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

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Date:   Thu, 16 Sep 2021 16:47:08 +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 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 ?
> 
>> +		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);
>> +		/*
>> +		 * It's safe to drop ref before request completion, inflight
>> +		 * request will ensure q_usage_counter won't be zero.
>> +		 */
> 
> The above comment is useless actually.

Will remove the comments.

Thanks,
Kuai
>