net/netfilter/xt_TCPMSS.c

   1 /*
   2  * This is a module which is used for setting the MSS option in TCP packets.
   3  *
   4  * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
   5  * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
   6  *
   7  * This program is free software; you can redistribute it and/or modify
   8  * it under the terms of the GNU General Public License version 2 as
   9  * published by the Free Software Foundation.
  10  */
  11 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  12 #include <linux/module.h>
  13 #include <linux/skbuff.h>
  14 #include <linux/ip.h>
  15 #include <linux/gfp.h>
  16 #include <linux/ipv6.h>
  17 #include <linux/tcp.h>
  18 #include <net/dst.h>
  19 #include <net/flow.h>
  20 #include <net/ipv6.h>
  21 #include <net/route.h>
  22 #include <net/tcp.h>
  23
  24 #include <linux/netfilter_ipv4/ip_tables.h>
  25 #include <linux/netfilter_ipv6/ip6_tables.h>
  26 #include <linux/netfilter/x_tables.h>
  27 #include <linux/netfilter/xt_tcpudp.h>
  28 #include <linux/netfilter/xt_TCPMSS.h>
  29
  30 MODULE_LICENSE("GPL");
  31 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  32 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
  33 MODULE_ALIAS("ipt_TCPMSS");
  34 MODULE_ALIAS("ip6t_TCPMSS");
  35
  36 static inline unsigned int
  37 optlen(const u_int8_t *opt, unsigned int offset)
  38 {
  39         /* Beware zero-length options: make finite progress */
  40         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  41                 return 1;
  42         else
  43                 return opt[offset+1];
  44 }
  45
  46 static u_int32_t tcpmss_reverse_mtu(struct net *net,
  47                                     const struct sk_buff *skb,
  48                                     unsigned int family)
  49 {
  50         struct flowi fl;
  51         const struct nf_afinfo *ai;
  52         struct rtable *rt = NULL;
  53         u_int32_t mtu     = ~0U;
  54
  55         if (family == PF_INET) {
  56                 struct flowi4 *fl4 = &fl.u.ip4;
  57                 memset(fl4, 0, sizeof(*fl4));
  58                 fl4->daddr = ip_hdr(skb)->saddr;
  59         } else {
  60                 struct flowi6 *fl6 = &fl.u.ip6;
  61
  62                 memset(fl6, 0, sizeof(*fl6));
  63                 fl6->daddr = ipv6_hdr(skb)->saddr;
  64         }
  65         rcu_read_lock();
  66         ai = nf_get_afinfo(family);
  67         if (ai != NULL)
  68                 ai->route(net, (struct dst_entry **)&rt, &fl, false);
  69         rcu_read_unlock();
  70
  71         if (rt != NULL) {
  72                 mtu = dst_mtu(&rt->dst);
  73                 dst_release(&rt->dst);
  74         }
  75         return mtu;
  76 }
  77
  78 static int
  79 tcpmss_mangle_packet(struct sk_buff *skb,
  80                      const struct xt_action_param *par,
  81                      unsigned int family,
  82                      unsigned int tcphoff,
  83                      unsigned int minlen)
  84 {
  85         const struct xt_tcpmss_info *info = par->targinfo;
  86         struct tcphdr *tcph;
  87         int len, tcp_hdrlen;
  88         unsigned int i;
  89         __be16 oldval;
  90         u16 newmss;
  91         u8 *opt;
  92
  93         /* This is a fragment, no TCP header is available */
  94         if (par->fragoff != 0)
  95                 return 0;
  96
  97         if (!skb_make_writable(skb, skb->len))
  98                 return -1;
  99
 100         len = skb->len - tcphoff;
 101         if (len < (int)sizeof(struct tcphdr))
 102                 return -1;
 103
 104         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 105         tcp_hdrlen = tcph->doff * 4;
 106
 107         if (len < tcp_hdrlen)
 108                 return -1;
 109
 110         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
 111                 struct net *net = xt_net(par);
 112                 unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
 113                 unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
 114
 115                 if (min_mtu <= minlen) {
 116                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
 117                                             min_mtu);
 118                         return -1;
 119                 }
 120                 newmss = min_mtu - minlen;
 121         } else
 122                 newmss = info->mss;
 123
 124         opt = (u_int8_t *)tcph;
 125         for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
 126                 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
 127                         u_int16_t oldmss;
 128
 129                         oldmss = (opt[i+2] << 8) | opt[i+3];
 130
 131                         /* Never increase MSS, even when setting it, as
 132                          * doing so results in problems for hosts that rely
 133                          * on MSS being set correctly.
