--- /dev/null
+/* Kernel module to match L2TP header parameters. */
+
+/* (C) 2013 James Chapman <jchapman@katalix.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/if_ether.h>
+#include <net/ip.h>
+#include <linux/ipv6.h>
+#include <net/ipv6.h>
+#include <net/udp.h>
+#include <linux/l2tp.h>
+
+#include <linux/netfilter_ipv4.h>
+#include <linux/netfilter_ipv6.h>
+#include <linux/netfilter_ipv4/ip_tables.h>
+#include <linux/netfilter_ipv6/ip6_tables.h>
+#include <linux/netfilter/x_tables.h>
+#include <linux/netfilter/xt_tcpudp.h>
+#include <linux/netfilter/xt_l2tp.h>
+
+/* L2TP header masks */
+#define L2TP_HDR_T_BIT 0x8000
+#define L2TP_HDR_L_BIT 0x4000
+#define L2TP_HDR_VER 0x000f
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("James Chapman <jchapman@katalix.com>");
+MODULE_DESCRIPTION("Xtables: L2TP header match");
+MODULE_ALIAS("ipt_l2tp");
+MODULE_ALIAS("ip6t_l2tp");
+
+/* The L2TP fields that can be matched */
+struct l2tp_data {
+ u32 tid;
+ u32 sid;
+ u8 type;
+ u8 version;
+};
+
+union l2tp_val {
+ __be16 val16[2];
+ __be32 val32;
+};
+
+static bool l2tp_match(const struct xt_l2tp_info *info, struct l2tp_data *data)
+{
+ if ((info->flags & XT_L2TP_TYPE) && (info->type != data->type))
+ return false;
+
+ if ((info->flags & XT_L2TP_VERSION) && (info->version != data->version))
+ return false;
+
+ /* Check tid only for L2TPv3 control or any L2TPv2 packets */
+ if ((info->flags & XT_L2TP_TID) &&
+ ((data->type == XT_L2TP_TYPE_CONTROL) || (data->version == 2)) &&
+ (info->tid != data->tid))
+ return false;
+
+ /* Check sid only for L2TP data packets */
+ if ((info->flags & XT_L2TP_SID) && (data->type == XT_L2TP_TYPE_DATA) &&
+ (info->sid != data->sid))
+ return false;
+
+ return true;
+}
+
+/* Parse L2TP header fields when UDP encapsulation is used. Handles
+ * L2TPv2 and L2TPv3. Note the L2TPv3 control and data packets have a
+ * different format. See
+ * RFC2661, Section 3.1, L2TPv2 Header Format
+ * RFC3931, Section 3.2.1, L2TPv3 Control Message Header
+ * RFC3931, Section 3.2.2, L2TPv3 Data Message Header
+ * RFC3931, Section 4.1.2.1, L2TPv3 Session Header over UDP
+ */
+static bool l2tp_udp_mt(const struct sk_buff *skb, struct xt_action_param *par, u16 thoff)
+{
+ const struct xt_l2tp_info *info = par->matchinfo;
+ int uhlen = sizeof(struct udphdr);
+ int offs = thoff + uhlen;
+ union l2tp_val *lh;
+ union l2tp_val lhbuf;
+ u16 flags;
+ struct l2tp_data data = { 0, };
+
+ if (par->fragoff != 0)
+ return false;
+
+ /* Extract L2TP header fields. The flags in the first 16 bits
+ * tell us where the other fields are.
+ */
+ lh = skb_header_pointer(skb, offs, 2, &lhbuf);
+ if (lh == NULL)
+ return false;
+
+ flags = ntohs(lh->val16[0]);
+ if (flags & L2TP_HDR_T_BIT)
+ data.type = XT_L2TP_TYPE_CONTROL;
+ else
+ data.type = XT_L2TP_TYPE_DATA;
+ data.version = (u8) flags & L2TP_HDR_VER;
+
+ /* Now extract the L2TP tid/sid. These are in different places
+ * for L2TPv2 (rfc2661) and L2TPv3 (rfc3931). For L2TPv2, we
+ * must also check to see if the length field is present,
+ * since this affects the offsets into the packet of the
+ * tid/sid fields.
