thermal/debugfs: Prevent use-after-free from occurring after cdev removal

Since thermal_debug_cdev_remove() does not run under cdev->lock, it can
run in parallel with thermal_debug_cdev_state_update() and it may free
the struct thermal_debugfs object used by the latter after it has been
checked against NULL.

If that happens, thermal_debug_cdev_state_update() will access memory
that has been freed already causing the kernel to crash.

Address this by using cdev->lock in thermal_debug_cdev_remove() around
the cdev->debugfs value check (in case the same cdev is removed at the
same time in two different threads) and its reset to NULL.

Fixes: 755113d76786 ("thermal/debugfs: Add thermal cooling device debugfs information")
Cc :6.8+ <[e-mail]> # 6.8+
Signed-off-by: Rafael J. Wysocki <[e-mail]>
Reviewed-by: Lukasz Luba <[e-mail]>

drivers/thermal/thermal_debugfs.c

···
  */
 void thermal_debug_cdev_remove(struct thermal_cooling_device *cdev)
 {
-	struct thermal_debugfs *thermal_dbg = cdev->debugfs;
+	struct thermal_debugfs *thermal_dbg;
 
-	if (!thermal_dbg)
+	mutex_lock(&cdev->lock);
+
+	thermal_dbg = cdev->debugfs;
+	if (!thermal_dbg) {
+		mutex_unlock(&cdev->lock);
 		return;
+	}
+
+	cdev->debugfs = NULL;
+
+	mutex_unlock(&cdev->lock);
 
 	mutex_lock(&thermal_dbg->lock);
 
 	thermal_debugfs_cdev_clear(&thermal_dbg->cdev_dbg);
-	cdev->debugfs = NULL;
 
 	mutex_unlock(&thermal_dbg->lock);