 134                          */
 135                         if (oldmss <= newmss)
 136                                 return 0;
 137
 138                         opt[i+2] = (newmss & 0xff00) >> 8;
 139                         opt[i+3] = newmss & 0x00ff;
 140
 141                         inet_proto_csum_replace2(&tcph->check, skb,
 142                                                  htons(oldmss), htons(newmss),
 143                                                  false);
 144                         return 0;
 145                 }
 146         }
 147
 148         /* There is data after the header so the option can't be added
 149          * without moving it, and doing so may make the SYN packet
 150          * itself too large. Accept the packet unmodified instead.
 151          */
 152         if (len > tcp_hdrlen)
 153                 return 0;
 154
 155         /*
 156          * MSS Option not found ?! add it..
 157          */
 158         if (skb_tailroom(skb) < TCPOLEN_MSS) {
 159                 if (pskb_expand_head(skb, 0,
 160                                      TCPOLEN_MSS - skb_tailroom(skb),
 161                                      GFP_ATOMIC))
 162                         return -1;
 163                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 164         }
 165
 166         skb_put(skb, TCPOLEN_MSS);
 167
 168         /*
 169          * IPv4: RFC 1122 states "If an MSS option is not received at
 170          * connection setup, TCP MUST assume a default send MSS of 536".
 171          * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
 172          * length IPv6 header of 60, ergo the default MSS value is 1220
 173          * Since no MSS was provided, we must use the default values
 174          */
 175         if (xt_family(par) == NFPROTO_IPV4)
 176                 newmss = min(newmss, (u16)536);
 177         else
 178                 newmss = min(newmss, (u16)1220);
 179
 180         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 181         memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
 182
 183         inet_proto_csum_replace2(&tcph->check, skb,
 184                                  htons(len), htons(len + TCPOLEN_MSS), true);
 185         opt[0] = TCPOPT_MSS;
 186         opt[1] = TCPOLEN_MSS;
 187         opt[2] = (newmss & 0xff00) >> 8;
 188         opt[3] = newmss & 0x00ff;
 189
 190         inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
 191
 192         oldval = ((__be16 *)tcph)[6];
 193         tcph->doff += TCPOLEN_MSS/4;
 194         inet_proto_csum_replace2(&tcph->check, skb,
 195                                  oldval, ((__be16 *)tcph)[6], false);
 196         return TCPOLEN_MSS;
 197 }
 198
 199 static unsigned int
 200 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
 201 {
 202         struct iphdr *iph = ip_hdr(skb);
 203         __be16 newlen;
 204         int ret;
 205
 206         ret = tcpmss_mangle_packet(skb, par,
 207                                    PF_INET,
 208                                    iph->ihl * 4,
 209                                    sizeof(*iph) + sizeof(struct tcphdr));
 210         if (ret < 0)
 211                 return NF_DROP;
 212         if (ret > 0) {
 213                 iph = ip_hdr(skb);
 214                 newlen = htons(ntohs(iph->tot_len) + ret);
 215                 csum_replace2(&iph->check, iph->tot_len, newlen);
 216                 iph->tot_len = newlen;
 217         }
 218         return XT_CONTINUE;
 219 }
 220
 221 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 222 static unsigned int
 223 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 224 {
 225         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 226         u8 nexthdr;
 227         __be16 frag_off, oldlen, newlen;
 228         int tcphoff;
 229         int ret;
 230
 231         nexthdr = ipv6h->nexthdr;
 232         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 233         if (tcphoff < 0)
 234                 return NF_DROP;
 235         ret = tcpmss_mangle_packet(skb, par,
 236                                    PF_INET6,
 237                                    tcphoff,
 238                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
 239         if (ret < 0)
 240                 return NF_DROP;
