drivers, connector: convert cn_callback_entry.refcnt from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable cn_callback_entry.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/connector.h b/include/linux/connector.h
index f8fe8637d771..032102b19645 100644
--- a/include/linux/connector.h
+++ b/include/linux/connector.h
@@ -22,7 +22,7 @@
 #define __CONNECTOR_H
 
 
-#include <linux/atomic.h>
+#include <linux/refcount.h>
 
 #include <linux/list.h>
 #include <linux/workqueue.h>
@@ -49,7 +49,7 @@ struct cn_callback_id {
 
 struct cn_callback_entry {
 	struct list_head callback_entry;
-	atomic_t refcnt;
+	refcount_t refcnt;
 	struct cn_queue_dev *pdev;
 
 	struct cn_callback_id id;