+ */
+ if (data.version == 3) {
+ lh = skb_header_pointer(skb, offs + 4, 4, &lhbuf);
+ if (lh == NULL)
+ return false;
+ if (data.type == XT_L2TP_TYPE_CONTROL)
+ data.tid = ntohl(lh->val32);
+ else
+ data.sid = ntohl(lh->val32);
+ } else if (data.version == 2) {
+ if (flags & L2TP_HDR_L_BIT)
+ offs += 2;
+ lh = skb_header_pointer(skb, offs + 2, 4, &lhbuf);
+ if (lh == NULL)
+ return false;
+ data.tid = (u32) ntohs(lh->val16[0]);
+ data.sid = (u32) ntohs(lh->val16[1]);
+ } else
+ return false;
+
+ return l2tp_match(info, &data);
+}
+
+/* Parse L2TP header fields for IP encapsulation (no UDP header).
+ * L2TPv3 data packets have a different form with IP encap. See
+ * RC3931, Section 4.1.1.1, L2TPv3 Session Header over IP.
+ * RC3931, Section 4.1.1.2, L2TPv3 Control and Data Traffic over IP.
+ */
+static bool l2tp_ip_mt(const struct sk_buff *skb, struct xt_action_param *par, u16 thoff)
+{
+ const struct xt_l2tp_info *info = par->matchinfo;
+ union l2tp_val *lh;
+ union l2tp_val lhbuf;
+ struct l2tp_data data = { 0, };
+
+ /* For IP encap, the L2TP sid is the first 32-bits. */
+ lh = skb_header_pointer(skb, thoff, sizeof(lhbuf), &lhbuf);
+ if (lh == NULL)
+ return false;
+ if (lh->val32 == 0) {
+ /* Must be a control packet. The L2TP tid is further
+ * into the packet.
+ */
+ data.type = XT_L2TP_TYPE_CONTROL;
+ lh = skb_header_pointer(skb, thoff + 8, sizeof(lhbuf),
+ &lhbuf);
+ if (lh == NULL)
+ return false;
+ data.tid = ntohl(lh->val32);
+ } else {
+ data.sid = ntohl(lh->val32);
+ data.type = XT_L2TP_TYPE_DATA;
+ }
+
+ data.version = 3;
+
+ return l2tp_match(info, &data);
+}
+
+static bool l2tp_mt4(const struct sk_buff *skb, struct xt_action_param *par)
+{
+ struct iphdr *iph = ip_hdr(skb);
+ u8 ipproto = iph->protocol;
+
+ /* l2tp_mt_check4 already restricts the transport protocol */
+ switch (ipproto) {
+ case IPPROTO_UDP:
+ return l2tp_udp_mt(skb, par, par->thoff);
+ case IPPROTO_L2TP:
+ return l2tp_ip_mt(skb, par, par->thoff);
+ }
+
+ return false;
+}
+
+#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
+static bool l2tp_mt6(const struct sk_buff *skb, struct xt_action_param *par)
+{
+ unsigned int thoff = 0;
+ unsigned short fragoff = 0;
+ int ipproto;
+
+ ipproto = ipv6_find_hdr(skb, &thoff, -1, &fragoff, NULL);
+ if (fragoff != 0)
+ return false;
+
+ /* l2tp_mt_check6 already restricts the transport protocol */
+ switch (ipproto) {
+ case IPPROTO_UDP:
+ return l2tp_udp_mt(skb, par, thoff);
+ case IPPROTO_L2TP:
+ return l2tp_ip_mt(skb, par, thoff);
+ }
+
+ return false;
+}
+#endif
+
+static int l2tp_mt_check(const struct xt_mtchk_param *par)
+{
+ const struct xt_l2tp_info *info = par->matchinfo;
+
+ /* Check for invalid flags */
+ if (info->flags & ~(XT_L2TP_TID | XT_L2TP_SID | XT_L2TP_VERSION |
+ XT_L2TP_TYPE)) {
+ pr_info("unknown flags: %x\n", info->flags);
+ return -EINVAL;
+ }
+
+ /* At least one of tid, sid or type=control must be specified */
+ if ((!(info->flags & XT_L2TP_TID)) &&
+ (!(info->flags & XT_L2TP_SID)) &&
+ ((!(info->flags & XT_L2TP_TYPE)) ||
