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

[yukuai (C)]

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().

Thanks,
Kuai