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).

NAK.

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