 241         if (ret > 0) {
 242                 ipv6h = ipv6_hdr(skb);
 243                 oldlen = ipv6h->payload_len;
 244                 newlen = htons(ntohs(oldlen) + ret);
 245                 if (skb->ip_summed == CHECKSUM_COMPLETE)
 246                         skb->csum = csum_add(csum_sub(skb->csum, oldlen),
 247                                              newlen);
 248                 ipv6h->payload_len = newlen;
 249         }
 250         return XT_CONTINUE;
 251 }
 252 #endif
 253
 254 /* Must specify -p tcp --syn */
 255 static inline bool find_syn_match(const struct xt_entry_match *m)
 256 {
 257         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
 258
 259         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 260             tcpinfo->flg_cmp & TCPHDR_SYN &&
 261             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 262                 return true;
 263
 264         return false;
 265 }
 266
 267 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
 268 {
 269         const struct xt_tcpmss_info *info = par->targinfo;
 270         const struct ipt_entry *e = par->entryinfo;
 271         const struct xt_entry_match *ematch;
 272
 273         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 274             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 275                            (1 << NF_INET_LOCAL_OUT) |
 276                            (1 << NF_INET_POST_ROUTING))) != 0) {
 277                 pr_info("path-MTU clamping only supported in "
 278                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
 279                 return -EINVAL;
 280         }
 281         if (par->nft_compat)
 282                 return 0;
 283
 284         xt_ematch_foreach(ematch, e)
 285                 if (find_syn_match(ematch))
 286                         return 0;
 287         pr_info("Only works on TCP SYN packets\n");
 288         return -EINVAL;
 289 }
 290
 291 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 292 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
 293 {
 294         const struct xt_tcpmss_info *info = par->targinfo;
 295         const struct ip6t_entry *e = par->entryinfo;
 296         const struct xt_entry_match *ematch;
 297
 298         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 299             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 300                            (1 << NF_INET_LOCAL_OUT) |
 301                            (1 << NF_INET_POST_ROUTING))) != 0) {
 302                 pr_info("path-MTU clamping only supported in "
 303                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
 304                 return -EINVAL;
 305         }
 306         if (par->nft_compat)
 307                 return 0;
 308
 309         xt_ematch_foreach(ematch, e)
 310                 if (find_syn_match(ematch))
 311                         return 0;
 312         pr_info("Only works on TCP SYN packets\n");
 313         return -EINVAL;
 314 }
 315 #endif
 316
 317 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
 318         {
 319                 .family         = NFPROTO_IPV4,
 320                 .name           = "TCPMSS",
 321                 .checkentry     = tcpmss_tg4_check,
 322                 .target         = tcpmss_tg4,
 323                 .targetsize     = sizeof(struct xt_tcpmss_info),
 324                 .proto          = IPPROTO_TCP,
 325                 .me             = THIS_MODULE,
 326         },
 327 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 328         {
 329                 .family         = NFPROTO_IPV6,
 330                 .name           = "TCPMSS",
 331                 .checkentry     = tcpmss_tg6_check,
 332                 .target         = tcpmss_tg6,
 333                 .targetsize     = sizeof(struct xt_tcpmss_info),
 334                 .proto          = IPPROTO_TCP,
 335                 .me             = THIS_MODULE,
 336         },
 337 #endif
 338 };
 339
 340 static int __init tcpmss_tg_init(void)
 341 {
 342         return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 343 }
 344
 345 static void __exit tcpmss_tg_exit(void)
 346 {
 347         xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 348 }
 349
 350 module_init(tcpmss_tg_init);
 351 module_exit(tcpmss_tg_exit);