l2tp: remove l2specific_len dependency in l2tp_core

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>

net/l2tp/l2tp_core.c

@@ -730,11 +730,9 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
 				 "%s: recv data ns=%u, session nr=%u\n",
 				 session->name, ns, session->nr);
 		}
+		ptr += 4;
 	}
 
-	/* Advance past L2-specific header, if present */
-	ptr += session->l2specific_len;
-
 	if (L2TP_SKB_CB(skb)->has_seq) {
 		/* Received a packet with sequence numbers. If we're the LNS,
 		 * check if we sre sending sequence numbers and if not,
@@ -1048,21 +1046,20 @@ static int l2tp_build_l2tpv3_header(struct l2tp_session *session, void *buf)
 		memcpy(bufp, &session->cookie[0], session->cookie_len);
 		bufp += session->cookie_len;
 	}
-	if (session->l2specific_len) {
-		if (session->l2specific_type == L2TP_L2SPECTYPE_DEFAULT) {
-			u32 l2h = 0;
-			if (session->send_seq) {
-				l2h = 0x40000000 | session->ns;
-				session->ns++;
-				session->ns &= 0xffffff;
-				l2tp_dbg(session, L2TP_MSG_SEQ,
-					 "%s: updated ns to %u\n",
-					 session->name, session->ns);
-			}
+	if (session->l2specific_type == L2TP_L2SPECTYPE_DEFAULT) {
+		u32 l2h = 0;
 
-			*((__be32 *) bufp) = htonl(l2h);
+		if (session->send_seq) {
+			l2h = 0x40000000 | session->ns;
+			session->ns++;
+			session->ns &= 0xffffff;
+			l2tp_dbg(session, L2TP_MSG_SEQ,
+				 "%s: updated ns to %u\n",
+				 session->name, session->ns);
 		}
-		bufp += session->l2specific_len;
+
+		*((__be32 *)bufp) = htonl(l2h);
+		bufp += 4;
 	}
 
 	return bufp - optr;
@@ -1719,7 +1716,7 @@ int l2tp_session_delete(struct l2tp_session *session)
 EXPORT_SYMBOL_GPL(l2tp_session_delete);
 
 /* We come here whenever a session's send_seq, cookie_len or
- * l2specific_len parameters are set.
+ * l2specific_type parameters are set.
  */
 void l2tp_session_set_header_len(struct l2tp_session *session, int version)
 {
@@ -1728,7 +1725,8 @@ void l2tp_session_set_header_len(struct l2tp_session *session, int version)
 		if (session->send_seq)
 			session->hdr_len += 4;
 	} else {
-		session->hdr_len = 4 + session->cookie_len + session->l2specific_len;
+		session->hdr_len = 4 + session->cookie_len;
+		session->hdr_len += l2tp_get_l2specific_len(session);
 		if (session->tunnel->encap == L2TP_ENCAPTYPE_UDP)
 			session->hdr_len += 4;
 	}

net/l2tp/l2tp_core.h

@@ -302,6 +302,17 @@ static inline void l2tp_session_dec_refcount(struct l2tp_session *session)
 		l2tp_session_free(session);
 }
 
+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))					\