linux-kernel - Re: [PATCH v7 6/6] nbd: fix uaf in nbd_handle

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Date:   Thu, 16 Sep 2021 17:14:55 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     <josef@...icpanda.com>, <axboe@...nel.dk>, <hch@...radead.org>,
        <linux-block@...r.kernel.org>, <nbd@...er.debian.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH v7 6/6] nbd: fix uaf in nbd_handle_reply()

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@...wei.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