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

A race condition may occur between the .disconnect function, which
is called when the device is disconnected, and the dvb_device_open()
function, which is called when the device node is open()ed.
This results in several types of UAFs.

The root cause of this is that you use the dvb_device_open() function,
which does not implement a conditional statement
that checks 'dvbnet->exit'.

So, add 'remove_mutex` to protect 'dvbnet->exit' and use
locked_dvb_net_open() function to check 'dvbnet->exit'.

[mchehab: fix a checkpatch warning]

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_net.c

···
 	return dvb_usercopy(file, cmd, arg, dvb_net_do_ioctl);
 }
 
+static int locked_dvb_net_open(struct inode *inode, struct file *file)
+{
+	struct dvb_device *dvbdev = file->private_data;
+	struct dvb_net *dvbnet = dvbdev->priv;
+	int ret;
+
+	if (mutex_lock_interruptible(&dvbnet->remove_mutex))
+		return -ERESTARTSYS;
+
+	if (dvbnet->exit) {
+		mutex_unlock(&dvbnet->remove_mutex);
+		return -ENODEV;
+	}
+
+	ret = dvb_generic_open(inode, file);
+
+	mutex_unlock(&dvbnet->remove_mutex);
+
+	return ret;
+}
+
 static int dvb_net_close(struct inode *inode, struct file *file)
 {
 	struct dvb_device *dvbdev = file->private_data;
 	struct dvb_net *dvbnet = dvbdev->priv;
 
+	mutex_lock(&dvbnet->remove_mutex);
+
 	dvb_generic_release(inode, file);
 
-	if(dvbdev->users == 1 && dvbnet->exit == 1)
+	if (dvbdev->users == 1 && dvbnet->exit == 1) {
+		mutex_unlock(&dvbnet->remove_mutex);
 		wake_up(&dvbdev->wait_queue);
+	} else {
+		mutex_unlock(&dvbnet->remove_mutex);
+	}
+
 	return 0;
 }
 
···
 static const struct file_operations dvb_net_fops = {
 	.owner = THIS_MODULE,
 	.unlocked_ioctl = dvb_net_ioctl,
-	.open =	dvb_generic_open,
+	.open =	locked_dvb_net_open,
 	.release = dvb_net_close,
 	.llseek = noop_llseek,
 };
···
 {
 	int i;
 
+	mutex_lock(&dvbnet->remove_mutex);
 	dvbnet->exit = 1;
+	mutex_unlock(&dvbnet->remove_mutex);
+
 	if (dvbnet->dvbdev->users < 1)
 		wait_event(dvbnet->dvbdev->wait_queue,
-				dvbnet->dvbdev->users==1);
+				dvbnet->dvbdev->users == 1);
 
 	dvb_unregister_device(dvbnet->dvbdev);
 
···
 	int i;
 
 	mutex_init(&dvbnet->ioctl_mutex);
+	mutex_init(&dvbnet->remove_mutex);
 	dvbnet->demux = dmx;
 
 	for (i=0; i<DVB_NET_DEVICES_MAX; i++)

include/media/dvb_net.h

···
  * @exit:		flag to indicate when the device is being removed.
  * @demux:		pointer to &struct dmx_demux.
  * @ioctl_mutex:	protect access to this struct.
+ * @remove_mutex:	mutex that avoids a race condition between a callback
+ *			called when the hardware is disconnected and the
+ *			file_operations of dvb_net.
  *
  * Currently, the core supports up to %DVB_NET_DEVICES_MAX (10) network
  * devices.
···
 	unsigned int exit:1;
 	struct dmx_demux *demux;
 	struct mutex ioctl_mutex;
+	struct mutex remove_mutex;
 };
 
 /**