media: rc: uevent sysfs file races with rc_unregister_device()

Only report uevent file contents if device still registered, else we
might read freed memory.

Reported-by: syzbot+ceef16277388d6f24898@syzkaller.appspotmail.com
Cc: Hillf Danton <hdanton@sina.com>
Cc: <stable@vger.kernel.org> # 4.16+
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/rc/rc-main.c

@@ -1601,25 +1601,25 @@ static void rc_dev_release(struct device *device)
 	kfree(dev);
 }
 
-#define ADD_HOTPLUG_VAR(fmt, val...)					\
-	do {								\
-		int err = add_uevent_var(env, fmt, val);		\
-		if (err)						\
-			return err;					\
-	} while (0)
-
 static int rc_dev_uevent(struct device *device, struct kobj_uevent_env *env)
 {
 	struct rc_dev *dev = to_rc_dev(device);
+	int ret = 0;
 
-	if (dev->rc_map.name)
-		ADD_HOTPLUG_VAR("NAME=%s", dev->rc_map.name);
-	if (dev->driver_name)
-		ADD_HOTPLUG_VAR("DRV_NAME=%s", dev->driver_name);
-	if (dev->device_name)
-		ADD_HOTPLUG_VAR("DEV_NAME=%s", dev->device_name);
+	mutex_lock(&dev->lock);
 
-	return 0;
+	if (!dev->registered)
+		ret = -ENODEV;
+	if (ret == 0 && dev->rc_map.name)
+		ret = add_uevent_var(env, "NAME=%s", dev->rc_map.name);
+	if (ret == 0 && dev->driver_name)
+		ret = add_uevent_var(env, "DRV_NAME=%s", dev->driver_name);
+	if (ret == 0 && dev->device_name)
+		ret = add_uevent_var(env, "DEV_NAME=%s", dev->device_name);
+
+	mutex_unlock(&dev->lock);
+
+	return ret;
 }
 
 /*
@@ -2011,14 +2011,14 @@ void rc_unregister_device(struct rc_dev *dev)
 	del_timer_sync(&dev->timer_keyup);
 	del_timer_sync(&dev->timer_repeat);
 
-	rc_free_rx_device(dev);
-
 	mutex_lock(&dev->lock);
 	if (dev->users && dev->close)
 		dev->close(dev);
 	dev->registered = false;
 	mutex_unlock(&dev->lock);
 
+	rc_free_rx_device(dev);
+
 	/*
 	 * lirc device should be freed with dev->registered = false, so
 	 * that userspace polling will get notified.