Re: [PATCH v6 6/6] nbd: fix uaf in nbd_handle_reply()

yukuai (C)
2021-09-15T08:41:18Z

On 2021/09/15 16:28, Ming Lei wrote:
> On Wed, Sep 15, 2021 at 04:15:37PM +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>
>> ---
>> block/blk-core.c | 1 +
>> drivers/block/nbd.c | 19 ++++++++++++++++++-
>> 2 files changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 5454db2fa263..2008e6903166 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -489,6 +489,7 @@ void blk_queue_exit(struct request_queue *q)
>> {
>> percpu_ref_put(&q->q_usage_counter);
>> }
>> +EXPORT_SYMBOL(blk_queue_exit);
>>
>> static void blk_queue_usage_counter_release(struct percpu_ref *ref)
>> {
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index 9a7bbf8ebe74..f065afcc7586 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;
>> @@ -834,13 +835,29 @@ static void recv_work(struct work_struct *work)
>> if (nbd_read_reply(nbd, args->index, &reply))
>> break;
>>
>> + /*
>> + * Get q_usage_counter can prevent accessing freed request
>> + * through blk_mq_tag_to_rq() in 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.
>> + */
>> + 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);
>> - if (IS_ERR(cmd))
>> + if (IS_ERR(cmd)) {
>> + blk_queue_exit(q);
>> break;
>> + }
>>
>> rq = blk_mq_rq_from_pdu(cmd);
>> if (likely(!blk_should_fake_timeout(rq->q)))
>> blk_mq_complete_request(rq);
>> + blk_queue_exit(q);
>
> You can simply call percpu_ref_put() directly just like what scsi_end_request()
> is doing.
>

Thanks for the adivce, will do that in next iteration. (hopefully the
last)

Best regards,
Kuai