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_debugfs.c b/net/l2tp/l2tp_debugfs.c
index 98a005d..53bae54 100644
--- a/net/l2tp/l2tp_debugfs.c
+++ b/net/l2tp/l2tp_debugfs.c
@@ -145,7 +145,7 @@
 		   "");
 	seq_printf(m, " %d sessions, refcnt %d/%d\n", session_count,
 		   tunnel->sock ? refcount_read(&tunnel->sock->sk_refcnt) : 0,
-		   atomic_read(&tunnel->ref_count));
+		   refcount_read(&tunnel->ref_count));
 	seq_printf(m, " %08x rx %ld/%ld/%ld rx %ld/%ld/%ld\n",
 		   tunnel->debug,
 		   atomic_long_read(&tunnel->stats.tx_packets),
@@ -170,7 +170,7 @@
 		   "");
 	if (session->send_seq || session->recv_seq)
 		seq_printf(m, "   nr %hu, ns %hu\n", session->nr, session->ns);
-	seq_printf(m, "   refcnt %d\n", atomic_read(&session->ref_count));
+	seq_printf(m, "   refcnt %d\n", refcount_read(&session->ref_count));
 	seq_printf(m, "   config %d/%d/%c/%c/%s/%s %08x %u\n",
 		   session->mtu, session->mru,
 		   session->recv_seq ? 'R' : '-',