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

[Christoph Hellwig <hch@infradead.org>]

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.