netdev - Re: size overflow in function qdisc_tree_decrease_qlen net/sched/sch

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Date:	Tue, 1 Dec 2015 14:40:47 -0800
From:	Cong Wang <cwang@...pensource.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	pageexec@...email.hu, Daniele Fucini <dfucini@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	David Miller <davem@...emloft.net>, spender@...ecurity.net,
	re.emese@...il.com
Subject: Re: size overflow in function qdisc_tree_decrease_qlen net/sched/sch_api.c

On Tue, Dec 1, 2015 at 12:06 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2015-12-01 at 11:17 -0800, Cong Wang wrote:
>> On Tue, Dec 1, 2015 at 11:09 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Tue, 2015-12-01 at 10:43 -0800, Cong Wang wrote:
>> >
>> >> This smells hacky... Another way to fix this is to hold the qdisc tree
>> >> lock in mq_dump(), since it is not a hot path (comparing with
>> >> enqueue/dequeue)?
>> >
>> > Really ? Which qdisc tree lock will protect you exactly ???
>> >
>> > Whole point of MQ is that each TX queue has its own lock.
>> >
>> > So multiple cpus can call qdisc_tree_decrease_qlen() at the same time,
>> > holding their own lock.
>> >
>> > Clearly modifying mq 'data' is wrong.
>>
>> Ah, yeah, but mq _seems_ also the only one who modifies sch->q.qlen
>> in ->dump(), which is the root cause of this bug. I am wondering if it should
>> just compute the qlen and return it without modifying sch->q.qlen.
>
> Sure, but then we still would get PAX underflows warnings ...
>
> Also need to take care of sch->qstats.drops += count;
>
> Also that would require a change of ->dump() api, since tc_fill_qdisc()
> does :
>
> if (q->ops->dump && q->ops->dump(q, skb) < 0)
>     goto nla_put_failure;
> qlen = q->q.qlen;
>
> Not sure it is worth the pain, changing signature of all ->dump()
> handlers...

Yeah, I am fully aware of that, your patch is a quick fix, I was trying
to see if there is any long-term fix for this.

>
>
> What about adding TCQ_F_NOPARENT and then :

This seems equivalent to your fix since TCQ_F_MQROOT implies
no parent:

        if (sch->parent != TC_H_ROOT)
                return -EOPNOTSUPP;

Again, your patch is fine, just want to check if there is any better fix.

Thanks.
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