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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         ai = nf_get_afinfo(family);
 66         if (ai != NULL)
 67                 ai->route(net, (struct dst_entry **)&rt, &fl, false);
 68 
 69         if (rt != NULL) {
 70                 mtu = dst_mtu(&rt->dst);
 71                 dst_release(&rt->dst);
 72         }
 73         return mtu;
 74 }
 75 
 76 static int
 77 tcpmss_mangle_packet(struct sk_buff *skb,
 78                      const struct xt_action_param *par,
 79                      unsigned int family,
 80                      unsigned int tcphoff,
 81                      unsigned int minlen)
 82 {
 83         const struct xt_tcpmss_info *info = par->targinfo;
 84         struct tcphdr *tcph;
 85         int len, tcp_hdrlen;
 86         unsigned int i;
 87         __be16 oldval;
 88         u16 newmss;
 89         u8 *opt;
 90 
 91         /* This is a fragment, no TCP header is available */
 92         if (par->fragoff != 0)
 93                 return 0;
 94 
 95         if (!skb_make_writable(skb, skb->len))
 96                 return -1;
 97 
 98         len = skb->len - tcphoff;
 99         if (len < (int)sizeof(struct tcphdr))
100                 return -1;
101 
102         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
103         tcp_hdrlen = tcph->doff * 4;
104 
105         if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
106                 return -1;
107 
108         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
109                 struct net *net = xt_net(par);
110                 unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
111                 unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
112 
113                 if (min_mtu <= minlen) {
114                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
115                                             min_mtu);
116                         return -1;
117                 }
118                 newmss = min_mtu - minlen;
119         } else
120                 newmss = info->mss;
121 
122         opt = (u_int8_t *)tcph;
123         for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
124                 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
125                         u_int16_t oldmss;
126 
127                         oldmss = (opt[i+2] << 8) | opt[i+3];
128 
129                         /* Never increase MSS, even when setting it, as
130                          * doing so results in problems for hosts that rely
131                          * on MSS being set correctly.
132                          */
133                         if (oldmss <= newmss)
134                                 return 0;
135 
136                         opt[i+2] = (newmss & 0xff00) >> 8;
137                         opt[i+3] = newmss & 0x00ff;
138 
139                         inet_proto_csum_replace2(&tcph->check, skb,
140                                                  htons(oldmss), htons(newmss),
141                                                  false);
142                         return 0;
143                 }
144         }
145 
146         /* There is data after the header so the option can't be added
147          * without moving it, and doing so may make the SYN packet
148          * itself too large. Accept the packet unmodified instead.
149          */
150         if (len > tcp_hdrlen)
151                 return 0;
152 
153         /* tcph->doff has 4 bits, do not wrap it to 0 */
154         if (tcp_hdrlen >= 15 * 4)
155                 return 0;
156 
157         /*
158          * MSS Option not found ?! add it..
159          */
160         if (skb_tailroom(skb) < TCPOLEN_MSS) {
161                 if (pskb_expand_head(skb, 0,
162                                      TCPOLEN_MSS - skb_tailroom(skb),
163                                      GFP_ATOMIC))
164                         return -1;
165                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
166         }
167 
168         skb_put(skb, TCPOLEN_MSS);
169 
170         /*
171          * IPv4: RFC 1122 states "If an MSS option is not received at
172          * connection setup, TCP MUST assume a default send MSS of 536".
