Messages in this thread | | | Subject | Re: [PATCH v6 6/6] nbd: fix uaf in nbd_handle_reply() | From | "yukuai (C)" <> | Date | Wed, 15 Sep 2021 16:41:11 +0800 |
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On 2021/09/15 16:28, Ming Lei wrote: > On Wed, Sep 15, 2021 at 04:15:37PM +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> >> --- >> block/blk-core.c | 1 + >> drivers/block/nbd.c | 19 ++++++++++++++++++- >> 2 files changed, 19 insertions(+), 1 deletion(-) >> >> diff --git a/block/blk-core.c b/block/blk-core.c >> index 5454db2fa263..2008e6903166 100644 >> --- a/block/blk-core.c >> +++ b/block/blk-core.c >> @@ -489,6 +489,7 @@ void blk_queue_exit(struct request_queue *q) >> { >> percpu_ref_put(&q->q_usage_counter); >> } >> +EXPORT_SYMBOL(blk_queue_exit); >> >> static void blk_queue_usage_counter_release(struct percpu_ref *ref) >> { >> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c >> index 9a7bbf8ebe74..f065afcc7586 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; >> @@ -834,13 +835,29 @@ static void recv_work(struct work_struct *work) >> if (nbd_read_reply(nbd, args->index, &reply)) >> break; >> >> + /* >> + * Get q_usage_counter can prevent accessing freed request >> + * through blk_mq_tag_to_rq() in 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. >> + */ >> + 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); >> - if (IS_ERR(cmd)) >> + if (IS_ERR(cmd)) { >> + blk_queue_exit(q); >> break; >> + } >> >> rq = blk_mq_rq_from_pdu(cmd); >> if (likely(!blk_should_fake_timeout(rq->q))) >> blk_mq_complete_request(rq); >> + blk_queue_exit(q); > > You can simply call percpu_ref_put() directly just like what scsi_end_request() > is doing. >
Thanks for the adivce, will do that in next iteration. (hopefully the last)
Best regards, Kuai
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