media: dvb-core: Fix use-after-free due to race condition at dvb_ca_en50221

If the device node of dvb_ca_en50221 is open() and the
device is disconnected, a UAF may occur when calling
close() on the device node.

The root cause is that wake_up() and wait_event() for
dvbdev->wait_queue are not implemented.

So implement wait_event() function in dvb_ca_en50221_release()
and add 'remove_mutex' which prevents race condition
for 'ca->exit'.

[mchehab: fix a checkpatch warning]

Link: https://lore.kernel.org/linux-media/20221121063308.GA33821@ubuntu
Signed-off-by: Hyunwoo Kim <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-core/dvb_ca_en50221.c

···
 
 	/* mutex serializing ioctls */
 	struct mutex ioctl_mutex;
+
+	/* A mutex used when a device is disconnected */
+	struct mutex remove_mutex;
+
+	/* Whether the device is disconnected */
+	int exit;
 };
 
 static void dvb_ca_private_free(struct dvb_ca_private *ca)
···
 
 	dprintk("%s\n", __func__);
 
-	if (!try_module_get(ca->pub->owner))
+	mutex_lock(&ca->remove_mutex);
+
+	if (ca->exit) {
+		mutex_unlock(&ca->remove_mutex);
+		return -ENODEV;
+	}
+
+	if (!try_module_get(ca->pub->owner)) {
+		mutex_unlock(&ca->remove_mutex);
 		return -EIO;
+	}
 
 	err = dvb_generic_open(inode, file);
 	if (err < 0) {
 		module_put(ca->pub->owner);
+		mutex_unlock(&ca->remove_mutex);
 		return err;
 	}
 
···
 
 	dvb_ca_private_get(ca);
 
+	mutex_unlock(&ca->remove_mutex);
 	return 0;
 }
 
···
 
 	dprintk("%s\n", __func__);
 
+	mutex_lock(&ca->remove_mutex);
+
 	/* mark the CA device as closed */
 	ca->open = 0;
 	dvb_ca_en50221_thread_update_delay(ca);
···
 	module_put(ca->pub->owner);
 
 	dvb_ca_private_put(ca);
+
+	if (dvbdev->users == 1 && ca->exit == 1) {
+		mutex_unlock(&ca->remove_mutex);
+		wake_up(&dvbdev->wait_queue);
+	} else {
+		mutex_unlock(&ca->remove_mutex);
+	}
 
 	return err;
 }
···
 	}
 
 	mutex_init(&ca->ioctl_mutex);
+	mutex_init(&ca->remove_mutex);
 
 	if (signal_pending(current)) {
 		ret = -EINTR;
···
 	int i;
 
 	dprintk("%s\n", __func__);
+
+	mutex_lock(&ca->remove_mutex);
+	ca->exit = 1;
+	mutex_unlock(&ca->remove_mutex);
+
+	if (ca->dvbdev->users < 1)
+		wait_event(ca->dvbdev->wait_queue,
+				ca->dvbdev->users == 1);
 
 	/* shutdown the thread if there was one */
 	kthread_stop(ca->thread);