linux-kernel - Re: net: suspicious RCU usage in nf

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Date:   Wed, 1 Feb 2017 15:29:54 -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 Wed, Feb 1, 2017 at 1:16 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-02-01 at 12:51 -0800, Cong Wang wrote:
>> On Tue, Jan 31, 2017 at 7:44 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Mon, 2017-01-30 at 22:19 -0800, Cong Wang wrote:
>> >
>> >>
>> >> 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.
>> >
>> > ipt_do_table() and similar netfilter entry points disable BH.
>> >
>> > Maybe it is done too late.
>>
>> I think we need a fix like the following one for minimum impact.
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 727b6fd..eee7d63 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1720,12 +1720,10 @@ EXPORT_SYMBOL(net_enable_timestamp);
>>  void net_disable_timestamp(void)
>>  {
>>  #ifdef HAVE_JUMP_LABEL
>> -       if (in_interrupt()) {
>> -               atomic_inc(&netstamp_needed_deferred);
>> -               return;
>> -       }
>> -#endif
>> +       atomic_inc(&netstamp_needed_deferred);
>> +#else
>>         static_key_slow_dec(&netstamp_needed);
>> +#endif
>>  }
>>  EXPORT_SYMBOL(net_disable_timestamp);
>
> This would permanently leave the kernel in the netstamp_needed state.
>
> I would prefer the patch using a process context to perform the
> cleanup ? Note there is a race window, but probably not a big deal.

Not sure if it is better. The difference is caught up in net_enable_timestamp(),
which is called setsockopt() path and sk_clone() path, so we could be
in netstamp_needed state for a long time too until user-space exercises
these paths.

I am feeling we probably need to get rid of netstamp_needed_deferred,
and simply defer the whole static_key_slow_dec(), like the attached patch
(compile only).

What do you think?

View attachment "net-timestamp.diff" of type "text/plain" (1483 bytes)