Re: [PATCH] ipmi: Move remove_work to dedicated workqueue

[Corey Minyard <minyard@acm.org>]

On Fri, Nov 12, 2021 at 06:54:13PM +0200, Ioanna Alifieraki wrote:
> Currently when removing an ipmi_user the removal is deferred as a work on
> the system's workqueue. Although this guarantees the free operation will
> occur in non atomic context, it can race with the ipmi_msghandler module
> removal (see [1]) . In case a remove_user work is scheduled for removal
> and shortly after ipmi_msghandler module is removed we can end up in a
> situation where the module is removed fist and when the work is executed
> the system crashes with :
> BUG: unable to handle page fault for address: ffffffffc05c3450
> PF: supervisor instruction fetch in kernel mode
> PF: error_code(0x0010) - not-present page
> because the pages of the module are gone. In cleanup_ipmi() there is no
> easy way to detect if there are any pending works to flush them before
> removing the module. This patch creates a separate workqueue and schedules
> the remove_work works on it. When removing the module the workqueue is
> flushed to avoid the race.

Yeah, this is an issue.  One comment below...

> 
> [1] https://bugs.launchpad.net/bugs/1950666
> 
> Cc: stable@vger.kernel.org
> Fixes: 3b9a907223d7 (ipmi: fix sleep-in-atomic in free_user at cleanup SRCU user->release_barrier)
> Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
> ---
>  drivers/char/ipmi/ipmi_msghandler.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/char/ipmi/ipmi_msghandler.c b/drivers/char/ipmi/ipmi_msghandler.c
> index deed355422f4..9e0ad2ccd3e0 100644
> --- a/drivers/char/ipmi/ipmi_msghandler.c
> +++ b/drivers/char/ipmi/ipmi_msghandler.c
> @@ -191,6 +191,8 @@ struct ipmi_user {
>  	struct work_struct remove_work;
>  };
>  
> +struct workqueue_struct *remove_work_wq;
> +
>  static struct ipmi_user *acquire_ipmi_user(struct ipmi_user *user, int *index)
>  	__acquires(user->release_barrier)
>  {
> @@ -1297,7 +1299,7 @@ static void free_user(struct kref *ref)
>  	struct ipmi_user *user = container_of(ref, struct ipmi_user, refcount);
>  
>  	/* SRCU cleanup must happen in task context. */
> -	schedule_work(&user->remove_work);
> +	queue_work(remove_work_wq, &user->remove_work);
>  }
>  
>  static void _ipmi_destroy_user(struct ipmi_user *user)
> @@ -5383,6 +5385,8 @@ static int ipmi_init_msghandler(void)
>  
>  	atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
>  
> +	remove_work_wq = create_singlethread_workqueue("ipmi-msghandler-remove-wq");
> +

Shouldn't you check the return value here?

-corey

>  	initialized = true;
>  
>  out:
> @@ -5408,6 +5412,9 @@ static void __exit cleanup_ipmi(void)
>  	int count;
>  
>  	if (initialized) {
> +		flush_workqueue(remove_work_wq);
> +		destroy_workqueue(remove_work_wq);
> +
>  		atomic_notifier_chain_unregister(&panic_notifier_list,
>  						 &panic_block);
>  
> -- 
> 2.17.1
>