Le 25/10/2023 à 15:07, George Stark a écrit :
> 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.
I see no point in using a device managed function if you have to call
the unregister function. All the purpose of device managed functions is
to avoid having to call an unregister function at the end.
>
> 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).
>
> George Stark (8):
> leds: powernv: explicitly unregister LEDs at module's shutdown
> leds: nic78bx: explicitly unregister LEDs at module's shutdown
> leds: an30259a: explicitly unregister LEDs at module's shutdown
> leds: mlxreg: explicitly unregister LEDs at module's shutdown
> leds: aw200xx: explicitly unregister LEDs at module's shutdown
> leds: aw2013: explicitly unregister LEDs at module's shutdown
> leds: lp3952: explicitly unregister LEDs at module's shutdown
> leds: lm3532: explicitly unregister LEDs at module's shutdown
>
> drivers/leds/leds-an30259a.c | 4 ++++
> drivers/leds/leds-aw200xx.c | 4 ++++
> drivers/leds/leds-aw2013.c | 4 ++++
> drivers/leds/leds-lm3532.c | 6 ++++++
> drivers/leds/leds-lp3952.c | 5 +++++
> drivers/leds/leds-mlxreg.c | 12 +++++++++++-
> drivers/leds/leds-nic78bx.c | 4 ++++
> drivers/leds/leds-powernv.c | 7 +++++++
> 8 files changed, 45 insertions(+), 1 deletion(-)
>
