| 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 | * |
| 6 | * This program is free software; you can redistribute it and/or modify |
| 7 | * it under the terms of the GNU General Public License version 2 as |
| 8 | * published by the Free Software Foundation. |
| 9 | */ |
| 10 | #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt |
| 11 | #include <linux/module.h> |
| 12 | #include <linux/skbuff.h> |
| 13 | #include <linux/ip.h> |
| 14 | #include <linux/gfp.h> |
| 15 | #include <linux/ipv6.h> |
| 16 | #include <linux/tcp.h> |
| 17 | #include <net/dst.h> |
| 18 | #include <net/flow.h> |
| 19 | #include <net/ipv6.h> |
| 20 | #include <net/route.h> |
| 21 | #include <net/tcp.h> |
| 22 | |
| 23 | #include <linux/netfilter_ipv4/ip_tables.h> |
| 24 | #include <linux/netfilter_ipv6/ip6_tables.h> |
| 25 | #include <linux/netfilter/x_tables.h> |
| 26 | #include <linux/netfilter/xt_tcpudp.h> |
| 27 | #include <linux/netfilter/xt_TCPMSS.h> |
| 28 | |
| 29 | MODULE_LICENSE("GPL"); |
| 30 | MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); |
| 31 | MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment"); |
| 32 | MODULE_ALIAS("ipt_TCPMSS"); |
| 33 | MODULE_ALIAS("ip6t_TCPMSS"); |
| 34 | |
| 35 | static inline unsigned int |
| 36 | optlen(const u_int8_t *opt, unsigned int offset) |
| 37 | { |
| 38 | /* Beware zero-length options: make finite progress */ |
| 39 | if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) |
| 40 | return 1; |
| 41 | else |
| 42 | return opt[offset+1]; |
| 43 | } |
| 44 | |
| 45 | static int |
| 46 | tcpmss_mangle_packet(struct sk_buff *skb, |
| 47 | const struct xt_tcpmss_info *info, |
| 48 | unsigned int in_mtu, |
| 49 | unsigned int tcphoff, |
| 50 | unsigned int minlen) |
| 51 | { |
| 52 | struct tcphdr *tcph; |
| 53 | unsigned int tcplen, i; |
| 54 | __be16 oldval; |
| 55 | u16 newmss; |
| 56 | u8 *opt; |
| 57 | |
| 58 | if (!skb_make_writable(skb, skb->len)) |
| 59 | return -1; |
| 60 | |
| 61 | tcplen = skb->len - tcphoff; |
| 62 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); |
| 63 | |
| 64 | /* Header cannot be larger than the packet */ |
| 65 | if (tcplen < tcph->doff*4) |
| 66 | return -1; |
| 67 | |
| 68 | if (info->mss == XT_TCPMSS_CLAMP_PMTU) { |
| 69 | if (dst_mtu(skb_dst(skb)) <= minlen) { |
| 70 | net_err_ratelimited("unknown or invalid path-MTU (%u)\n", |
| 71 | dst_mtu(skb_dst(skb))); |
| 72 | return -1; |
| 73 | } |
| 74 | if (in_mtu <= minlen) { |
| 75 | net_err_ratelimited("unknown or invalid path-MTU (%u)\n", |
| 76 | in_mtu); |
| 77 | return -1; |
| 78 | } |
| 79 | newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen; |
| 80 | } else |
| 81 | newmss = info->mss; |
| 82 | |
| 83 | opt = (u_int8_t *)tcph; |
| 84 | for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) { |
| 85 | if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS && |
| 86 | opt[i+1] == TCPOLEN_MSS) { |
| 87 | u_int16_t oldmss; |
| 88 | |
| 89 | oldmss = (opt[i+2] << 8) | opt[i+3]; |
| 90 | |
| 91 | /* Never increase MSS, even when setting it, as |
| 92 | * doing so results in problems for hosts that rely |
| 93 | * on MSS being set correctly. |
| 94 | */ |
| 95 | if (oldmss <= newmss) |
| 96 | return 0; |
| 97 | |
| 98 | opt[i+2] = (newmss & 0xff00) >> 8; |
| 99 | opt[i+3] = newmss & 0x00ff; |
| 100 | |
| 101 | inet_proto_csum_replace2(&tcph->check, skb, |