+ (info->type != XT_L2TP_TYPE_CONTROL))) {
+ pr_info("invalid flags combination: %x\n", info->flags);
+ return -EINVAL;
+ }
+
+ /* If version 2 is specified, check that incompatible params
+ * are not supplied
+ */
+ if (info->flags & XT_L2TP_VERSION) {
+ if ((info->version < 2) || (info->version > 3)) {
+ pr_info("wrong L2TP version: %u\n", info->version);
+ return -EINVAL;
+ }
+
+ if (info->version == 2) {
+ if ((info->flags & XT_L2TP_TID) &&
+ (info->tid > 0xffff)) {
+ pr_info("v2 tid > 0xffff: %u\n", info->tid);
+ return -EINVAL;
+ }
+ if ((info->flags & XT_L2TP_SID) &&
+ (info->sid > 0xffff)) {
+ pr_info("v2 sid > 0xffff: %u\n", info->sid);
+ return -EINVAL;
+ }
+ }
+ }
+
+ return 0;
+}
+
+static int l2tp_mt_check4(const struct xt_mtchk_param *par)
+{
+ const struct xt_l2tp_info *info = par->matchinfo;
+ const struct ipt_entry *e = par->entryinfo;
+ const struct ipt_ip *ip = &e->ip;
+ int ret;
+
+ ret = l2tp_mt_check(par);
+ if (ret != 0)
+ return ret;
+
+ if ((ip->proto != IPPROTO_UDP) &&
+ (ip->proto != IPPROTO_L2TP)) {
+ pr_info("missing protocol rule (udp|l2tpip)\n");
+ return -EINVAL;
+ }
+
+ if ((ip->proto == IPPROTO_L2TP) &&
+ (info->version == 2)) {
+ pr_info("v2 doesn't support IP mode\n");
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
+#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
+static int l2tp_mt_check6(const struct xt_mtchk_param *par)
+{
+ const struct xt_l2tp_info *info = par->matchinfo;
+ const struct ip6t_entry *e = par->entryinfo;
+ const struct ip6t_ip6 *ip = &e->ipv6;
+ int ret;
+
+ ret = l2tp_mt_check(par);
+ if (ret != 0)
+ return ret;
+
+ if ((ip->proto != IPPROTO_UDP) &&
+ (ip->proto != IPPROTO_L2TP)) {
+ pr_info("missing protocol rule (udp|l2tpip)\n");
+ return -EINVAL;
+ }
+
+ if ((ip->proto == IPPROTO_L2TP) &&
+ (info->version == 2)) {
+ pr_info("v2 doesn't support IP mode\n");
+ return -EINVAL;
+ }
+
+ return 0;
+}
+#endif
+
+static struct xt_match l2tp_mt_reg[] __read_mostly = {
+ {
+ .name = "l2tp",
+ .revision = 0,
+ .family = NFPROTO_IPV4,
+ .match = l2tp_mt4,
+ .matchsize = XT_ALIGN(sizeof(struct xt_l2tp_info)),
+ .checkentry = l2tp_mt_check4,
+ .hooks = ((1 << NF_INET_PRE_ROUTING) |
+ (1 << NF_INET_LOCAL_IN) |
+ (1 << NF_INET_LOCAL_OUT) |
+ (1 << NF_INET_FORWARD)),
+ .me = THIS_MODULE,
+ },
+#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
+ {
+ .name = "l2tp",
+ .revision = 0,
+ .family = NFPROTO_IPV6,
+ .match = l2tp_mt6,
+ .matchsize = XT_ALIGN(sizeof(struct xt_l2tp_info)),
+ .checkentry = l2tp_mt_check6,
+ .hooks = ((1 << NF_INET_PRE_ROUTING) |
+ (1 << NF_INET_LOCAL_IN) |
+ (1 << NF_INET_LOCAL_OUT) |
+ (1 << NF_INET_FORWARD)),
+ .me = THIS_MODULE,
+ },
+#endif
+};
+
+static int __init l2tp_mt_init(void)
+{
+ return xt_register_matches(&l2tp_mt_reg[0], ARRAY_SIZE(l2tp_mt_reg));
+}
+
+static void __exit l2tp_mt_exit(void)
+{
+ xt_unregister_matches(&l2tp_mt_reg[0], ARRAY_SIZE(l2tp_mt_reg));
+}
+
+module_init(l2tp_mt_init);
+module_exit(l2tp_mt_exit);
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