[PATCH net 1/2] l2tp: reject creation of non-PPP sessions on L2TPv2 tunnels

[Guillaume Nault]

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.na...@alphalink.fr>
---
 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 6616c9fd292f..5b9900889e31 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -553,6 +553,12 @@ 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_DATA_SEQ])
                        cfg.data_seq = 
nla_get_u8(info->attrs[L2TP_ATTR_DATA_SEQ]);
-- 
2.17.1