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

On Tue, Sep 14, 2021 at 05:19:31PM +0800, yukuai (C) wrote:

On 在 2021/09/14 15:46, Ming Lei wrote:

quoted

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