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

["yukuai (C)" <yukuai3@huawei.com>]

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