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 16:44:01 +0800 |
| |
On 2021/09/15 16:20, Christoph Hellwig wrote: > On Wed, Sep 15, 2021 at 04:15:37PM +0800, Yu Kuai wrote: >> +++ 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); > > These needs to be an EXPORT_SYMBOL_GPL. But more importantly it > needs to be a separate properly documented patch, and this function > needs to grow a kerneldoc comment as well. > >> + /* >> + * 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; >> + } > > And this needs a properly documented wrapper as well. > >> + >> 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); > > That being said I can't say I like how this exposed block layer > internals. We don't really need a reference to the queue here > anywhere, you just use it as a dumb debug check. If we really want to > reuse (abuse?) q_usage_counter a helper to just grab a reference and > immediately drop it might be a better fit. > .
Hi,
The uaf is because blk_mq_sched_free_requests() can concurrent with nbd_read_stat(), and hold the ref during nbd_read_stat() can break the concurrency, thus the ref can't be dropped immediately.
I'll use percpu_ref_put() directly as Ming suggestted.
Thanks, Kuai
| |