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

["yukuai (C)" <yukuai3@huawei.com>]

On 2021/09/14 22:37, Ming Lei wrote:
> 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().

Hi Ming

This apporch is a good idea, however we should not get q_usage_counter
in reccv_work(), because It will block freeze queue.

How about get q_usage_counter in nbd_read_stat(), and put in error path
or after request completion?

Thanks
Kuai