drm/xe/hwmon: fix uaf on unload

It doesn't look like you can mix and match devm_ and drmmm_ for a
managed resource. For drmmm the resources are all tracked in drm with
its own list, and there is only one devm_ resource for the entire list.
If the driver itself also adds some of its own devm resources, then
those will be released first. In the case of hwmon the devm_kzalloc will
be freed before the drmmm_ action to destroy the mutex allocated within,
leading to uaf.

Since hwmon itself wants to use devm, rather use that for the mutex
destroy.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/766
Signed-off-by: Matthew Auld <[e-mail]>
Cc: Badal Nilawar <[e-mail]>
Cc: Rodrigo Vivi <[e-mail]>
Reviewed-by: Rodrigo Vivi <[e-mail]>
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/xe/xe_hwmon.c

···
 		xe_hwmon_energy_get(hwmon, &energy);
 }
 
+static void xe_hwmon_mutex_destroy(void *arg)
+{
+	struct xe_hwmon *hwmon = arg;
+
+	mutex_destroy(&hwmon->hwmon_lock);
+}
+
 void xe_hwmon_register(struct xe_device *xe)
 {
 	struct device *dev = xe->drm.dev;
···
 
 	xe->hwmon = hwmon;
 
-	drmm_mutex_init(&xe->drm, &hwmon->hwmon_lock);
+	mutex_init(&hwmon->hwmon_lock);
+	if (devm_add_action_or_reset(dev, xe_hwmon_mutex_destroy, hwmon))
+		return;
 
 	/* primary GT to access device level properties */
 	hwmon->gt = xe->tiles[0].primary_gt;