linux-kernel - Re: [Question] null pointer risk of kernel workqueue

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Date:   Fri, 20 Oct 2017 14:57:18 +0800
From:   tanxiaofei <tanxiaofei@...wei.com>
To:     Tejun Heo <tj@...nel.org>
CC:     <jiangshanlai@...il.com>, <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>,
        John Garry <john.garry@...wei.com>, <huawei.libin@...wei.com>
Subject: Re: [Question] null pointer risk of kernel workqueue

Hi Tejun,

Any comments about this?

Thanks

On 2017/9/27 17:17, tanxiaofei wrote:
> 
> 
> On 2017/9/25 23:25, Tejun Heo wrote:
>> Hello,
>>
>> On Sat, Sep 23, 2017 at 05:04:24PM +0800, tanxiaofei wrote:
>>> Hi Tejun & Jiangshan,
>>>
>>> I find an null pointer risk in the code of workqueue. Here is description:
>>>
>>> If draining, __queue_work() will call the function is_chained_work() to do some checks.
>>> In is_chained_work(), worker->current_pwq is used directly. It should be not safe.
>>> http://elixir.free-electrons.com/linux/latest/source/kernel/workqueue.c#L1384
>>>
>>> If you check the thread function of this worker, worker_thread(), you will find worker->current_pwq
>>> is null when one work is done or ready to be processed.
>>> This issue may happen only if we queue work during executing drain_workqueue().
>>> http://elixir.free-electrons.com/linux/latest/source/kernel/workqueue.c#L2173
>>
>> Hmmm? I don't get it.  worker->current_pwq is guaranteed to be set
>> while a work function is being executed and the chained check can only
>> get there iff the the worker is executing a work function.
>>
> 
> Hi Tejun,
> 
> Thanks for your quick reply.
> Hmm, but i think interrupt preemption could make this happen.
> For example, the scenario that all following conditions are met.
> 1.The handler of an interrupt call queue_work() to queue some works, and the workqueue is draining.
> 2.An worker thread is interrupted by this interrupt.
> 3.The worker thread is being executed, and at the very moment,worker->current_pwq
> is set to null.(This is possible and please check function worker_thread() carefully).
> 
> Thanks.
> 
> 
> 
> 
> 
> .
>