net/netfilter/xt_TCPMSS.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * This is a module which is used for setting the MSS option in TCP packets.
   4  *
   5  * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
   6  * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
   7  */
   8 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
   9 #include <linux/module.h>
  10 #include <linux/skbuff.h>
  11 #include <linux/ip.h>
  12 #include <linux/gfp.h>
  13 #include <linux/ipv6.h>
  14 #include <linux/tcp.h>
  15 #include <net/dst.h>
  16 #include <net/flow.h>
  17 #include <net/ipv6.h>
  18 #include <net/route.h>
  19 #include <net/tcp.h>
  20
  21 #include <linux/netfilter_ipv4/ip_tables.h>
  22 #include <linux/netfilter_ipv6/ip6_tables.h>
  23 #include <linux/netfilter/x_tables.h>
  24 #include <linux/netfilter/xt_tcpudp.h>
  25 #include <linux/netfilter/xt_TCPMSS.h>
  26
  27 MODULE_LICENSE("GPL");
  28 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  29 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
  30 MODULE_ALIAS("ipt_TCPMSS");
  31 MODULE_ALIAS("ip6t_TCPMSS");
  32
  33 static inline unsigned int
  34 optlen(const u_int8_t *opt, unsigned int offset)
  35 {
  36         /* Beware zero-length options: make finite progress */
  37         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  38                 return 1;
  39         else
  40                 return opt[offset+1];
  41 }
  42
  43 static u_int32_t tcpmss_reverse_mtu(struct net *net,
  44                                     const struct sk_buff *skb,
  45                                     unsigned int family)
  46 {
  47         struct flowi fl;
  48         struct rtable *rt = NULL;
  49         u_int32_t mtu     = ~0U;
  50
  51         if (family == PF_INET) {
  52                 struct flowi4 *fl4 = &fl.u.ip4;
  53                 memset(fl4, 0, sizeof(*fl4));
  54                 fl4->daddr = ip_hdr(skb)->saddr;
  55         } else {
  56                 struct flowi6 *fl6 = &fl.u.ip6;
  57
  58                 memset(fl6, 0, sizeof(*fl6));
  59                 fl6->daddr = ipv6_hdr(skb)->saddr;
  60         }
  61
  62         nf_route(net, (struct dst_entry **)&rt, &fl, false, family);
  63         if (rt != NULL) {
  64                 mtu = dst_mtu(&rt->dst);
  65                 dst_release(&rt->dst);
  66         }
  67         return mtu;
  68 }
  69
  70 static int
  71 tcpmss_mangle_packet(struct sk_buff *skb,
  72                      const struct xt_action_param *par,
  73                      unsigned int family,
  74                      unsigned int tcphoff,
  75                      unsigned int minlen)
  76 {
  77         const struct xt_tcpmss_info *info = par->targinfo;
  78         struct tcphdr *tcph;
  79         int len, tcp_hdrlen;
  80         unsigned int i;
  81         __be16 oldval;
  82         u16 newmss;
  83         u8 *opt;
  84
  85         /* This is a fragment, no TCP header is available */
  86         if (par->fragoff != 0)
  87                 return 0;
  88
  89         if (skb_ensure_writable(skb, skb->len))
  90                 return -1;
  91
  92         len = skb->len - tcphoff;
  93         if (len < (int)sizeof(struct tcphdr))
  94                 return -1;
  95
  96         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  97         tcp_hdrlen = tcph->doff * 4;
  98
  99         if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
 100                 return -1;
 101
 102         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
 103                 struct net *net = xt_net(par);
 104                 unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
 105                 unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
 106
 107                 if (min_mtu <= minlen) {
 108                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
 109                                             min_mtu);
 110                         return -1;
 111                 }
 112                 newmss = min_mtu - minlen;
 113         } else
 114                 newmss = info->mss;
 115
 116         opt = (u_int8_t *)tcph;
 117         for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
 118                 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
 119                         u_int16_t oldmss;
 120
 121                         oldmss = (opt[i+2] << 8) | opt[i+3];
 122
 123                         /* Never increase MSS, even when setting it, as
 124                          * doing so results in problems for hosts that rely
 125                          * on MSS being set correctly.
 126                          */
 127                         if (oldmss <= newmss)
 128                                 return 0;
 129
 130                         opt[i+2] = (newmss & 0xff00) >> 8;
 131                         opt[i+3] = newmss & 0x00ff;
 132
 133                         inet_proto_csum_replace2(&tcph->check, skb,
 134                                                  htons(oldmss), htons(newmss),
 135                                                  false);
 136                         return 0;
 137                 }
 138         }
 139
 140         /* There is data after the header so the option can't be added
 141          * without moving it, and doing so may make the SYN packet
 142          * itself too large. Accept the packet unmodified instead.
 143          */
 144         if (len > tcp_hdrlen)
 145                 return 0;
 146
 147         /* tcph->doff has 4 bits, do not wrap it to 0 */
 148         if (tcp_hdrlen >= 15 * 4)
 149                 return 0;
 150
 151         /*
 152          * MSS Option not found ?! add it..
 153          */
 154         if (skb_tailroom(skb) < TCPOLEN_MSS) {
 155                 if (pskb_expand_head(skb, 0,
 156                                      TCPOLEN_MSS - skb_tailroom(skb),
 157                                      GFP_ATOMIC))
 158                         return -1;
 159                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 160         }
 161
 162         skb_put(skb, TCPOLEN_MSS);
 163
 164         /*
 165          * IPv4: RFC 1122 states "If an MSS option is not received at
 166          * connection setup, TCP MUST assume a default send MSS of 536".