173          * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
174          * length IPv6 header of 60, ergo the default MSS value is 1220
175          * Since no MSS was provided, we must use the default values
176          */
177         if (xt_family(par) == NFPROTO_IPV4)
178                 newmss = min(newmss, (u16)536);
179         else
180                 newmss = min(newmss, (u16)1220);
181 
182         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
183         memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
184 
185         inet_proto_csum_replace2(&tcph->check, skb,
186                                  htons(len), htons(len + TCPOLEN_MSS), true);
187         opt[0] = TCPOPT_MSS;
188         opt[1] = TCPOLEN_MSS;
189         opt[2] = (newmss & 0xff00) >> 8;
190         opt[3] = newmss & 0x00ff;
191 
192         inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
193 
194         oldval = ((__be16 *)tcph)[6];
195         tcph->doff += TCPOLEN_MSS/4;
196         inet_proto_csum_replace2(&tcph->check, skb,
197                                  oldval, ((__be16 *)tcph)[6], false);
198         return TCPOLEN_MSS;
199 }
200 
201 static unsigned int
202 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
203 {
204         struct iphdr *iph = ip_hdr(skb);
205         __be16 newlen;
206         int ret;
207 
208         ret = tcpmss_mangle_packet(skb, par,
209                                    PF_INET,
210                                    iph->ihl * 4,
211                                    sizeof(*iph) + sizeof(struct tcphdr));
212         if (ret < 0)
213                 return NF_DROP;
214         if (ret > 0) {
215                 iph = ip_hdr(skb);
216                 newlen = htons(ntohs(iph->tot_len) + ret);
217                 csum_replace2(&iph->check, iph->tot_len, newlen);
218                 iph->tot_len = newlen;
219         }
220         return XT_CONTINUE;
221 }
222 
223 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
224 static unsigned int
225 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
226 {
227         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
228         u8 nexthdr;
229         __be16 frag_off, oldlen, newlen;
230         int tcphoff;
231         int ret;
232 
233         nexthdr = ipv6h->nexthdr;
234         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
235         if (tcphoff < 0)
236                 return NF_DROP;
237         ret = tcpmss_mangle_packet(skb, par,
238                                    PF_INET6,
239                                    tcphoff,
240                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
241         if (ret < 0)
242                 return NF_DROP;
243         if (ret > 0) {
244                 ipv6h = ipv6_hdr(skb);
245                 oldlen = ipv6h->payload_len;
246                 newlen = htons(ntohs(oldlen) + ret);
247                 if (skb->ip_summed == CHECKSUM_COMPLETE)
248                         skb->csum = csum_add(csum_sub(skb->csum, oldlen),
249                                              newlen);
250                 ipv6h->payload_len = newlen;
251         }
252         return XT_CONTINUE;
253 }
254 #endif
255 
256 /* Must specify -p tcp --syn */
257 static inline bool find_syn_match(const struct xt_entry_match *m)
258 {
259         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
260 
261         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
262             tcpinfo->flg_cmp & TCPHDR_SYN &&
263             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
264                 return true;
265 
266         return false;
267 }
268 
269 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
270 {
271         const struct xt_tcpmss_info *info = par->targinfo;
272         const struct ipt_entry *e = par->entryinfo;
273         const struct xt_entry_match *ematch;
274 
275         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
276             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
277                            (1 << NF_INET_LOCAL_OUT) |
278                            (1 << NF_INET_POST_ROUTING))) != 0) {
279                 pr_info("path-MTU clamping only supported in "
280                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
281                 return -EINVAL;
282         }
283         if (par->nft_compat)
284                 return 0;
285 
286         xt_ematch_foreach(ematch, e)
287                 if (find_syn_match(ematch))
288                         return 0;
289         pr_info("Only works on TCP SYN packets\n");
290         return -EINVAL;
291 }
292 
293 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
294 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
295 {
296         const struct xt_tcpmss_info *info = par->targinfo;
297         const struct ip6t_entry *e = par->entryinfo;
298         const struct xt_entry_match *ematch;
299 
300         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
301             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
302                            (1 << NF_INET_LOCAL_OUT) |
303                            (1 << NF_INET_POST_ROUTING))) != 0) {
304                 pr_info("path-MTU clamping only supported in "
305                         "FORWARD, OUTPUT and POSTROUTING hooks\n");
306                 return -EINVAL;
307         }
308         if (par->nft_compat)
309                 return 0;
310 
311         xt_ematch_foreach(ematch, e)
312                 if (find_syn_match(ematch))
313                         return 0;
314         pr_info("Only works on TCP SYN packets\n");
315         return -EINVAL;
316 }
317 #endif
318 
319 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
320         {
321                 .family         = NFPROTO_IPV4,
322                 .name           = "TCPMSS",
323                 .checkentry     = tcpmss_tg4_check,
324                 .target         = tcpmss_tg4,
325                 .targetsize     = sizeof(struct xt_tcpmss_info),
326                 .proto          = IPPROTO_TCP,
327                 .me             = THIS_MODULE,
328         },
329 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
330         {
331                 .family         = NFPROTO_IPV6,
332                 .name           = "TCPMSS",
333                 .checkentry     = tcpmss_tg6_check,
334                 .target         = tcpmss_tg6,
335                 .targetsize     = sizeof(struct xt_tcpmss_info),
336                 .proto          = IPPROTO_TCP,
337                 .me             = THIS_MODULE,
338         },
339 #endif
340 };
341 
342 static int __init tcpmss_tg_init(void)
343 {
344         return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
345 }
346 
347 static void __exit tcpmss_tg_exit(void)
348 {
349         xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
350 }
351 
352 module_init(tcpmss_tg_init);
353 module_exit(tcpmss_tg_exit);
354 

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