net/netfilter/xt_l2tp.c

   1 /* Kernel module to match L2TP header parameters. */
   2
   3 /* (C) 2013      James Chapman <jchapman@katalix.com>
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License version 2 as
   7  * published by the Free Software Foundation.
   8  */
   9
  10 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  11 #include <linux/module.h>
  12 #include <linux/skbuff.h>
  13 #include <linux/if_ether.h>
  14 #include <net/ip.h>
  15 #include <linux/ipv6.h>
  16 #include <net/ipv6.h>
  17 #include <net/udp.h>
  18 #include <linux/l2tp.h>
  19
  20 #include <linux/netfilter_ipv4.h>
  21 #include <linux/netfilter_ipv6.h>
  22 #include <linux/netfilter_ipv4/ip_tables.h>
  23 #include <linux/netfilter_ipv6/ip6_tables.h>
  24 #include <linux/netfilter/x_tables.h>
  25 #include <linux/netfilter/xt_tcpudp.h>
  26 #include <linux/netfilter/xt_l2tp.h>
  27
  28 /* L2TP header masks */
  29 #define L2TP_HDR_T_BIT  0x8000
  30 #define L2TP_HDR_L_BIT  0x4000
  31 #define L2TP_HDR_VER    0x000f
  32
  33 MODULE_LICENSE("GPL");
  34 MODULE_AUTHOR("James Chapman <jchapman@katalix.com>");
  35 MODULE_DESCRIPTION("Xtables: L2TP header match");
  36 MODULE_ALIAS("ipt_l2tp");
  37 MODULE_ALIAS("ip6t_l2tp");
  38
  39 /* The L2TP fields that can be matched */
  40 struct l2tp_data {
  41         u32 tid;
  42         u32 sid;
  43         u8 type;
  44         u8 version;
  45 };
  46
  47 union l2tp_val {
  48         __be16 val16[2];
  49         __be32 val32;
  50 };
  51
  52 static bool l2tp_match(const struct xt_l2tp_info *info, struct l2tp_data *data)
  53 {
  54         if ((info->flags & XT_L2TP_TYPE) && (info->type != data->type))
  55                 return false;
  56
  57         if ((info->flags & XT_L2TP_VERSION) && (info->version != data->version))
  58                 return false;
  59
  60         /* Check tid only for L2TPv3 control or any L2TPv2 packets */
  61         if ((info->flags & XT_L2TP_TID) &&
  62             ((data->type == XT_L2TP_TYPE_CONTROL) || (data->version == 2)) &&
  63             (info->tid != data->tid))
  64                 return false;
  65
  66         /* Check sid only for L2TP data packets */
  67         if ((info->flags & XT_L2TP_SID) && (data->type == XT_L2TP_TYPE_DATA) &&
  68             (info->sid != data->sid))
  69                 return false;
  70
  71         return true;
  72 }
  73
  74 /* Parse L2TP header fields when UDP encapsulation is used. Handles
  75  * L2TPv2 and L2TPv3. Note the L2TPv3 control and data packets have a
  76  * different format. See
  77  * RFC2661, Section 3.1, L2TPv2 Header Format
  78  * RFC3931, Section 3.2.1, L2TPv3 Control Message Header
  79  * RFC3931, Section 3.2.2, L2TPv3 Data Message Header
  80  * RFC3931, Section 4.1.2.1, L2TPv3 Session Header over UDP
  81  */
  82 static bool l2tp_udp_mt(const struct sk_buff *skb, struct xt_action_param *par, u16 thoff)
  83 {
  84         const struct xt_l2tp_info *info = par->matchinfo;
  85         int uhlen = sizeof(struct udphdr);
  86         int offs = thoff + uhlen;
  87         union l2tp_val *lh;
  88         union l2tp_val lhbuf;
  89         u16 flags;
  90         struct l2tp_data data = { 0, };
  91
  92         if (par->fragoff != 0)
  93                 return false;
  94
  95         /* Extract L2TP header fields. The flags in the first 16 bits
  96          * tell us where the other fields are.
  97          */
  98         lh = skb_header_pointer(skb, offs, 2, &lhbuf);
  99         if (lh == NULL)
 100                 return false;
 101
 102         flags = ntohs(lh->val16[0]);
 103         if (flags & L2TP_HDR_T_BIT)
 104                 data.type = XT_L2TP_TYPE_CONTROL;
 105         else
 106                 data.type = XT_L2TP_TYPE_DATA;
 107         data.version = (u8) flags & L2TP_HDR_VER;
 108
 109         /* Now extract the L2TP tid/sid. These are in different places
 110          * for L2TPv2 (rfc2661) and L2TPv3 (rfc3931). For L2TPv2, we
 111          * must also check to see if the length field is present,
 112          * since this affects the offsets into the packet of the
 113          * tid/sid fields.