| 102 | htons(oldmss), htons(newmss), |
| 103 | 0); |
| 104 | return 0; |
| 105 | } |
| 106 | } |
| 107 | |
| 108 | /* There is data after the header so the option can't be added |
| 109 | without moving it, and doing so may make the SYN packet |
| 110 | itself too large. Accept the packet unmodified instead. */ |
| 111 | if (tcplen > tcph->doff*4) |
| 112 | return 0; |
| 113 | |
| 114 | /* |
| 115 | * MSS Option not found ?! add it.. |
| 116 | */ |
| 117 | if (skb_tailroom(skb) < TCPOLEN_MSS) { |
| 118 | if (pskb_expand_head(skb, 0, |
| 119 | TCPOLEN_MSS - skb_tailroom(skb), |
| 120 | GFP_ATOMIC)) |
| 121 | return -1; |
| 122 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); |
| 123 | } |
| 124 | |
| 125 | skb_put(skb, TCPOLEN_MSS); |
| 126 | |
| 127 | opt = (u_int8_t *)tcph + sizeof(struct tcphdr); |
| 128 | memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr)); |
| 129 | |
| 130 | inet_proto_csum_replace2(&tcph->check, skb, |
| 131 | htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1); |
| 132 | opt[0] = TCPOPT_MSS; |
| 133 | opt[1] = TCPOLEN_MSS; |
| 134 | opt[2] = (newmss & 0xff00) >> 8; |
| 135 | opt[3] = newmss & 0x00ff; |
| 136 | |
| 137 | inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0); |
| 138 | |
| 139 | oldval = ((__be16 *)tcph)[6]; |
| 140 | tcph->doff += TCPOLEN_MSS/4; |
| 141 | inet_proto_csum_replace2(&tcph->check, skb, |
| 142 | oldval, ((__be16 *)tcph)[6], 0); |
| 143 | return TCPOLEN_MSS; |
| 144 | } |
| 145 | |
| 146 | static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb, |
| 147 | unsigned int family) |
| 148 | { |
| 149 | struct flowi fl; |
| 150 | const struct nf_afinfo *ai; |
| 151 | struct rtable *rt = NULL; |
| 152 | u_int32_t mtu = ~0U; |
| 153 | |
| 154 | if (family == PF_INET) { |
| 155 | struct flowi4 *fl4 = &fl.u.ip4; |
| 156 | memset(fl4, 0, sizeof(*fl4)); |
| 157 | fl4->daddr = ip_hdr(skb)->saddr; |
| 158 | } else { |
| 159 | struct flowi6 *fl6 = &fl.u.ip6; |
| 160 | |
| 161 | memset(fl6, 0, sizeof(*fl6)); |
| 162 | fl6->daddr = ipv6_hdr(skb)->saddr; |
| 163 | } |
| 164 | rcu_read_lock(); |
| 165 | ai = nf_get_afinfo(family); |
| 166 | if (ai != NULL) |
| 167 | ai->route(&init_net, (struct dst_entry **)&rt, &fl, false); |
| 168 | rcu_read_unlock(); |
| 169 | |
| 170 | if (rt != NULL) { |
| 171 | mtu = dst_mtu(&rt->dst); |
| 172 | dst_release(&rt->dst); |
| 173 | } |
| 174 | return mtu; |
| 175 | } |
| 176 | |
| 177 | static unsigned int |
| 178 | tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par) |
| 179 | { |
| 180 | struct iphdr *iph = ip_hdr(skb); |
| 181 | __be16 newlen; |
| 182 | int ret; |
| 183 | |
| 184 | ret = tcpmss_mangle_packet(skb, par->targinfo, |
| 185 | tcpmss_reverse_mtu(skb, PF_INET), |
| 186 | iph->ihl * 4, |
| 187 | sizeof(*iph) + sizeof(struct tcphdr)); |
| 188 | if (ret < 0) |
| 189 | return NF_DROP; |
| 190 | if (ret > 0) { |
| 191 | iph = ip_hdr(skb); |
| 192 | newlen = htons(ntohs(iph->tot_len) + ret); |
| 193 | csum_replace2(&iph->check, iph->tot_len, newlen); |
| 194 | iph->tot_len = newlen; |
| 195 | } |
| 196 | return XT_CONTINUE; |
| 197 | } |
| 198 | |
| 199 | #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) |
| 200 | static unsigned int |
| 201 | tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par) |
| 202 | { |
| 203 | struct ipv6hdr *ipv6h = ipv6_hdr(skb); |
