media: dvb-core: Fix use-after-free on race condition at dvb_frontend

If the device node of dvb_frontend is open() and the device is
disconnected, many kinds of UAFs may occur when calling close()
on the device node.

The root cause of this is that wake_up() for dvbdev->wait_queue
is implemented in the dvb_frontend_release() function, but
wait_event() is not implemented in the dvb_frontend_stop() function.

So, implement wait_event() function in dvb_frontend_stop() and
add 'remove_mutex' which prevents race condition for 'fe->exit'.

[mchehab: fix a couple of checkpatch warnings and some mistakes at the error handling logic]

Link: https://lore.kernel.org/linux-media/[e-mail]
Signed-off-by: Hyunwoo Kim <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-core/dvb_frontend.c

···
 
 	dev_dbg(fe->dvb->device, "%s:\n", __func__);
 
+	mutex_lock(&fe->remove_mutex);
+
 	if (fe->exit != DVB_FE_DEVICE_REMOVED)
 		fe->exit = DVB_FE_NORMAL_EXIT;
 	mb();
 
-	if (!fepriv->thread)
+	if (!fepriv->thread) {
+		mutex_unlock(&fe->remove_mutex);
 		return;
+	}
 
 	kthread_stop(fepriv->thread);
+
+	mutex_unlock(&fe->remove_mutex);
+
+	if (fepriv->dvbdev->users < -1) {
+		wait_event(fepriv->dvbdev->wait_queue,
+			   fepriv->dvbdev->users == -1);
+	}
 
 	sema_init(&fepriv->sem, 1);
 	fepriv->state = FESTATE_IDLE;
···
 	struct dvb_adapter *adapter = fe->dvb;
 	int ret;
 
+	mutex_lock(&fe->remove_mutex);
+
 	dev_dbg(fe->dvb->device, "%s:\n", __func__);
-	if (fe->exit == DVB_FE_DEVICE_REMOVED)
-		return -ENODEV;
+	if (fe->exit == DVB_FE_DEVICE_REMOVED) {
+		ret = -ENODEV;
+		goto err_remove_mutex;
+	}
 
 	if (adapter->mfe_shared == 2) {
 		mutex_lock(&adapter->mfe_lock);
···
 			if (adapter->mfe_dvbdev &&
 			    !adapter->mfe_dvbdev->writers) {
 				mutex_unlock(&adapter->mfe_lock);
-				return -EBUSY;
+				ret = -EBUSY;
+				goto err_remove_mutex;
 			}
 			adapter->mfe_dvbdev = dvbdev;
 		}
···
 			while (mferetry-- && (mfedev->users != -1 ||
 					      mfepriv->thread)) {
 				if (msleep_interruptible(500)) {
-					if (signal_pending(current))
-						return -EINTR;
+					if (signal_pending(current)) {
+						ret = -EINTR;
+						goto err_remove_mutex;
+					}
 				}
 			}
 
···
 				if (mfedev->users != -1 ||
 				    mfepriv->thread) {
 					mutex_unlock(&adapter->mfe_lock);
-					return -EBUSY;
+					ret = -EBUSY;
+					goto err_remove_mutex;
 				}
 				adapter->mfe_dvbdev = dvbdev;
 			}
···
 
 	if (adapter->mfe_shared)
 		mutex_unlock(&adapter->mfe_lock);
+
+	mutex_unlock(&fe->remove_mutex);
 	return ret;
 
 err3:
···
 err0:
 	if (adapter->mfe_shared)
 		mutex_unlock(&adapter->mfe_lock);
+
+err_remove_mutex:
+	mutex_unlock(&fe->remove_mutex);
 	return ret;
 }
 
···
 	struct dvb_frontend *fe = dvbdev->priv;
 	struct dvb_frontend_private *fepriv = fe->frontend_priv;
 	int ret;
+
+	mutex_lock(&fe->remove_mutex);
 
 	dev_dbg(fe->dvb->device, "%s:\n", __func__);
 
···
 		}
 		mutex_unlock(&fe->dvb->mdev_lock);
 #endif
-		if (fe->exit != DVB_FE_NO_EXIT)
-			wake_up(&dvbdev->wait_queue);
 		if (fe->ops.ts_bus_ctrl)
 			fe->ops.ts_bus_ctrl(fe, 0);
+
+		if (fe->exit != DVB_FE_NO_EXIT) {
+			mutex_unlock(&fe->remove_mutex);
+			wake_up(&dvbdev->wait_queue);
+		} else {
+			mutex_unlock(&fe->remove_mutex);
+		}
+
+	} else {
+		mutex_unlock(&fe->remove_mutex);
 	}
 
 	dvb_frontend_put(fe);
···
 	fepriv = fe->frontend_priv;
 
 	kref_init(&fe->refcount);
+	mutex_init(&fe->remove_mutex);
 
 	/*
 	 * After initialization, there need to be two references: one

include/media/dvb_frontend.h

···
  * @id:			Frontend ID
  * @exit:		Used to inform the DVB core that the frontend
  *			thread should exit (usually, means that the hardware
- *			got disconnected.
+ *			got disconnected).
+ * @remove_mutex:	mutex that avoids a race condition between a callback
+ *			called when the hardware is disconnected and the
+ *			file_operations of dvb_frontend.
  */
 
 struct dvb_frontend {
···
 	int (*callback)(void *adapter_priv, int component, int cmd, int arg);
 	int id;
 	unsigned int exit;
+	struct mutex remove_mutex;
 };
 
 /**