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

yukuai (C)
2021-09-15T08:44:06Z

On 2021/09/15 16:20, Christoph Hellwig wrote:
> On Wed, Sep 15, 2021 at 04:15:37PM +0800, Yu Kuai wrote:
>> +++ 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);
>
> These needs to be an EXPORT_SYMBOL_GPL. But more importantly it
> needs to be a separate properly documented patch, and this function
> needs to grow a kerneldoc comment as well.
>
>> + /*
>> + * 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;
>> + }
>
> And this needs a properly documented wrapper as well.
>
>> +
>> 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);
>
> That being said I can't say I like how this exposed block layer
> internals. We don't really need a reference to the queue here
> anywhere, you just use it as a dumb debug check. If we really want to
> reuse (abuse?) q_usage_counter a helper to just grab a reference and
> immediately drop it might be a better fit.
> .

Hi,

The uaf is because blk_mq_sched_free_requests() can concurrent with
nbd_read_stat(), and hold the ref during nbd_read_stat() can break the
concurrency, thus the ref can't be dropped immediately.

I'll use percpu_ref_put() directly as Ming suggestted.

Thanks,
Kuai