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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 = 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);
352 

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