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

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

[Yuhao Jiang]

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>
---
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;
-- 
2.34.1

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

[Hans de Goede]

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

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

[Yuhao Jiang]

Hi Hans,

Thanks for the feedback! I've submitted it in patch v3:
https://lore.kernel.org/all/20251022200704.2655507-1-danisjiang@gmail.com/.

Best regards,
Yuhao

On Wed, Oct 22, 2025 at 4:28 AM Hans de Goede <hansg@kernel.org> wrote:
>
> 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
>
>