On Tue, Sep 14, 2021 at 05:08:00PM +0800, yukuai (C) wrote: > On 2021/09/14 15:46, Ming Lei wrote: > > On Tue, Sep 14, 2021 at 03:13:38PM +0800, yukuai (C) wrote: > > > On 2021/09/14 14:44, Ming Lei wrote: > > > > On Tue, Sep 14, 2021 at 11:11:06AM +0800, yukuai (C) wrote: > > > > > On 2021/09/14 9:11, Ming Lei wrote: > > > > > > On Thu, Sep 09, 2021 at 10:12:55PM +0800, Yu Kuai wrote: > > > > > > > blk_mq_tag_to_rq() can only ensure to return valid request in > > > > > > > following situation: > > > > > > > > > > > > > > 1) client send request message to server first > > > > > > > submit_bio > > > > > > > ... > > > > > > > blk_mq_get_tag > > > > > > > ... > > > > > > > blk_mq_get_driver_tag > > > > > > > ... > > > > > > > nbd_queue_rq > > > > > > > nbd_handle_cmd > > > > > > > nbd_send_cmd > > > > > > > > > > > > > > 2) client receive respond message from server > > > > > > > recv_work > > > > > > > nbd_read_stat > > > > > > > blk_mq_tag_to_rq > > > > > > > > > > > > > > If step 1) is missing, blk_mq_tag_to_rq() will return a stale > > > > > > > request, which might be freed. Thus convert to use > > > > > > > blk_mq_find_and_get_req() to make sure the returned request is not > > > > > > > freed. > > > > > > > > > > > > But NBD_CMD_INFLIGHT has been added for checking if the reply is > > > > > > expected, do we still need blk_mq_find_and_get_req() for covering > > > > > > this issue? BTW, request and its payload is pre-allocated, so there > > > > > > isn't real use-after-free. > > > > > > > > > > Hi, Ming > > > > > > > > > > Checking NBD_CMD_INFLIGHT relied on the request founded by tag is valid, > > > > > not the other way round. > > > > > > > > > > nbd_read_stat > > > > > req = blk_mq_tag_to_rq() > > > > > cmd = blk_mq_rq_to_pdu(req) > > > > > mutex_lock(cmd->lock) > > > > > checking NBD_CMD_INFLIGHT > > > > > > > > Request and its payload is pre-allocated, and either req->ref or cmd->lock can > > > > serve the same purpose here. Once cmd->lock is held, you can check if the cmd is > > > > inflight or not. If it isn't inflight, just return -ENOENT. Is there any > > > > problem to handle in this way? > > > > > > Hi, Ming > > > > > > in nbd_read_stat: > > > > > > 1) get a request by tag first > > > 2) get nbd_cmd by the request > > > 3) hold cmd->lock and check if cmd is inflight > > > > > > If we want to check if the cmd is inflight in step 3), we have to do > > > setp 1) and 2) first. As I explained in patch 0, blk_mq_tag_to_rq() > > > can't make sure the returned request is not freed: > > > > > > nbd_read_stat > > > blk_mq_sched_free_requests > > > blk_mq_free_rqs > > > blk_mq_tag_to_rq > > > -> get rq before clear mapping > > > blk_mq_clear_rq_mapping > > > __free_pages -> rq is freed > > > blk_mq_request_started -> UAF > > > > If the above can happen, blk_mq_find_and_get_req() may not fix it too, just > > Hi, Ming > > Why can't blk_mq_find_and_get_req() fix it? I can't think of any > scenario that might have problem currently. The principle behind blk_mq_find_and_get_req() is that if one request's ref is grabbed, the queue's usage counter is guaranteed to be grabbed, and this way isn't straight-forward. Yeah, it can fix the issue, but I don't think it is good to call it in fast path cause tags->lock is required. > > > 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; > > + > > We can't make sure freeze_queue is called before this, thus this approch > can't fix the problem, right? > nbd_read_stat > blk_mq_tag_to_rq > elevator_switch > blk_mq_freeze_queue(q); > elevator_switch_mq > elevator_exit > blk_mq_sched_free_requests > blk_mq_request_started -> UAF No, blk_mq_freeze_queue() waits until .q_usage_counter becomes zero, so there won't be any concurrent nbd_read_stat() during switching elevator if ->q_usage_counter is grabbed in recv_work(). Thanks, Ming
