Re: [patch v8 7/7] nbd: fix uaf in nbd_handle_reply()

yukuai (C)
2021-09-16T14:05:51Z

On 2021/09/16 21:55, Ming Lei write:
> On Thu, Sep 16, 2021 at 09:10:37PM +0800, yukuai (C) wrote:
>> On 2021/09/16 20:58, Ming Lei wrote:
>>> On Thu, Sep 16, 2021 at 05:33:50PM +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 | 14 ++++++++++++++
>>>> 1 file changed, 14 insertions(+)
>>>>
>>>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>>>> index 69dc5eac9ad3..b3a47fc6237f 100644
>>>> --- a/drivers/block/nbd.c
>>>> +++ b/drivers/block/nbd.c
>>>> @@ -825,6 +825,7 @@ 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;
>>>> @@ -835,7 +836,20 @@ static void recv_work(struct work_struct *work)
>>>> if (nbd_read_reply(nbd, args->index, &reply))
>>>> break;
>>>> + /*
>>>> + * 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.
>>>> + */
>>>> + 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);
>>>> + percpu_ref_put(&q->q_usage_counter);
>>>> if (IS_ERR(cmd))
>>>> break;
>>>
>>> The refcount needs to be grabbed when completing the request because
>>> the request may be completed from other code path, then the request pool
>>> will be freed from that code path when the request is referred.
>>
>> Hi,
>>
>> The request can't complete concurrently, thus put ref here is safe.
>>
>> There used to be a commet here that I tried to explain it... It's fine
>> to me to move it behind anyway.
>
> Never see such comment. cmd->lock isn't held here, so I believe
> concurrent completion is possible here.
>

After patch 2, __test_and_clear_bit(NBD_CMD_INFLIGHT) must pass
while cmd->lock is held before completing the request, thus request
completion won't concurrent...

Thanks,
Kuai