linux-kernel - Re: [PATCH] Input: cyttsp4_core - change del_timer_sync() to timer_shutdown

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Message-ID: <ZFBbmml9FjXICHUv@google.com>
Date:   Mon, 1 May 2023 17:38:50 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Duoming Zhou <duoming@....edu.cn>
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        linus.walleij@...aro.org
Subject: Re: [PATCH] Input: cyttsp4_core - change del_timer_sync() to
 timer_shutdown_sync()

On Fri, Apr 21, 2023 at 04:29:19PM +0800, Duoming Zhou wrote:
> The watchdog_timer can schedule tx_timeout_task and watchdog_work
> can also arm watchdog_timer. The process is shown below:
> 
> ----------- timer schedules work ------------
> cyttsp4_watchdog_timer() //timer handler
>   schedule_work(&cd->watchdog_work)
> 
> ----------- work arms timer ------------
> cyttsp4_watchdog_work() //workqueue callback function
>   cyttsp4_start_wd_timer()
>     mod_timer(&cd->watchdog_timer, ...)
> 
> Although del_timer_sync() and cancel_work_sync() are called in
> cyttsp4_remove(), the timer and workqueue could still be rearmed.
> As a result, the possible use after free bugs could happen. The
> process is shown below:
> 
>   (cleanup routine)           |  (timer and workqueue routine)
> cyttsp4_remove()              | cyttsp4_watchdog_timer() //timer
>   cyttsp4_stop_wd_timer()     |   schedule_work()
>     del_timer_sync()          |
>                               | cyttsp4_watchdog_work() //worker
>                               |   cyttsp4_start_wd_timer()
>                               |     mod_timer()
>     cancel_work_sync()        |
>                               | cyttsp4_watchdog_timer() //timer
>                               |   schedule_work()
>     del_timer_sync()          |
>   kfree(cd) //FREE            |
>                               | cyttsp4_watchdog_work() // reschedule!
>                               |   cd-> //USE
> 
> This patch changes del_timer_sync() to timer_shutdown_sync(),
> which could prevent rearming of the timer from the workqueue.
> 
> Fixes: 17fb1563d69b ("Input: cyttsp4 - add core driver for Cypress TMA4XX touchscreen devices")
> Signed-off-by: Duoming Zhou <duoming@....edu.cn>

Applied, thank you.

-- 
Dmitry