Messages in this thread | | | Subject | Re: [PATCH v7 6/6] nbd: fix uaf in nbd_handle_reply() | From | "yukuai (C)" <> | Date | Thu, 16 Sep 2021 16:47:08 +0800 |
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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@huawei.com> >> --- >> 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 ? > >> + if (!percpu_ref_tryget(&q->q_usage_counter)) { >> + dev_err(disk_to_dev(nbd->disk), "%s: no io inflight\n", >> + __func__); >> + break; >> + } >> + >> cmd = nbd_handle_reply(nbd, args->index, &reply); >> + /* >> + * It's safe to drop ref before request completion, inflight >> + * request will ensure q_usage_counter won't be zero. >> + */ > > The above comment is useless actually.
Will remove the comments.
Thanks, Kuai >
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