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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, "David S. Miller" <davem@davemloft.net>,
        "Guillaume Nault" <g.nault@alphalink.fr>
Date:   Sun, 11 Nov 2018 19:49:05 +0000
Message-ID: <lsq.1541965745.367561099@decadent.org.uk>
Subject: [PATCH 3.16 143/366] l2tp: reject creation of non-PPP sessions on
 L2TPv2 tunnels
In-Reply-To: <lsq.1541965744.387173642@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Guillaume Nault <g.nault@alphalink.fr>

commit de9bada5d389903f4faf33980e6a95a2911c7e6d upstream.

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: Ben Hutchings <ben@decadent.org.uk>
---
 net/l2tp/l2tp_netlink.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -460,6 +460,12 @@ static int l2tp_nl_cmd_session_create(st
 		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]);