[PATCH] leds: class: disable sysfs before unregistering LED devices

[David Lee <david.lee@trailofbits.com>]

Closing a uleds device removes the LED class device and then frees the
struct uleds_device that contains it. The LED class device is also exposed
through sysfs, where attributes such as trigger, brightness, delay_on, and
delay_off can remain open across unregister.

That creates a lifetime race. led_classdev_unregister() tears down the LED
trigger, software blink timer, brightness state, and trigger-specific sysfs
groups, but it does not first block LED sysfs writers. An already-open sysfs
attribute can therefore enter the LED trigger or timer paths while unregister
is tearing down the same embedded struct led_classdev.

For uleds this can become a use-after-free: after unregister returns,
uleds_release() frees the containing struct uleds_device, while stale sysfs
or timer paths may still access led_classdev fields embedded in that freed
object. KASAN reports use-after-free reads and writes in __run_timers(),
led_timer_function(), and uleds_brightness_set().

Fix this by entering the existing LED sysfs exclusion protocol at the start
of unregister. Take led_cdev->led_access, set LED_SYSFS_DISABLE, perform the
trigger, timer, brightness, and trigger-sysfs teardown, then release the
mutex. A racing sysfs writer either completes before unregister starts or
sees the disabled state and returns -EBUSY instead of touching an object that
the driver may free.

Fixes: e381322b0190 ("leds: Introduce userspace LED class driver")
Cc: stable@vger.kernel.org
Signed-off-by: David Lee <david.lee@trailofbits.com>
Assisted-by: Codex:gpt-5.5
---
Trail of Bits has a minimal PoC that triggers this crash on a custom
kernel build, which can be shared further if needed.

drivers/leds/led-class.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
index a51b0ed53886..a697d6740011 100644
--- a/drivers/leds/led-class.c
+++ b/drivers/leds/led-class.c
@@ -616,6 +616,9 @@ void led_classdev_unregister(struct led_classdev *led_cdev)
 	if (IS_ERR_OR_NULL(led_cdev->dev))
 		return;
 
+	mutex_lock(&led_cdev->led_access);
+	led_sysfs_disable(led_cdev);
+
 #ifdef CONFIG_LEDS_TRIGGERS
 	down_write(&led_cdev->trigger_lock);
 	if (led_cdev->trigger)
@@ -636,6 +639,8 @@ void led_classdev_unregister(struct led_classdev *led_cdev)
 	if (led_cdev->flags & LED_BRIGHT_HW_CHANGED)
 		led_remove_brightness_hw_changed(led_cdev);
 
+	mutex_unlock(&led_cdev->led_access);
+
 	device_unregister(led_cdev->dev);
 
 	down_write(&leds_list_lock);