Re: [PATCH v2] ACPI: video: Fix use-after-free in acpi_video_switch_brightness()

[Hans de Goede <hansg@kernel.org>]

Hi Yuhao,

On 22-Oct-25 6:25 AM, Yuhao Jiang wrote:
> The switch_brightness_work delayed work accesses device->brightness
> and device->backlight, which are freed by
> acpi_video_dev_unregister_backlight() during device removal.
> 
> If the work executes after acpi_video_bus_unregister_backlight()
> frees these resources, it causes a use-after-free when
> acpi_video_switch_brightness() dereferences device->brightness or
> device->backlight.
> 
> Fix this by calling cancel_delayed_work_sync() for each device's
> switch_brightness_work before unregistering its backlight resources.
> This ensures the work completes before the memory is freed.
> 
> Fixes: 8ab58e8e7e097 ("ACPI / video: Fix backlight taking 2 steps on a brightness up/down keypress")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yuhao Jiang <danisjiang@gmail.com>

Thank you for your patch, this is a good catch.

> ---
> Changes in v2:
> - Move cancel_delayed_work_sync() to acpi_video_bus_unregister_backlight()
>   instead of acpi_video_bus_put_devices() for better logic clarity and to
>   prevent potential UAF of device->brightness
> - Correct Fixes tag to point to 8ab58e8e7e097 which introduced the delayed work
> - Link to v1: https://lore.kernel.org/all/20251022040859.2102914-1-danisjiang@gmail.com
> ---
>  drivers/acpi/acpi_video.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
> index 103f29661576..64709658bdc4 100644
> --- a/drivers/acpi/acpi_video.c
> +++ b/drivers/acpi/acpi_video.c
> @@ -1869,8 +1869,10 @@ static int acpi_video_bus_unregister_backlight(struct acpi_video_bus *video)
>  	error = unregister_pm_notifier(&video->pm_nb);
>  
>  	mutex_lock(&video->device_list_lock);
> -	list_for_each_entry(dev, &video->video_device_list, entry)
> +	list_for_each_entry(dev, &video->video_device_list, entry) {
> +		cancel_delayed_work_sync(&dev->switch_brightness_work);
>  		acpi_video_dev_unregister_backlight(dev);
> +	}
>  	mutex_unlock(&video->device_list_lock);
>  
>  	video->backlight_registered = false;

As you mention in your changelog, the cancel_delayed_work_sync() needs
to happen before acpi_video_dev_unregister_backlight().

Since this needs to happen before unregistering things I think it would be
more logical to put the cancel_delayed_work_sync(&dev->switch_brightness_work);
call inside acpi_video_bus_remove_notify_handler().

So do the cancel in the loop there, directly after the
acpi_video_dev_remove_notify_handler(dev) call which removes the handler
which queues the work.

E.g. make the loop inside acpi_video_bus_remove_notify_handler() look like
this:

        mutex_lock(&video->device_list_lock);
        list_for_each_entry(dev, &video->video_device_list, entry) {
                acpi_video_dev_remove_notify_handler(dev);
                cancel_delayed_work_sync(&dev->switch_brightness_work);
        }
        mutex_unlock(&video->device_list_lock);

This cancels the work a bit earlier, but more importantly this feels
like the more logical place to put the cancel call.

Regards,

Hans