netdev - Re: net: suspicious RCU usage in nf

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Date:   Mon, 30 Jan 2017 22:19:52 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Eric Dumazet <edumazet@...gle.com>,
        James Morris <jmorris@...ei.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Patrick McHardy <kaber@...sh.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net: suspicious RCU usage in nf_hook

On Fri, Jan 27, 2017 at 5:31 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2017-01-27 at 17:00 -0800, Cong Wang wrote:
>> On Fri, Jan 27, 2017 at 3:35 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > Oh well, I forgot to submit the official patch I think, Jan 9th.
>> >
>> > https://groups.google.com/forum/#!topic/syzkaller/BhyN5OFd7sQ
>> >
>>
>> Hmm, but why only fragments need skb_orphan()? It seems like
>> any kfree_skb() inside a nf hook needs to have a preceding
>> skb_orphan().
>
>
>>
>> Also, I am not convinced it is similar to commit 8282f27449bf15548
>> which is on RX path.
>
> Well, we clearly see IPv6 reassembly being part of the equation in both
> cases.

Yeah, of course. My worry is that this problem is more than just
IPv6 reassembly.

>
> I was replying to first part of the splat [1], which was already
> diagnosed and had a non official patch.
>
> use after free is also a bug, regardless of jump label being used or
> not.
>
> I still do not really understand this nf_hook issue, I thought we were
> disabling BH in netfilter.

It is a different warning from use-after-free, this one is about sleep
in atomic context, mutex lock is acquired with RCU read lock held.


>
> So the in_interrupt() check in net_disable_timestamp() should trigger,
> this was the intent of netstamp_needed_deferred existence.
>
> Not sure if we can test for rcu_read_lock() as well.
>

The context is process context (TX path before hitting qdisc), and
BH is not disabled, so in_interrupt() doesn't catch it. Hmm, this
makes me thinking maybe we really need to disable BH in this
case for nf_hook()? But it is called in RX path too, and BH is
already disabled there.