HID: hiddev: fix race between hiddev_disconnect and hiddev_release

When hiddev_disconnect() runs with chardev open, it will proceed with
usbhid_close(). When userspace in parallel runs the hiddev_release(),
it sees !hiddev->exists (as it has been already set so by
hiddev_disconnect()) and kfrees hiddev while hiddev_disconnect() hasn't
finished yet.

Serialize the access to hiddev->exists and hiddev->open by existancelock.

Reported-by: [e-mail]
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/hid/usbhid/hiddev.c

···
 	list_del(&list->node);
 	spin_unlock_irqrestore(&list->hiddev->list_lock, flags);
 
+	mutex_lock(&list->hiddev->existancelock);
 	if (!--list->hiddev->open) {
 		if (list->hiddev->exist) {
 			usbhid_close(list->hiddev->hid);
···
 	}
 
 	kfree(list);
+	mutex_unlock(&list->hiddev->existancelock);
 
 	return 0;
 }
···
 	list_add_tail(&list->node, &hiddev->list);
 	spin_unlock_irq(&list->hiddev->list_lock);
 
+	mutex_lock(&hiddev->existancelock);
 	if (!list->hiddev->open++)
 		if (list->hiddev->exist) {
 			struct hid_device *hid = hiddev->hid;
 			res = usbhid_get_power(hid);
 			if (res < 0) {
 				res = -EIO;
-				goto bail;
+				goto bail_unlock;
 			}
 			usbhid_open(hid);
 		}
+	mutex_unlock(&hiddev->existancelock);
 	return 0;
+bail_unlock:
+	mutex_unlock(&hiddev->existancelock);
 bail:
 	file->private_data = NULL;
 	kfree(list);
···
 
 	mutex_lock(&hiddev->existancelock);
 	hiddev->exist = 0;
-	mutex_unlock(&hiddev->existancelock);
 
 	usb_deregister_dev(usbhid->intf, &hiddev_class);
 
···
 	} else {
 		kfree(hiddev);
 	}
+	mutex_unlock(&hiddev->existancelock);
 }