net, l2tp: convert l2tp_tunnel.ref_count from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index eec5ad2..da58fad 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -7,6 +7,7 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
+#include <linux/refcount.h>
 
 #ifndef _L2TP_CORE_H_
 #define _L2TP_CORE_H_
@@ -177,7 +178,7 @@ struct l2tp_tunnel {
 	struct list_head	list;		/* Keep a list of all tunnels */
 	struct net		*l2tp_net;	/* the net we belong to */
 
-	atomic_t		ref_count;
+	refcount_t		ref_count;
 #ifdef CONFIG_DEBUG_FS
 	void (*show)(struct seq_file *m, void *arg);
 #endif