netdev - Re: [PATCH net] sch_htb: fix crash on init failure

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Date:   Tue, 29 Aug 2017 11:13:38 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Roopa Prabhu <roopa@...ulusnetworks.com>
Subject: Re: [PATCH net] sch_htb: fix crash on init failure

On Tue, Aug 29, 2017 at 11:09 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-08-29 at 20:58 +0300, Nikolay Aleksandrov wrote:
>> The commit below added a call to the ->destroy() callback for all qdiscs
>> which failed in their ->init(), but some were not prepared for such
>> change and can't handle partially initialized qdisc. HTB is one of them
>> and if any error occurs before the qdisc watchdog timer and qdisc work are
>> initialized then we can hit either a null ptr deref (timer->base) when
>> canceling in ->destroy or lockdep error info about trying to register
>> a non-static key and a stack dump. So to fix these two move the watchdog
>> timer and workqueue init before anything that can err out.
>> To reproduce userspace needs to send broken htb qdisc create request,
>> tested with a modified tc (q_htb.c).
>
>> Note that probably this bug goes further back because the default qdisc
>> handling always calls ->destroy on init failure too.
>>
>> Fixes: 87b60cfacf9f ("net_sched: fix error recovery at qdisc creation")
>> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>> ---
>> Always calling qdisc destroy on init failure in the default qdisc handling
>> was added in commit 0fbbeb1ba43b. I'm not sure if I should include that
>> one as fixes tag.
>
> Well, we probably need to audit init/destroy not only in net/sched, but
> other parts of networking stack.
>
> What about the qdisc_skb_head_init(&q->direct_queue) call ?

It just zero the pointers:

static inline void qdisc_skb_head_init(struct qdisc_skb_head *qh)
{
        qh->head = NULL;
        qh->tail = NULL;
        qh->qlen = 0;
}

And qdisc is already kzalloc()'ed.