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Linux/net/netfilter/xt_TCPMSS.c

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  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 = par->net;
112                 unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
113 
114                 if (dst_mtu(skb_dst(skb)) <= minlen) {
115                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
116                                             dst_mtu(skb_dst(skb)));
117                         return -1;
118                 }
119                 if (in_mtu <= minlen) {
120                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
121                                             in_mtu);
122                         return -1;
123                 }
124                 newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
125         } else
126                 newmss = info->mss;
127 
128         opt = (u_int8_t *)tcph;
129         for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
130                 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
131                         u_int16_t oldmss;
132 
133                         oldmss = (opt[i+2] << 8) | opt[i+3];
134 
135                         /* Never increase MSS, even when setting it, as
136                          * doing so results in problems for hosts that rely
137                          * on MSS being set correctly.
138                          */
139                         if (oldmss <= newmss)
140                                 return 0;
141 
142                         opt[i+2] = (newmss & 0xff00) >> 8;
143                         opt[i+3] = newmss & 0x00ff;
144 
145                         inet_proto_csum_replace2(&tcph->check, skb,
146                                                  htons(oldmss), htons(newmss),
147                                                  false);
148                         return 0;
149                 }
150         }
151 
152         /* There is data after the header so the option can't be added
153          * without moving it, and doing so may make the SYN packet
154          * itself too large. Accept the packet unmodified instead.
155          */
156         if (len > tcp_hdrlen)
157                 return 0;
158 
159         /*
160          * MSS Option not found ?! add it..
161          */
162         if (skb_tailroom(skb) < TCPOLEN_MSS) {
163                 if (pskb_expand_head(skb, 0,
164                                      TCPOLEN_MSS - skb_tailroom(skb),
165                                      GFP_ATOMIC))
166                         return -1;
167                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
168         }
169 
170         skb_put(skb, TCPOLEN_MSS);
171 
172         /*
173          * IPv4: RFC 1122 states "If an MSS option is not received at
174          * connection setup, TCP MUST assume a default send MSS of 536".
175          * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
176          * length IPv6 header of 60, ergo the default MSS value is 1220
177          * Since no MSS was provided, we must use the default values
178          */
179         if (par->family == NFPROTO_IPV4)
180                 newmss = min(newmss, (u16)536);
181         else
182                 newmss = min(newmss, (u16)1220);
183 
184         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
185         memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
186 
187         inet_proto_csum_replace2(&tcph->check, skb,
188                                  htons(len), htons(len + TCPOLEN_MSS), true);
189         opt[0] = TCPOPT_MSS;
190         opt[1] = TCPOLEN_MSS;
191         opt[2] = (newmss & 0xff00) >> 8;
192         opt[3] = newmss & 0x00ff;
193 
194         inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
195 
196         oldval = ((__be16 *)tcph)[6];
197         tcph->doff += TCPOLEN_MSS/4;
198         inet_proto_csum_replace2(&tcph->check, skb,
199                                  oldval, ((__be16 *)tcph)[6], false);
200         return TCPOLEN_MSS;
201 }
202 
203 static unsigned int
204 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
205 {
206         struct iphdr *iph = ip_hdr(skb);
207         __be16 newlen;
208         int ret;
209 
210         ret = tcpmss_mangle_packet(skb, par,
211                                    PF_INET,
212                                    iph->ihl * 4,
213                                    sizeof(*iph) + sizeof(struct tcphdr));
214         if (ret < 0)
215                 return NF_DROP;
216         if (ret > 0) {
217                 iph = ip_hdr(skb);
218                 newlen = htons(ntohs(iph->tot_len) + ret);
219                 csum_replace2(&iph->check, iph->tot_len, newlen);
220                 iph->tot_len = newlen;
221         }
222         return XT_CONTINUE;
223 }
224 
225 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
226 static unsigned int
227 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
228 {
229         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
230         u8 nexthdr;
231         __be16 frag_off, oldlen, newlen;
232         int tcphoff;
233         int ret;
234 
235         nexthdr = ipv6h->nexthdr;
236         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
237         if (tcphoff < 0)
238                 return NF_DROP;
239         ret = tcpmss_mangle_packet(skb, par,
240                                    PF_INET6,
241                                    tcphoff,
242                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
243         if (ret < 0)
244                 return NF_DROP;
245         if (ret > 0) {
246                 ipv6h = ipv6_hdr(skb);
247                 oldlen = ipv6h->payload_len;
248                 newlen = htons(ntohs(oldlen) + ret);
249                 if (skb->ip_summed == CHECKSUM_COMPLETE)
250                         skb->csum = csum_add(csum_sub(skb->csum, oldlen),
251                                              newlen);
252                 ipv6h->payload_len = newlen;
253         }
254         return XT_CONTINUE;
255 }
256 #endif
257 
258 /* Must specify -p tcp --syn */
259 static inline bool find_syn_match(const struct xt_entry_match *m)
260 {
261         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
262 
263         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
264             tcpinfo->flg_cmp & TCPHDR_SYN &&
265             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
266                 return true;
267 
268         return false;
269 }
270 
271 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
272 {
273         const struct xt_tcpmss_info *info = par->targinfo;
274         const struct ipt_entry *e = par->entryinfo;
275         const struct xt_entry_match *ematch;
276 
277         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
278             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
279                            (1 << NF_INET_LOCAL_OUT) |
280                            (1 << NF_INET_POST_ROUTING))) != 0) {
281                 pr_info("path-MTU clamping only supported in "
282                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
283                 return -EINVAL;
284         }
285         if (par->nft_compat)
286                 return 0;
287 
288         xt_ematch_foreach(ematch, e)
289                 if (find_syn_match(ematch))
290                         return 0;
291         pr_info("Only works on TCP SYN packets\n");
292         return -EINVAL;
293 }
294 
295 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
296 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
297 {
298         const struct xt_tcpmss_info *info = par->targinfo;
299         const struct ip6t_entry *e = par->entryinfo;
300         const struct xt_entry_match *ematch;
301 
302         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
303             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
304                            (1 << NF_INET_LOCAL_OUT) |
305                            (1 << NF_INET_POST_ROUTING))) != 0) {
306                 pr_info("path-MTU clamping only supported in "
307                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
308                 return -EINVAL;
309         }
310         if (par->nft_compat)
311                 return 0;
312 
313         xt_ematch_foreach(ematch, e)
314                 if (find_syn_match(ematch))
315                         return 0;
316         pr_info("Only works on TCP SYN packets\n");
317         return -EINVAL;
318 }
319 #endif
320 
321 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
322         {
323                 .family         = NFPROTO_IPV4,
324                 .name           = "TCPMSS",
325                 .checkentry     = tcpmss_tg4_check,
326                 .target         = tcpmss_tg4,
327                 .targetsize     = sizeof(struct xt_tcpmss_info),
328                 .proto          = IPPROTO_TCP,
329                 .me             = THIS_MODULE,
330         },
331 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
332         {
333                 .family         = NFPROTO_IPV6,
334                 .name           = "TCPMSS",
335                 .checkentry     = tcpmss_tg6_check,
336                 .target         = tcpmss_tg6,
337                 .targetsize     = sizeof(struct xt_tcpmss_info),
338                 .proto          = IPPROTO_TCP,
339                 .me             = THIS_MODULE,
340         },
341 #endif
342 };
343 
344 static int __init tcpmss_tg_init(void)
345 {
346         return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
347 }
348 
349 static void __exit tcpmss_tg_exit(void)
350 {
351         xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
352 }
353 
354 module_init(tcpmss_tg_init);
355 module_exit(tcpmss_tg_exit);
356 

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