 114          */
 115         if (data.version == 3) {
 116                 lh = skb_header_pointer(skb, offs + 4, 4, &lhbuf);
 117                 if (lh == NULL)
 118                         return false;
 119                 if (data.type == XT_L2TP_TYPE_CONTROL)
 120                         data.tid = ntohl(lh->val32);
 121                 else
 122                         data.sid = ntohl(lh->val32);
 123         } else if (data.version == 2) {
 124                 if (flags & L2TP_HDR_L_BIT)
 125                         offs += 2;
 126                 lh = skb_header_pointer(skb, offs + 2, 4, &lhbuf);
 127                 if (lh == NULL)
 128                         return false;
 129                 data.tid = (u32) ntohs(lh->val16[0]);
 130                 data.sid = (u32) ntohs(lh->val16[1]);
 131         } else
 132                 return false;
 133
 134         return l2tp_match(info, &data);
 135 }
 136
 137 /* Parse L2TP header fields for IP encapsulation (no UDP header).
 138  * L2TPv3 data packets have a different form with IP encap. See
 139  * RC3931, Section 4.1.1.1, L2TPv3 Session Header over IP.
 140  * RC3931, Section 4.1.1.2, L2TPv3 Control and Data Traffic over IP.
 141  */
 142 static bool l2tp_ip_mt(const struct sk_buff *skb, struct xt_action_param *par, u16 thoff)
 143 {
 144         const struct xt_l2tp_info *info = par->matchinfo;
 145         union l2tp_val *lh;
 146         union l2tp_val lhbuf;
 147         struct l2tp_data data = { 0, };
 148
 149         /* For IP encap, the L2TP sid is the first 32-bits. */
 150         lh = skb_header_pointer(skb, thoff, sizeof(lhbuf), &lhbuf);
 151         if (lh == NULL)
 152                 return false;
 153         if (lh->val32 == 0) {
 154                 /* Must be a control packet. The L2TP tid is further
 155                  * into the packet.
 156                  */
 157                 data.type = XT_L2TP_TYPE_CONTROL;
 158                 lh = skb_header_pointer(skb, thoff + 8, sizeof(lhbuf),
 159                                         &lhbuf);
 160                 if (lh == NULL)
 161                         return false;
 162                 data.tid = ntohl(lh->val32);
 163         } else {
 164                 data.sid = ntohl(lh->val32);
 165                 data.type = XT_L2TP_TYPE_DATA;
 166         }
 167
 168         data.version = 3;
 169
 170         return l2tp_match(info, &data);
 171 }
 172
 173 static bool l2tp_mt4(const struct sk_buff *skb, struct xt_action_param *par)
 174 {
 175         struct iphdr *iph = ip_hdr(skb);
 176         u8 ipproto = iph->protocol;
 177
 178         /* l2tp_mt_check4 already restricts the transport protocol */
 179         switch (ipproto) {
 180         case IPPROTO_UDP:
 181                 return l2tp_udp_mt(skb, par, par->thoff);
 182         case IPPROTO_L2TP:
 183                 return l2tp_ip_mt(skb, par, par->thoff);
 184         }
 185
 186         return false;
 187 }
 188
 189 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 190 static bool l2tp_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 191 {
 192         unsigned int thoff = 0;
 193         unsigned short fragoff = 0;
 194         int ipproto;
 195
 196         ipproto = ipv6_find_hdr(skb, &thoff, -1, &fragoff, NULL);
 197         if (fragoff != 0)
 198                 return false;
 199
 200         /* l2tp_mt_check6 already restricts the transport protocol */
 201         switch (ipproto) {
 202         case IPPROTO_UDP:
 203                 return l2tp_udp_mt(skb, par, thoff);
 204         case IPPROTO_L2TP:
 205                 return l2tp_ip_mt(skb, par, thoff);
 206         }
 207
 208         return false;
 209 }
 210 #endif
 211
 212 static int l2tp_mt_check(const struct xt_mtchk_param *par)
 213 {
 214         const struct xt_l2tp_info *info = par->matchinfo;
 215
 216         /* Check for invalid flags */
 217         if (info->flags & ~(XT_L2TP_TID | XT_L2TP_SID | XT_L2TP_VERSION |
 218                             XT_L2TP_TYPE)) {
 219                 pr_info_ratelimited("unknown flags: %x\n", info->flags);
 220                 return -EINVAL;
 221         }
 222
 223         /* At least one of tid, sid or type=control must be specified */
 224         if ((!(info->flags & XT_L2TP_TID)) &&
 225             (!(info->flags & XT_L2TP_SID)) &&
 226             ((!(info->flags & XT_L2TP_TYPE)) ||
 227              (info->type != XT_L2TP_TYPE_CONTROL))) {
 228                 pr_info_ratelimited("invalid flags combination: %x\n",
 229                                     info->flags);
 230                 return -EINVAL;
 231         }
 232
 233         /* If version 2 is specified, check that incompatible params
