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>

1 files changed, 17 insertions, 14 deletions

diff --git a/include/media/dvbdev.h b/include/media/dvbdev.h
index 6ccff7c6fa6b..fad9871157e2 100644
--- a/include/media/dvbdev.h
+++ b/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;
@@ -192,6 +193,20 @@ struct dvb_device {
 };
 
 /**
+ * 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
  *
  * @adap:	pointer to struct 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);