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SubjectRe: [PATCH v7 6/6] nbd: fix uaf in nbd_handle_reply()
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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 ?
>
>> + 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
>

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