ -813,6 +817,7 @@ static void recv_work(struct work_struct *work)
> >   		if (likely(!blk_should_fake_timeout(rq->q)))
> >   			blk_mq_complete_request(rq);
> >   	}
> > +	blk_queue_exit(q);
> >   	nbd_config_put(nbd);
> >   	atomic_dec(&config->recv_threads);
> >   	wake_up(&config->recv_wq);
> > 
> 
> Hi, Ming
> 
> This apporch is wrong.
> 
> If blk_mq_freeze_queue() is called, and nbd is waiting for all
> request to complete. percpu_ref_tryget() will fail here, and deadlock
> will occur because request can't complete in recv_work().

No, percpu_ref_tryget() won't fail until ->q_usage_counter is zero, when
it is perfectly fine to do nothing in recv_work().

Thanks,
Ming