l2tp: double-check l2specific_type provided by userspace

Add sanity check on l2specific_type provided by userspace in
l2tp_nl_cmd_session_create() since just L2TP_L2SPECTYPE_DEFAULT and
L2TP_L2SPECTYPE_NONE are currently supported.
Moreover explicitly set l2specific_type to L2TP_L2SPECTYPE_DEFAULT
only if the userspace does not provide a value for it

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_netlink.c

@@ -550,9 +550,16 @@ static int l2tp_nl_cmd_session_create(struct sk_buff *skb, struct genl_info *inf
 		if (info->attrs[L2TP_ATTR_DATA_SEQ])
 			cfg.data_seq = nla_get_u8(info->attrs[L2TP_ATTR_DATA_SEQ]);
 
-		cfg.l2specific_type = L2TP_L2SPECTYPE_DEFAULT;
-		if (info->attrs[L2TP_ATTR_L2SPEC_TYPE])
+		if (info->attrs[L2TP_ATTR_L2SPEC_TYPE]) {
 			cfg.l2specific_type = nla_get_u8(info->attrs[L2TP_ATTR_L2SPEC_TYPE]);
+			if (cfg.l2specific_type != L2TP_L2SPECTYPE_DEFAULT &&
+			    cfg.l2specific_type != L2TP_L2SPECTYPE_NONE) {
+				ret = -EINVAL;
+				goto out_tunnel;
+			}
+		} else {
+			cfg.l2specific_type = L2TP_L2SPECTYPE_DEFAULT;
+		}
 
 		cfg.l2specific_len = 4;
 		if (info->attrs[L2TP_ATTR_L2SPEC_LEN])