l2tp: initialise session's refcount before making it reachable

commit 9ee369a405c57613d7c83a3967780c3e30c52ecc upstream.

Sessions must be fully initialised before calling
l2tp_session_add_to_tunnel(). Otherwise, there's a short time frame
where partially initialised sessions can be accessed by external users.

Backporting Notes

l2tp_core.c: moving code that had been converted from atomic to
refcount_t by an earlier change (which isn't being included in this
patch series).

Fixes: dbdbc73b4478 ("l2tp: fix duplicate session creation")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index a9d4d42..7f72957 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1847,6 +1847,8 @@
 		l2tp_session_set_header_len(session, tunnel->version);
+		l2tp_session_inc_refcount(session);
 		err = l2tp_session_add_to_tunnel(tunnel, session);
 		if (err) {
@@ -1854,10 +1856,6 @@
 			return ERR_PTR(err);
-		/* Bump the reference count. The session context is deleted
-		 * only when this drops to zero.
-		 */
-		l2tp_session_inc_refcount(session);
 		/* Ensure tunnel socket isn't deleted */