linux-kernel - Re: [PATCH] workqueue: Check for null pointer return from get_work

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Date:   Mon, 12 Dec 2022 12:55:16 -1000
From:   Tejun Heo <tj@...nel.org>
To:     John Moon <quic_johmoo@...cinc.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: Check for null pointer return from
 get_work_pwq()

On Wed, Dec 07, 2022 at 04:53:44PM -0800, John Moon wrote:
> We've encountered a kernel panic with the following stack trace:
> 
> -> ret_from_fork
>  -> kthread
>   -> worker_thread
>    -> process_one_work
>     -> pwq_dec_nr_in_flight
>      -> pwq_activate_inactive_work
> 
> The issue was narrowed down to a null pointer dereference within
> pwq_activate_inactive_work() stemming from the return value of
> get_work_pwq() which may return NULL, but was not checked for
> null return prior to use.
> 
> While fixing the issue, other dereferences of get_work_pwq()'s
> return value were found without a null check.
> 
> Add null pointer checks to the calling functions that need them.

At that point the work item must have pwq assigned - see insert_work(), so
this can't be the root cause. It's just papering over a bug somewhere else
(e.g. the work item got freed or written over somehow).

Thanks.

-- 
tejun