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

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Date:   Tue, 29 Aug 2017 11:14:03 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Cc:     netdev@...r.kernel.org, edumazet@...gle.com, jhs@...atatu.com,
        xiyou.wangcong@...il.com, jiri@...nulli.us,
        roopa@...ulusnetworks.com
Subject: Re: [PATCH net] sch_htb: fix crash on init failure

On Tue, 2017-08-29 at 11:09 -0700, Eric Dumazet 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 ?
> 
> I am surprised you do not crash in __skb_queue_purge(&q->direct_queue);

Oh, this is because skb_peek() is happy if queue->next is NULL.