Re: [PATCH] media: s5p-mfc: use timer_shutdown_sync() for the self-rearming watchdog

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 30.07.2026 09:15, Fan Wu wrote:
> The MFC watchdog timer (s5p_mfc_watchdog) is self-rearming: every
> callback re-arms itself with add_timer() and, on a timeout, schedules
> watchdog_work. s5p_mfc_remove() tears this down with timer_delete_sync()
> followed by flush_work().
>
> timer_delete_sync() dequeues the timer and waits for a callback that is
> already running, but it does not prevent a subsequent re-arm. Because the
> watchdog callback re-arms via add_timer(), this is the wrong shutdown
> primitive: once the wait returns nothing guarantees a just-queued re-arm
> will not fire later, dereferencing dev (clock on/off, deinit_hw,
> load_firmware, init_hw) after video_unregister_device(), DMA teardown and
> s5p_mfc_final_pm() have run, a potential use-after-free.
>
> Use timer_shutdown_sync(), which puts the timer into a shutdown state so
> that any add_timer()/mod_timer() from the callback becomes a no-op and no
> later firing is possible. Switch flush_work() to cancel_work_sync() so any
> watchdog_work that was already queued before the timer was stopped is
> cancelled rather than allowed to run to completion against the torn-down
> device.
>
> This issue was found by an in-house static analysis tool. No runtime
> reproducer is available.
>
> Fixes: af9357467810 ("[media] MFC: Add MFC 5.1 V4L2 driver")
> Cc: stable@vger.kernel.org
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Andrzej Hajda <andrzej.hajda@intel.com>
> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> Assisted-by: Codex:gpt-5.6
> Signed-off-by: Fan Wu <fanwu01@zju.edu.cn>
Reviewed-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
>  drivers/media/platform/samsung/s5p-mfc/s5p_mfc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/platform/samsung/s5p-mfc/s5p_mfc.c b/drivers/media/platform/samsung/s5p-mfc/s5p_mfc.c
> index 32eb402d439c..03e7eac8d1f7 100644
> --- a/drivers/media/platform/samsung/s5p-mfc/s5p_mfc.c
> +++ b/drivers/media/platform/samsung/s5p-mfc/s5p_mfc.c
> @@ -1474,8 +1474,8 @@ static void s5p_mfc_remove(struct platform_device *pdev)
>  	}
>  	mutex_unlock(&dev->mfc_mutex);
>  
> -	timer_delete_sync(&dev->watchdog_timer);
> -	flush_work(&dev->watchdog_work);
> +	timer_shutdown_sync(&dev->watchdog_timer);
> +	cancel_work_sync(&dev->watchdog_work);
>  
>  	video_unregister_device(dev->vfd_enc);
>  	video_unregister_device(dev->vfd_dec);

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland