Re: [PATCH v7 6/6] nbd: fix uaf in nbd_handle_reply()

["yukuai (C)" <yukuai3@huawei.com>]

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