Re: [PATCH v6 6/6] nbd: fix uaf in nbd_handle_reply()

[yukuai (C)]

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@xxxxxxxxxx>
> > ---
> >   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