linux-kernel - Re: [PATCH net] cls_matchall: avoid panic when receiving a packet before filter set

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Message-ID: <vbf8svnq59y.fsf@mellanox.com>
Date:   Fri, 3 May 2019 09:03:11 +0000
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Matteo Croce <mcroce@...hat.com>
CC:     Vlad Buslov <vladbu@...lanox.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] cls_matchall: avoid panic when receiving a packet
 before filter set

On Thu 02 May 2019 at 03:48, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Wed, May 1, 2019 at 2:27 AM Matteo Croce <mcroce@...hat.com> wrote:
>>
>> On Tue, Apr 30, 2019 at 11:25 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>> >
>> > On Mon, Apr 29, 2019 at 10:38 AM Matteo Croce <mcroce@...hat.com> wrote:
>> > >
>> > > When a matchall classifier is added, there is a small time interval in
>> > > which tp->root is NULL. If we receive a packet in this small time slice
>> > > a NULL pointer dereference will happen, leading to a kernel panic:
>> >
>> > Hmm, why not just check tp->root against NULL in mall_classify()?
>> >
>> > Also, which is the offending commit here? Please add a Fixes: tag.
>> >
>> > Thanks.
>>
>> Hi,
>>
>> I just want to avoid an extra check which would be made for every packet.
>> Probably the benefit over a check is negligible, but it's still a
>> per-packet thing.
>> If you prefer a simple check, I can make a v2 that way.
>
> Yeah, I think that is better, you can add an unlikely() for performance
> concern, as NULL is a rare case.
>
>
>>
>> For the fixes tag, I didn't put it as I'm not really sure about the
>> offending commit. I guess it's the following, what do you think?
>>
>> commit ed76f5edccc98fa66f2337f0b3b255d6e1a568b7
>> Author: Vlad Buslov <vladbu@...lanox.com>
>> Date:   Mon Feb 11 10:55:38 2019 +0200
>>
>>     net: sched: protect filter_chain list with filter_chain_lock mutex
>
> I think you are right, this is the commit introduced the code
> that inserts the tp before fully initializing it. Please Cc Vlad
> for your v2, in case we blame a wrong commit here.
>
>
> BTW, it looks like cls_cgroup needs a same fix. Please audit
> other tc filters as well.
>
> Thanks!

Sorry for late response. This is indeed the offending commit that should
be referenced by fixes tag.

Thanks for fixing this, Matteo!