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 17:14:55 +0800 |
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On 2021/09/16 17:06, Ming Lei wrote: > 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@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 ? > > 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 > */
Will use these words.
Thanks, Kuai
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