On Thu, Sep 16, 2021 at 04:47:08PM +0800, yukuai (C) wrote: > On 2021/09/16 16:04, Ming Lei wrote: > > On Wed, Sep 15, 2021 at 05:20:10PM +0800, Yu Kuai wrote: > > > There is a problem that nbd_handle_reply() might access freed request: > > > > > > 1) At first, a normal io is submitted and completed with scheduler: > > > > > > internel_tag = blk_mq_get_tag -> get tag from sched_tags > > > blk_mq_rq_ctx_init > > > sched_tags->rq[internel_tag] = sched_tag->static_rq[internel_tag] > > > ... > > > blk_mq_get_driver_tag > > > __blk_mq_get_driver_tag -> get tag from tags > > > tags->rq[tag] = sched_tag->static_rq[internel_tag] > > > > > > So, both tags->rq[tag] and sched_tags->rq[internel_tag] are pointing > > > to the request: sched_tags->static_rq[internal_tag]. Even if the > > > io is finished. > > > > > > 2) nbd server send a reply with random tag directly: > > > > > > recv_work > > > nbd_handle_reply > > > blk_mq_tag_to_rq(tags, tag) > > > rq = tags->rq[tag] > > > > > > 3) if the sched_tags->static_rq is freed: > > > > > > blk_mq_sched_free_requests > > > blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i) > > > -> step 2) access rq before clearing rq mapping > > > blk_mq_clear_rq_mapping(set, tags, hctx_idx); > > > __free_pages() -> rq is freed here > > > > > > 4) Then, nbd continue to use the freed request in nbd_handle_reply > > > > > > Fix the problem by get 'q_usage_counter' before blk_mq_tag_to_rq(), > > > thus request is ensured not to be freed because 'q_usage_counter' is > > > not zero. > > > > > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx> > > > --- > > > drivers/block/nbd.c | 18 ++++++++++++++++++ > > > 1 file changed, 18 insertions(+) > > > > > > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c > > > index 9a7bbf8ebe74..3e8b70b5d4f9 100644 > > > --- a/drivers/block/nbd.c > > > +++ b/drivers/block/nbd.c > > > @@ -824,6 +824,7 @@ 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_sock *nsock; > > > struct nbd_cmd *cmd; > > > struct request *rq; > > > @@ -834,7 +835,24 @@ static void recv_work(struct work_struct *work) > > > if (nbd_read_reply(nbd, args->index, &reply)) > > > break; > > > + /* > > > + * Grab ref of q_usage_counter can prevent request being freed > > > + * during nbd_handle_reply(). If q_usage_counter is zero, then > > > + * no request is inflight, which means something is wrong since > > > + * we expect to find a request to complete here. > > > + */ > > > > The above comment is wrong, the purpose is simply for avoiding request > > pool freed, such as elevator switching won't happen once > > ->q_usage_counter is grabbed. So no any request UAF can be triggered > > when calling into nbd_handle_reply(). > > Do you mean the comment about q_usage_counter is zero is wrong ? How about the following words? /* * Grab .q_usage_counter so request pool won't go away, then no request * use-after-free is possible during nbd_handle_reply(). If queue is frozen, * there won't be any inflight requests, we needn't to handle the incoming * garbage message */ Thanks, Ming
