[PATCH net 2/4] l2tp: only accept PPP sessions in pppol2tp_connect()

[Guillaume Nault]

l2tp_session_priv() returns a struct pppol2tp_session pointer only for
PPPoL2TP sessions. In particular, if the session is an L2TP_PWTYPE_ETH
pseudo-wire, l2tp_session_priv() returns a pointer to an l2tp_eth_sess
structure, which is much smaller than struct pppol2tp_session. This
leads to invalid memory dereference when trying to lock ps->sk_lock.

Fixes: d9e31d17ceba ("l2tp: Add L2TP ethernet pseudowire support")
Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
---
 net/l2tp/l2tp_ppp.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 270a0a999eaf..8b3b6947a07d 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -734,6 +734,12 @@ static int pppol2tp_connect(struct socket *sock, struct 
sockaddr *uservaddr,
        session = l2tp_session_get(sock_net(sk), tunnel, session_id);
        if (session) {
                drop_refcnt = true;
+
+               if (session->pwtype != L2TP_PWTYPE_PPP) {
+                       error = -EPROTOTYPE;
+                       goto end;
+               }
+
                ps = l2tp_session_priv(session);
 
                /* Using a pre-existing session is fine as long as it hasn't
-- 
2.17.1