 234          * are not supplied
 235          */
 236         if (info->flags & XT_L2TP_VERSION) {
 237                 if ((info->version < 2) || (info->version > 3)) {
 238                         pr_info_ratelimited("wrong L2TP version: %u\n",
 239                                             info->version);
 240                         return -EINVAL;
 241                 }
 242
 243                 if (info->version == 2) {
 244                         if ((info->flags & XT_L2TP_TID) &&
 245                             (info->tid > 0xffff)) {
 246                                 pr_info_ratelimited("v2 tid > 0xffff: %u\n",
 247                                                     info->tid);
 248                                 return -EINVAL;
 249                         }
 250                         if ((info->flags & XT_L2TP_SID) &&
 251                             (info->sid > 0xffff)) {
 252                                 pr_info_ratelimited("v2 sid > 0xffff: %u\n",
 253                                                     info->sid);
 254                                 return -EINVAL;
 255                         }
 256                 }
 257         }
 258
 259         return 0;
 260 }
 261
 262 static int l2tp_mt_check4(const struct xt_mtchk_param *par)
 263 {
 264         const struct xt_l2tp_info *info = par->matchinfo;
 265         const struct ipt_entry *e = par->entryinfo;
 266         const struct ipt_ip *ip = &e->ip;
 267         int ret;
 268
 269         ret = l2tp_mt_check(par);
 270         if (ret != 0)
 271                 return ret;
 272
 273         if ((ip->proto != IPPROTO_UDP) &&
 274             (ip->proto != IPPROTO_L2TP)) {
 275                 pr_info_ratelimited("missing protocol rule (udp|l2tpip)\n");
 276                 return -EINVAL;
 277         }
 278
 279         if ((ip->proto == IPPROTO_L2TP) &&
 280             (info->version == 2)) {
 281                 pr_info_ratelimited("v2 doesn't support IP mode\n");
 282                 return -EINVAL;
 283         }
 284
 285         return 0;
 286 }
 287
 288 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 289 static int l2tp_mt_check6(const struct xt_mtchk_param *par)
 290 {
 291         const struct xt_l2tp_info *info = par->matchinfo;
 292         const struct ip6t_entry *e = par->entryinfo;
 293         const struct ip6t_ip6 *ip = &e->ipv6;
 294         int ret;
 295
 296         ret = l2tp_mt_check(par);
 297         if (ret != 0)
 298                 return ret;
 299
 300         if ((ip->proto != IPPROTO_UDP) &&
 301             (ip->proto != IPPROTO_L2TP)) {
 302                 pr_info_ratelimited("missing protocol rule (udp|l2tpip)\n");
 303                 return -EINVAL;
 304         }
 305
 306         if ((ip->proto == IPPROTO_L2TP) &&
 307             (info->version == 2)) {
 308                 pr_info_ratelimited("v2 doesn't support IP mode\n");
 309                 return -EINVAL;
 310         }
 311
 312         return 0;
 313 }
 314 #endif
 315
 316 static struct xt_match l2tp_mt_reg[] __read_mostly = {
 317         {
 318                 .name      = "l2tp",
 319                 .revision  = 0,
 320                 .family    = NFPROTO_IPV4,
 321                 .match     = l2tp_mt4,
 322                 .matchsize = XT_ALIGN(sizeof(struct xt_l2tp_info)),
 323                 .checkentry = l2tp_mt_check4,
 324                 .hooks     = ((1 << NF_INET_PRE_ROUTING) |
 325                               (1 << NF_INET_LOCAL_IN) |
 326                               (1 << NF_INET_LOCAL_OUT) |
 327                               (1 << NF_INET_FORWARD)),
 328                 .me        = THIS_MODULE,
 329         },
 330 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 331         {
 332                 .name      = "l2tp",
 333                 .revision  = 0,
 334                 .family    = NFPROTO_IPV6,
 335                 .match     = l2tp_mt6,
 336                 .matchsize = XT_ALIGN(sizeof(struct xt_l2tp_info)),
 337                 .checkentry = l2tp_mt_check6,
 338                 .hooks     = ((1 << NF_INET_PRE_ROUTING) |
 339                               (1 << NF_INET_LOCAL_IN) |
 340                               (1 << NF_INET_LOCAL_OUT) |
 341                               (1 << NF_INET_FORWARD)),
 342                 .me        = THIS_MODULE,
 343         },
 344 #endif
 345 };
 346
 347 static int __init l2tp_mt_init(void)
 348 {
 349         return xt_register_matches(&l2tp_mt_reg[0], ARRAY_SIZE(l2tp_mt_reg));
 350 }
 351
 352 static void __exit l2tp_mt_exit(void)
 353 {
 354         xt_unregister_matches(&l2tp_mt_reg[0], ARRAY_SIZE(l2tp_mt_reg));
 355 }
 356
 357 module_init(l2tp_mt_init);
 358 module_exit(l2tp_mt_exit);