[PATCH 0/8] devm_led_classdev_register() usage problem

Andy Shevchenko andriy.shevchenko at intel.com
Sat Nov 25 02:28:57 AEDT 2023

On Wed, Oct 25, 2023 at 04:07:29PM +0300, George Stark wrote:
> Lots of drivers use devm_led_classdev_register() to register their led objects
> and let the kernel free those leds at the driver's remove stage.
> It can lead to a problem due to led_classdev_unregister()
> implementation calls led_set_brightness() to turn off the led.
> led_set_brightness() may call one of the module's brightness_set callbacks.
> If that callback uses module's resources allocated without using devm funcs()
> then those resources will be already freed at module's remove() callback and
> we may have use-after-free situation.
> Here is an example:
> module_probe()
> {
>     devm_led_classdev_register(module_brightness_set_cb);
>     mutex_init(&mutex);
> }
> module_brightness_set_cb()
> {
>     mutex_lock(&mutex);
>     do_set_brightness();
>     mutex_unlock(&mutex);
> }
> module_remove()
> {
>     mutex_destroy(&mutex);
> }
> at rmmod:
> module_remove()
>     ->mutex_destroy(&mutex);
> devres_release_all()
>     ->led_classdev_unregister();
>         ->led_set_brightness();
>             ->module_brightness_set_cb();
>                  ->mutex_lock(&mutex);  /* use-after-free */
> I think it's an architectural issue and should be discussed thoroughly.
> Some thoughts about fixing it as a start:
> 1) drivers can use devm_led_classdev_unregister() to explicitly free leds before
> dependend resources are freed. devm_led_classdev_register() remains being useful
> to simplify probe implementation.
> As a proof of concept I examined all drivers from drivers/leds and prepared
> patches where it's needed. Sometimes it was not as clean as just calling
> devm_led_classdev_unregister() because several drivers do not track
> their leds object at all - they can call devm_led_classdev_register() and drop the
> returned pointer. In that case I used devres group API.
> Drivers outside drivers/leds should be checked too after discussion.
> 2) remove led_set_brightness from led_classdev_unregister() and force the drivers
> to turn leds off at shutdown. May be add check that led's brightness is 0
> at led_classdev_unregister() and put a warning to dmesg if it's not.
> Actually in many cases it doesn't really need to turn off the leds manually one-by-one
> if driver shutdowns whole led controller. For the last case to disable the warning
> new flag can be brought in e.g LED_AUTO_OFF_AT_SHUTDOWN (similar to LED_RETAIN_AT_SHUTDOWN).


Just fix the drivers by wrapping mutex_destroy() into devm, There are many
doing so. You may be brave enough to introduce devm_mutex_init() somewhere
in include/linux/device*

With Best Regards,
Andy Shevchenko

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