l2tp: remove l2specific_len dependency in l2tp_core

commit 62e7b6a57c7b9bf3c6fd99418eeec05b08a85c38 upstream.

Remove l2specific_len dependency while building l2tpv3 header or
parsing the received frame since default L2-Specific Sublayer is
always four bytes long and we don't need to rely on a user supplied
value.
Moreover in l2tp netlink code there are no sanity checks to
enforce the relation between l2specific_len and l2specific_type,
so sending a malformed netlink message is possible to set
l2specific_type to L2TP_L2SPECTYPE_DEFAULT (or even
L2TP_L2SPECTYPE_NONE) and set l2specific_len to a value greater than
4 leaking memory on the wire and sending corrupted frames.

Reviewed-by: Guillaume Nault <g.nault@alphalink.fr>
Tested-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 86356a2..a78a17f 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -314,6 +314,17 @@
 #define l2tp_session_dec_refcount(s) l2tp_session_dec_refcount_1(s)
 #endif
 
+static inline int l2tp_get_l2specific_len(struct l2tp_session *session)
+{
+	switch (session->l2specific_type) {
+	case L2TP_L2SPECTYPE_DEFAULT:
+		return 4;
+	case L2TP_L2SPECTYPE_NONE:
+	default:
+		return 0;
+	}
+}
+
 #define l2tp_printk(ptr, type, func, fmt, ...)				\
 do {									\
 	if (((ptr)->debug) & (type))					\