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).
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(-)
--
2.38.4
