2017-09-23 09:05:13

by Xiaofei Tan

Subject: [Question] null pointer risk of kernel workqueue

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

There are very few places to call drain_workqueue() in the whole linux kernel.
I think that's why no one noticed this risk.

Xiaofei Tan
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.

2017-09-25 15:25:41

by Tejun Heo

Subject: Re: [Question] null pointer risk of kernel workqueue

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.

Thanks.

--
tejun

2017-09-27 09:17:35

by Xiaofei Tan

Subject: Re: [Question] null pointer risk of kernel workqueue

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.