| 204 | u8 nexthdr; |
| 205 | __be16 frag_off; |
| 206 | int tcphoff; |
| 207 | int ret; |
| 208 | |
| 209 | nexthdr = ipv6h->nexthdr; |
| 210 | tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off); |
| 211 | if (tcphoff < 0) |
| 212 | return NF_DROP; |
| 213 | ret = tcpmss_mangle_packet(skb, par->targinfo, |
| 214 | tcpmss_reverse_mtu(skb, PF_INET6), |
| 215 | tcphoff, |
| 216 | sizeof(*ipv6h) + sizeof(struct tcphdr)); |
| 217 | if (ret < 0) |
| 218 | return NF_DROP; |
| 219 | if (ret > 0) { |
| 220 | ipv6h = ipv6_hdr(skb); |
| 221 | ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret); |
| 222 | } |
| 223 | return XT_CONTINUE; |
| 224 | } |
| 225 | #endif |
| 226 | |
| 227 | /* Must specify -p tcp --syn */ |
| 228 | static inline bool find_syn_match(const struct xt_entry_match *m) |
| 229 | { |
| 230 | const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data; |
| 231 | |
| 232 | if (strcmp(m->u.kernel.match->name, "tcp") == 0 && |
| 233 | tcpinfo->flg_cmp & TCPHDR_SYN && |
| 234 | !(tcpinfo->invflags & XT_TCP_INV_FLAGS)) |
| 235 | return true; |
| 236 | |
| 237 | return false; |
| 238 | } |
| 239 | |
| 240 | static int tcpmss_tg4_check(const struct xt_tgchk_param *par) |
| 241 | { |
| 242 | const struct xt_tcpmss_info *info = par->targinfo; |
| 243 | const struct ipt_entry *e = par->entryinfo; |
| 244 | const struct xt_entry_match *ematch; |
| 245 | |
| 246 | if (info->mss == XT_TCPMSS_CLAMP_PMTU && |
| 247 | (par->hook_mask & ~((1 << NF_INET_FORWARD) | |
| 248 | (1 << NF_INET_LOCAL_OUT) | |
| 249 | (1 << NF_INET_POST_ROUTING))) != 0) { |
| 250 | pr_info("path-MTU clamping only supported in " |
| 251 | "FORWARD, OUTPUT and POSTROUTING hooks\n"); |
| 252 | return -EINVAL; |
| 253 | } |
| 254 | xt_ematch_foreach(ematch, e) |
| 255 | if (find_syn_match(ematch)) |
| 256 | return 0; |
| 257 | pr_info("Only works on TCP SYN packets\n"); |
| 258 | return -EINVAL; |
| 259 | } |
| 260 | |
| 261 | #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) |
| 262 | static int tcpmss_tg6_check(const struct xt_tgchk_param *par) |
| 263 | { |
| 264 | const struct xt_tcpmss_info *info = par->targinfo; |
| 265 | const struct ip6t_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 | #endif |
| 283 | |
| 284 | static struct xt_target tcpmss_tg_reg[] __read_mostly = { |
| 285 | { |
| 286 | .family = NFPROTO_IPV4, |
| 287 | .name = "TCPMSS", |
| 288 | .checkentry = tcpmss_tg4_check, |
| 289 | .target = tcpmss_tg4, |
| 290 | .targetsize = sizeof(struct xt_tcpmss_info), |
| 291 | .proto = IPPROTO_TCP, |
| 292 | .me = THIS_MODULE, |
| 293 | }, |
| 294 | #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) |
| 295 | { |
| 296 | .family = NFPROTO_IPV6, |
| 297 | .name = "TCPMSS", |
| 298 | .checkentry = tcpmss_tg6_check, |
| 299 | .target = tcpmss_tg6, |
| 300 | .targetsize = sizeof(struct xt_tcpmss_info), |
| 301 | .proto = IPPROTO_TCP, |
| 302 | .me = THIS_MODULE, |
| 303 | }, |
| 304 | #endif |
| 305 | }; |
| 306 | |
| 307 | static int __init tcpmss_tg_init(void) |
| 308 | { |
| 309 | return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
| 310 | } |
| 311 | |
| 312 | static void __exit tcpmss_tg_exit(void) |
| 313 | { |
| 314 | xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); |
| 315 | } |
| 316 | |
| 317 | module_init(tcpmss_tg_init); |
| 318 | module_exit(tcpmss_tg_exit); |