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 int
  47 tcpmss_mangle_packet(struct sk_buff *skb,
  48                      const struct xt_action_param *par,
  49                      unsigned int in_mtu,
  50                      unsigned int tcphoff,
  51                      unsigned int minlen)
  52 {
  53         const struct xt_tcpmss_info *info = par->targinfo;
  54         struct tcphdr *tcph;
  55         int len, tcp_hdrlen;
  56         unsigned int i;
  57         __be16 oldval;
  58         u16 newmss;
  59         u8 *opt;
  60
  61         /* This is a fragment, no TCP header is available */
  62         if (par->fragoff != 0)
  63                 return 0;
  64
  65         if (!skb_make_writable(skb, skb->len))
  66                 return -1;
  67
  68         len = skb->len - tcphoff;
  69         if (len < (int)sizeof(struct tcphdr))
  70                 return -1;
  71
  72         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  73         tcp_hdrlen = tcph->doff * 4;
  74
  75         if (len < tcp_hdrlen)
  76                 return -1;
  77
  78         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
  79                 if (dst_mtu(skb_dst(skb)) <= minlen) {
  80                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
  81                                             dst_mtu(skb_dst(skb)));
  82                         return -1;
  83                 }
  84                 if (in_mtu <= minlen) {
  85                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
  86                                             in_mtu);
  87                         return -1;
  88                 }
  89                 newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
  90         } else
  91                 newmss = info->mss;
  92
  93         opt = (u_int8_t *)tcph;
  94         for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
  95                 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
  96                         u_int16_t oldmss;
  97
  98                         oldmss = (opt[i+2] << 8) | opt[i+3];
  99
 100                         /* Never increase MSS, even when setting it, as
 101                          * doing so results in problems for hosts that rely
 102                          * on MSS being set correctly.
 103                          */
 104                         if (oldmss <= newmss)
 105                                 return 0;
 106
 107                         opt[i+2] = (newmss & 0xff00) >> 8;
 108                         opt[i+3] = newmss & 0x00ff;
 109
 110                         inet_proto_csum_replace2(&tcph->check, skb,
 111                                                  htons(oldmss), htons(newmss),
 112                                                  0);
 113                         return 0;
 114                 }
 115         }
 116
 117         /* There is data after the header so the option can't be added
 118          * without moving it, and doing so may make the SYN packet
 119          * itself too large. Accept the packet unmodified instead.
 120          */
 121         if (len > tcp_hdrlen)
 122                 return 0;
 123
 124         /*
 125          * MSS Option not found ?! add it..
 126          */
 127         if (skb_tailroom(skb) < TCPOLEN_MSS) {
 128                 if (pskb_expand_head(skb, 0,
 129                                      TCPOLEN_MSS - skb_tailroom(skb),
 130                                      GFP_ATOMIC))
 131                         return -1;
 132                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 133         }
 134
 135         skb_put(skb, TCPOLEN_MSS);
 136
 137         /*
 138          * IPv4: RFC 1122 states "If an MSS option is not received at
 139          * connection setup, TCP MUST assume a default send MSS of 536".
