netdev - Re: [PATCH nf-next v2 1/2] netfilter: Fix potential null pointer dereference

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Message-ID: <CA+6hz4o4jA4quVRWdG8Tcb7miEQxBpK8bvOcsLW2LX74jYvSQg@mail.gmail.com>
Date:   Wed, 28 Sep 2016 15:56:40 +0800
From:   Feng Gao <gfree.wind@...il.com>
To:     Liping Zhang <zlpnobody@...il.com>
Cc:     Aaron Conole <aconole@...heb.org>,
        Netfilter Developer Mailing List 
        <netfilter-devel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Florian Westphal <fw@...len.de>,
        Pablo Neira Ayuso <pablo@...filter.org>
Subject: Re: [PATCH nf-next v2 1/2] netfilter: Fix potential null pointer dereference

Hi Liping,

On Wed, Sep 28, 2016 at 11:13 AM, Liping Zhang <zlpnobody@...il.com> wrote:
> 2016-09-28 11:08 GMT+08:00 Liping Zhang <zlpnobody@...il.com>:
>> Hi Feng,
>>
>> 2016-09-28 9:23 GMT+08:00 Feng Gao <gfree.wind@...il.com>:
>>> Hi Aaraon,
>>>
>>> On Tue, Sep 27, 2016 at 9:38 PM, Aaron Conole <aconole@...heb.org> wrote:
>>>> It's possible for nf_hook_entry_head to return NULL if two
>>>> nf_unregister_net_hook calls happen simultaneously with a single hook
>>>
>>> The critical region of nf_unregister_net_hook is protected by &nf_hook_mutex.
>>> When it would be called simultaneously?
>>
>> This is unrelated to race condition.
>>
>> Suppose that only the last nf_hook_entry exist, and two callers want to do
>> un-register work.
>>
>> The first one will remove it successfully, after the end of the work, the
>> second one will enter the critical section, but it will see the NULL pointer.
>> Because the last nf_hook_entry was already removed by the first one.
>>
>>>
>>> Regards
>>> Feng
>>>
>>>> entry in the list.  This fix ensures that no null pointer dereference
>>>> could occur when such a race happens.
>>>>
>>>> Signed-off-by: Aaron Conole <aconole@...heb.org>
>
> I read the commit log again, I think the description here is a
> little confusing indeed.

Yes. I doesn't check if the list head always exists, just learn the
patch from commit log.
It confuses me indeed.

Regards
Feng