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[Question] null pointer risk of kernel workqueue

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  [Question] null pointer risk of kernel workqueue tanxiaofei <tanxiaofei@huawei.com> - 2017-09-23 11:10 +0200
    Re: [Question] null pointer risk of kernel workqueue Tejun Heo <tj@kernel.org> - 2017-09-25 17:30 +0200

#1737982 — [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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#1739117

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

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