Messages in this thread | | | Subject | Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req() | From | "yukuai (C)" <> | Date | Wed, 15 Sep 2021 09:54:09 +0800 |
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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
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