# Re: \[PATCH\] net:sched fix array-index-out-of-bounds in taprio\_change

- From: Pavel Skripkin \<hidden\>
- Date: 2021-08-11 08:35:44 
- List: netdev
- Page: https://ratatoskr.run/netdev/2021/08/11209124
- Thread: https://ratatoskr.run/netdev/2021/08/11209119/t.md

```text
On 8/11/21 11:30 AM, Pavel Skripkin wrote:
> On 8/11/21 10:44 AM, Eric Dumazet wrote:
>>
>>
>> On 8/11/21 7:10 AM, tcs.kernel@gmail.com wrote:
>>> From: Haimin Zhang <redacted>
>>>
>>> syzbot report an array-index-out-of-bounds in taprio_change
>>> index 16 is out of range for type '__u16 [16]'
>>> that's because mqprio->num_tc is lager than TC_MAX_QUEUE,so we check
>>> the return value of netdev_set_num_tc.
>>>
>>> Reported-by: <redacted>
>>> Signed-off-by: Haimin Zhang <redacted>
>>> ---
>>>  net/sched/sch_taprio.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
>>> index 9c79374..1ab2fc9 100644
>>> --- a/net/sched/sch_taprio.c
>>> +++ b/net/sched/sch_taprio.c
>>> @@ -1513,7 +1513,9 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
>>>  	taprio_set_picos_per_byte(dev, q);
>>>  
>>>  	if (mqprio) {
>>> -		netdev_set_num_tc(dev, mqprio->num_tc);
>>> +		err = netdev_set_num_tc(dev, mqprio->num_tc);
>>> +		if (err)
>>> +			goto free_sched;
>>>  		for (i = 0; i < mqprio->num_tc; i++)
>>>  			netdev_set_tc_queue(dev, i,
>>>  					    mqprio->count[i],
>>>
>>
>> When was the bug added ?
>>
>> Hint: Please provide a Fixes: tag
>>
>> taprio_parse_mqprio_opt() already checks :
>>
>> /* Verify num_tc is not out of max range */
>> if (qopt->num_tc > TC_MAX_QUEUE) {
>>      NL_SET_ERR_MSG(extack, "Number of traffic classes is outside valid range");
>>      return -EINVAL;
>> }
>>
>> So what is happening exactly ?
>>
>>
>
> Hi, Eric!
>
> I've looked into this bug, but I decided to write a reproducer for it
> first. Unfortunately, I didn't finish it yesterday, but I have an idea
> about what happened:
>
> taprio_parse_mqprio_opt() may return 0 before qopt->num_tc check:
>
> 	/* If num_tc is already set, it means that the user already
> 	 * configured the mqprio part
> 	 */
> 	if (dev->num_tc)
> 		return 0;
>
> Then taprio_mqprio_cmp() fails here:
>
> 	if (!mqprio || mqprio->num_tc != dev->num_tc)
> 		return -1;
>
> That's why we won't get shift-out-of-bound in taprio_mqprio_cmp().

			  ^^^^^^^^^^^^^^^^^

			array-index-out-of-bounds

Sorry for confusion



With regards,
Pavel Skripkin
```