netdev - Re: [PATCH 3/5] net: sched: taprio: Fix null pointer deref bug

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Date:   Tue, 23 Apr 2019 19:21:42 +0000
From:   "Guedes, Andre" <andre.guedes@...el.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Gomes, Vinicius" <vinicius.gomes@...el.com>
Subject: Re: [PATCH 3/5] net: sched: taprio: Fix null pointer deref bug


> On Apr 22, 2019, at 12:36 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> 
> On Mon, Apr 22, 2019 at 12:24 PM Guedes, Andre <andre.guedes@...el.com> wrote:
>> 
>> 
>>> On Apr 22, 2019, at 11:07 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>> 
>>> On Mon, Apr 22, 2019 at 11:04 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
>>>> 
>>>> On Fri, Apr 19, 2019 at 6:06 PM Andre Guedes <andre.guedes@...el.com> wrote:
>>>>> 
>>>>> If 'entry' is NULL we WARN_ON() but dereference the pointer anyway,
>>>>> generating a null pointer dereference bug. This patch fixes should_
>>>>> restart_cycle() so we return if the pointer is NULL.
>>>> 
>>>> Hmm, while you are on it, how is it possible to have entry==NULL
>>>> for should_restart_cycle(). It is only called in advance_sched()
>>>> where entry is already checked against NULL right before it,
>>>> so for me, entry is always NULL at the point of calling
>>>> should_restart_cycle().
>>> 
>>> I meant, 'entry' is always non-NULL here... I typed too fast.
>> 
>> Your assessment is correct. I believe the WARN_ON() was added as a defensive practice to prevent null pointer dereference in case someone misuse that helper in the future.
> 
> Yeah, so we can just remove it. :)

Fine by me. In that case, the function should_restart_cycle() will be just a dummy wrapper on list_is_last() so we should probably get rid of it and call list_is_last() within advance_sched().