Messages in this thread | | | Subject | Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req() | From | "yukuai (C)" <> | Date | Tue, 14 Sep 2021 15:13:38 +0800 |
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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
Thanks, Kuai
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