media: dvbdev: adopts refcnt to avoid UAF

dvb_unregister_device() is known that prone to use-after-free.
That is, the cleanup from dvb_unregister_device() releases the dvb_device
even if there are pointers stored in file->private_data still refer to it.

This patch adds a reference counter into struct dvb_device and delays its
deallocation until no pointer refers to the object.

Link: https://lore.kernel.org/linux-media/20220807145952.10368-1-linma@zju.edu.cn
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/dvb-core/dvb_ca_en50221.c

@@ -157,7 +157,7 @@ static void dvb_ca_private_free(struct dvb_ca_private *ca)
 {
 	unsigned int i;
 
-	dvb_free_device(ca->dvbdev);
+	dvb_device_put(ca->dvbdev);
 	for (i = 0; i < ca->slot_count; i++)
 		vfree(ca->slot_info[i].rx_buffer.data);
 

drivers/media/dvb-core/dvb_frontend.c

@@ -136,7 +136,7 @@ static void __dvb_frontend_free(struct dvb_frontend *fe)
 	struct dvb_frontend_private *fepriv = fe->frontend_priv;
 
 	if (fepriv)
-		dvb_free_device(fepriv->dvbdev);
+		dvb_device_put(fepriv->dvbdev);
 
 	dvb_frontend_invoke_release(fe, fe->ops.release);
 

drivers/media/dvb-core/dvbdev.c

@@ -97,7 +97,7 @@ static int dvb_device_open(struct inode *inode, struct file *file)
 		new_fops = fops_get(dvbdev->fops);
 		if (!new_fops)
 			goto fail;
-		file->private_data = dvbdev;
+		file->private_data = dvb_device_get(dvbdev);
 		replace_fops(file, new_fops);
 		if (file->f_op->open)
 			err = file->f_op->open(inode, file);
@@ -161,6 +161,9 @@ int dvb_generic_release(struct inode *inode, struct file *file)
 	}
 
 	dvbdev->users++;
+
+	dvb_device_put(dvbdev);
+
 	return 0;
 }
 EXPORT_SYMBOL(dvb_generic_release);
@@ -479,6 +482,7 @@ int dvb_register_device(struct dvb_adapter *adap, struct dvb_device **pdvbdev,
 		return -ENOMEM;
 	}
 
+	kref_init(&dvbdev->ref);
 	memcpy(dvbdev, template, sizeof(struct dvb_device));
 	dvbdev->type = type;
 	dvbdev->id = id;
@@ -510,7 +514,7 @@ int dvb_register_device(struct dvb_adapter *adap, struct dvb_device **pdvbdev,
 #endif
 
 	dvbdev->minor = minor;
-	dvb_minors[minor] = dvbdev;
+	dvb_minors[minor] = dvb_device_get(dvbdev);
 	up_write(&minor_rwsem);
 
 	ret = dvb_register_media_device(dvbdev, type, minor, demux_sink_pads);
@@ -555,6 +559,7 @@ void dvb_remove_device(struct dvb_device *dvbdev)
 
 	down_write(&minor_rwsem);
 	dvb_minors[dvbdev->minor] = NULL;
+	dvb_device_put(dvbdev);
 	up_write(&minor_rwsem);
 
 	dvb_media_device_free(dvbdev);
@@ -566,21 +571,34 @@ void dvb_remove_device(struct dvb_device *dvbdev)
 EXPORT_SYMBOL(dvb_remove_device);
 
 
-void dvb_free_device(struct dvb_device *dvbdev)
+static void dvb_free_device(struct kref *ref)
 {
-	if (!dvbdev)
-		return;
+	struct dvb_device *dvbdev = container_of(ref, struct dvb_device, ref);
 
 	kfree (dvbdev->fops);
 	kfree (dvbdev);
 }
-EXPORT_SYMBOL(dvb_free_device);
+
+
+struct dvb_device *dvb_device_get(struct dvb_device *dvbdev)
+{
+	kref_get(&dvbdev->ref);
+	return dvbdev;
+}
+EXPORT_SYMBOL(dvb_device_get);
+
+
+void dvb_device_put(struct dvb_device *dvbdev)
+{
+	if (dvbdev)
+		kref_put(&dvbdev->ref, dvb_free_device);
+}
 
 
 void dvb_unregister_device(struct dvb_device *dvbdev)
 {
 	dvb_remove_device(dvbdev);
-	dvb_free_device(dvbdev);
+	dvb_device_put(dvbdev);
 }
 EXPORT_SYMBOL(dvb_unregister_device);
 

include/media/dvbdev.h

@@ -160,6 +160,7 @@ struct dvb_adapter {
  */
 struct dvb_device {
 	struct list_head list_head;
+	struct kref ref;
 	const struct file_operations *fops;
 	struct dvb_adapter *adapter;
 	enum dvb_device_type type;
@@ -191,6 +192,20 @@ struct dvb_device {
 	void *priv;
 };
 
+/**
+ * dvb_device_get - Increase dvb_device reference
+ *
+ * @dvbdev:	pointer to struct dvb_device
+ */
+struct dvb_device *dvb_device_get(struct dvb_device *dvbdev);
+
+/**
+ * dvb_device_get - Decrease dvb_device reference
+ *
+ * @dvbdev:	pointer to struct dvb_device
+ */
+void dvb_device_put(struct dvb_device *dvbdev);
+
 /**
  * dvb_register_adapter - Registers a new DVB adapter
  *
@@ -235,29 +250,17 @@ int dvb_register_device(struct dvb_adapter *adap,
 /**
  * dvb_remove_device - Remove a registered DVB device
  *
- * This does not free memory.  To do that, call dvb_free_device().
+ * This does not free memory. dvb_free_device() will do that when
+ * reference counter is empty
  *
  * @dvbdev:	pointer to struct dvb_device
  */
 void dvb_remove_device(struct dvb_device *dvbdev);
 
-/**
- * dvb_free_device - Free memory occupied by a DVB device.
- *
- * Call dvb_unregister_device() before calling this function.
- *
- * @dvbdev:	pointer to struct dvb_device
- */
-void dvb_free_device(struct dvb_device *dvbdev);
 
 /**
  * dvb_unregister_device - Unregisters a DVB device
  *
- * This is a combination of dvb_remove_device() and dvb_free_device().
- * Using this function is usually a mistake, and is often an indicator
- * for a use-after-free bug (when a userspace process keeps a file
- * handle to a detached device).
- *
  * @dvbdev:	pointer to struct dvb_device
  */
 void dvb_unregister_device(struct dvb_device *dvbdev);