 140          * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
 141          * length IPv6 header of 60, ergo the default MSS value is 1220
 142          * Since no MSS was provided, we must use the default values
 143          */
 144         if (par->family == NFPROTO_IPV4)
 145                 newmss = min(newmss, (u16)536);
 146         else
 147                 newmss = min(newmss, (u16)1220);
 148
 149         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 150         memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
 151
 152         inet_proto_csum_replace2(&tcph->check, skb,
 153                                  htons(len), htons(len + TCPOLEN_MSS), 1);
 154         opt[0] = TCPOPT_MSS;
 155         opt[1] = TCPOLEN_MSS;
 156         opt[2] = (newmss & 0xff00) >> 8;
 157         opt[3] = newmss & 0x00ff;
 158
 159         inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
 160
 161         oldval = ((__be16 *)tcph)[6];
 162         tcph->doff += TCPOLEN_MSS/4;
 163         inet_proto_csum_replace2(&tcph->check, skb,
 164                                  oldval, ((__be16 *)tcph)[6], 0);
 165         return TCPOLEN_MSS;
 166 }
 167
 168 static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
 169                                     unsigned int family)
 170 {
 171         struct flowi fl;
 172         const struct nf_afinfo *ai;
 173         struct rtable *rt = NULL;
 174         u_int32_t mtu     = ~0U;
 175
 176         if (family == PF_INET) {
 177                 struct flowi4 *fl4 = &fl.u.ip4;
 178                 memset(fl4, 0, sizeof(*fl4));
 179                 fl4->daddr = ip_hdr(skb)->saddr;
 180         } else {
 181                 struct flowi6 *fl6 = &fl.u.ip6;
 182
 183                 memset(fl6, 0, sizeof(*fl6));
 184                 fl6->daddr = ipv6_hdr(skb)->saddr;
 185         }
 186         rcu_read_lock();
 187         ai = nf_get_afinfo(family);
 188         if (ai != NULL)
 189                 ai->route(&init_net, (struct dst_entry **)&rt, &fl, false);
 190         rcu_read_unlock();
 191
 192         if (rt != NULL) {
 193                 mtu = dst_mtu(&rt->dst);
 194                 dst_release(&rt->dst);
 195         }
 196         return mtu;
 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                                    tcpmss_reverse_mtu(skb, 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;
 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                                    tcpmss_reverse_mtu(skb, 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                 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
 244         }
 245         return XT_CONTINUE;
 246 }
 247 #endif
 248
 249 /* Must specify -p tcp --syn */
 250 static inline bool find_syn_match(const struct xt_entry_match *m)
 251 {
 252         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
 253
 254         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 255             tcpinfo->flg_cmp & TCPHDR_SYN &&
 256             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 257                 return true;
 258
 259         return false;
 260 }
 261
 262 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
 263 {
 264         const struct xt_tcpmss_info *info = par->targinfo;
 265         const struct ipt_entry *e = par->entryinfo;
 266         const struct xt_entry_match *ematch;
 267
 268         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 269             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 270                            (1 << NF_INET_LOCAL_OUT) |
 271                            (1 << NF_INET_POST_ROUTING))) != 0) {
 272                 pr_info("path-MTU clamping only supported in "
 273                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
 274                 return -EINVAL;
 275         }
 276         xt_ematch_foreach(ematch, e)
 277                 if (find_syn_match(ematch))
 278                         return 0;
 279         pr_info("Only works on TCP SYN packets\n");
 280         return -EINVAL;
 281 }
 282
 283 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 284 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
 285 {
 286         const struct xt_tcpmss_info *info = par->targinfo;
 287         const struct ip6t_entry *e = par->entryinfo;
 288         const struct xt_entry_match *ematch;
 289
 290         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 291             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 292                            (1 << NF_INET_LOCAL_OUT) |
 293                            (1 << NF_INET_POST_ROUTING))) != 0) {
 294                 pr_info("path-MTU clamping only supported in "
 295                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
 296                 return -EINVAL;
 297         }
 298         xt_ematch_foreach(ematch, e)
 299                 if (find_syn_match(ematch))
 300                         return 0;
 301         pr_info("Only works on TCP SYN packets\n");
 302         return -EINVAL;
 303 }
 304 #endif
 305
 306 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
 307         {
 308                 .family         = NFPROTO_IPV4,
 309                 .name           = "TCPMSS",
 310                 .checkentry     = tcpmss_tg4_check,
 311                 .target         = tcpmss_tg4,
 312                 .targetsize     = sizeof(struct xt_tcpmss_info),
 313                 .proto          = IPPROTO_TCP,
 314                 .me             = THIS_MODULE,
 315         },
 316 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 317         {
 318                 .family         = NFPROTO_IPV6,
 319                 .name           = "TCPMSS",
 320                 .checkentry     = tcpmss_tg6_check,
 321                 .target         = tcpmss_tg6,
 322                 .targetsize     = sizeof(struct xt_tcpmss_info),
 323                 .proto          = IPPROTO_TCP,
 324                 .me             = THIS_MODULE,
 325         },
 326 #endif
 327 };
 328
 329 static int __init tcpmss_tg_init(void)
 330 {
 331         return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 332 }
 333
 334 static void __exit tcpmss_tg_exit(void)
 335 {
 336         xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 337 }
 338
 339 module_init(tcpmss_tg_init);
 340 module_exit(tcpmss_tg_exit);