From 5dd691d13dc4e921aedbf5b53267d950d741b2c5 Mon Sep 17 00:00:00 2001
From: Guillaume Nault <g.nault@alphalink.fr>
Date: Fri, 15 Jun 2018 15:39:17 +0200
Subject: [PATCH 4/4] l2tp: reject creation of non-PPP sessions on L2TPv2
 tunnels
Git-commit: de9bada5d389903f4faf33980e6a95a2911c7e6d
Patch-mainline: v4.18-rc1
References: git-fixes

The /proc/net/pppol2tp handlers (pppol2tp_seq_*()) iterate over all
L2TPv2 tunnels, and rightfully expect that only PPP sessions can be
found there. However, l2tp_netlink accepts creating Ethernet sessions
regardless of the underlying tunnel version.

This confuses pppol2tp_seq_session_show(), which expects that
l2tp_session_priv() returns a pppol2tp_session structure. When the
session is an Ethernet pseudo-wire, a struct l2tp_eth_sess is returned
instead. This leads to invalid memory access when
pppol2tp_session_get_sock() later tries to dereference ps->sk.

Fixes: d9e31d17ceba ("l2tp: Add L2TP ethernet pseudowire support")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Denis Kirjanov <denis.kirjanov@suse.com>
---
 net/l2tp/l2tp_netlink.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index 2377aa16c3a7..6cd7ffdef633 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -550,6 +550,12 @@ static int l2tp_nl_cmd_session_create(struct sk_buff *skb, struct genl_info *inf
 		goto out_tunnel;
 	}
 
+	/* L2TPv2 only accepts PPP pseudo-wires */
+	if (tunnel->version == 2 && cfg.pw_type != L2TP_PWTYPE_PPP) {
+		ret = -EPROTONOSUPPORT;
+		goto out_tunnel;
+	}
+
 	if (tunnel->version > 2) {
 		if (info->attrs[L2TP_ATTR_OFFSET])
 			cfg.offset = nla_get_u16(info->attrs[L2TP_ATTR_OFFSET]);
-- 
2.16.4