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

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Date:   Wed, 28 Sep 2016 11:13:07 +0800
From:   Liping Zhang <zlpnobody@...il.com>
To:     Feng Gao <gfree.wind@...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

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.