 167          * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
 168          * length IPv6 header of 60, ergo the default MSS value is 1220
 169          * Since no MSS was provided, we must use the default values
 170          */
 171         if (xt_family(par) == NFPROTO_IPV4)
 172                 newmss = min(newmss, (u16)536);
 173         else
 174                 newmss = min(newmss, (u16)1220);
 175
 176         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 177         memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
 178
 179         inet_proto_csum_replace2(&tcph->check, skb,
 180                                  htons(len), htons(len + TCPOLEN_MSS), true);
 181         opt[0] = TCPOPT_MSS;
 182         opt[1] = TCPOLEN_MSS;
 183         opt[2] = (newmss & 0xff00) >> 8;
 184         opt[3] = newmss & 0x00ff;
 185
 186         inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
 187
 188         oldval = ((__be16 *)tcph)[6];
 189         tcph->doff += TCPOLEN_MSS/4;
 190         inet_proto_csum_replace2(&tcph->check, skb,
 191                                  oldval, ((__be16 *)tcph)[6], false);
 192         return TCPOLEN_MSS;
 193 }
 194
 195 static unsigned int
 196 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
 197 {
 198         struct iphdr *iph = ip_hdr(skb);
 199         __be16 newlen;
 200         int ret;
 201
 202         ret = tcpmss_mangle_packet(skb, par,
 203                                    PF_INET,
 204                                    iph->ihl * 4,
 205                                    sizeof(*iph) + sizeof(struct tcphdr));
 206         if (ret < 0)
 207                 return NF_DROP;
 208         if (ret > 0) {
 209                 iph = ip_hdr(skb);
 210                 newlen = htons(ntohs(iph->tot_len) + ret);
 211                 csum_replace2(&iph->check, iph->tot_len, newlen);
 212                 iph->tot_len = newlen;
 213         }
 214         return XT_CONTINUE;
 215 }
 216
 217 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 218 static unsigned int
 219 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 220 {
 221         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 222         u8 nexthdr;
 223         __be16 frag_off, oldlen, newlen;
 224         int tcphoff;
 225         int ret;
 226
 227         nexthdr = ipv6h->nexthdr;
 228         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 229         if (tcphoff < 0)
 230                 return NF_DROP;
 231         ret = tcpmss_mangle_packet(skb, par,
 232                                    PF_INET6,
 233                                    tcphoff,
 234                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
 235         if (ret < 0)
 236                 return NF_DROP;
 237         if (ret > 0) {
 238                 ipv6h = ipv6_hdr(skb);
 239                 oldlen = ipv6h->payload_len;
 240                 newlen = htons(ntohs(oldlen) + ret);
 241                 if (skb->ip_summed == CHECKSUM_COMPLETE)
 242                         skb->csum = csum_add(csum_sub(skb->csum, oldlen),
 243                                              newlen);
 244                 ipv6h->payload_len = newlen;
 245         }
 246         return XT_CONTINUE;
 247 }
 248 #endif
 249
 250 /* Must specify -p tcp --syn */
 251 static inline bool find_syn_match(const struct xt_entry_match *m)
 252 {
 253         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
 254
 255         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
 256             tcpinfo->flg_cmp & TCPHDR_SYN &&
 257             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
 258                 return true;
 259
 260         return false;
 261 }
 262
 263 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
 264 {
 265         const struct xt_tcpmss_info *info = par->targinfo;
 266         const struct ipt_entry *e = par->entryinfo;
 267         const struct xt_entry_match *ematch;
 268
 269         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 270             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 271                            (1 << NF_INET_LOCAL_OUT) |
 272                            (1 << NF_INET_POST_ROUTING))) != 0) {
 273                 pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
 274                 return -EINVAL;
 275         }
 276         if (par->nft_compat)
 277                 return 0;
 278
 279         xt_ematch_foreach(ematch, e)
 280                 if (find_syn_match(ematch))
 281                         return 0;
 282         pr_info_ratelimited("Only works on TCP SYN packets\n");
 283         return -EINVAL;
 284 }
 285
 286 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 287 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
 288 {
 289         const struct xt_tcpmss_info *info = par->targinfo;
 290         const struct ip6t_entry *e = par->entryinfo;
 291         const struct xt_entry_match *ematch;
 292
 293         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
 294             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
 295                            (1 << NF_INET_LOCAL_OUT) |
 296                            (1 << NF_INET_POST_ROUTING))) != 0) {
 297                 pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
 298                 return -EINVAL;
 299         }
 300         if (par->nft_compat)
 301                 return 0;
 302
 303         xt_ematch_foreach(ematch, e)
 304                 if (find_syn_match(ematch))
 305                         return 0;
 306         pr_info_ratelimited("Only works on TCP SYN packets\n");
 307         return -EINVAL;
 308 }
 309 #endif
 310
 311 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
 312         {
 313                 .family         = NFPROTO_IPV4,
 314                 .name           = "TCPMSS",
 315                 .checkentry     = tcpmss_tg4_check,
 316                 .target         = tcpmss_tg4,
 317                 .targetsize     = sizeof(struct xt_tcpmss_info),
 318                 .proto          = IPPROTO_TCP,
 319                 .me             = THIS_MODULE,
 320         },
 321 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 322         {
 323                 .family         = NFPROTO_IPV6,
 324                 .name           = "TCPMSS",
 325                 .checkentry     = tcpmss_tg6_check,
 326                 .target         = tcpmss_tg6,
 327                 .targetsize     = sizeof(struct xt_tcpmss_info),
 328                 .proto          = IPPROTO_TCP,
 329                 .me             = THIS_MODULE,
 330         },
 331 #endif
 332 };
 333
 334 static int __init tcpmss_tg_init(void)
 335 {
 336         return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 337 }
 338
 339 static void __exit tcpmss_tg_exit(void)
 340 {
 341         xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
 342 }
 343
 344 module_init(tcpmss_tg_init);
 345 module_exit(tcpmss_tg_exit);