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Linux/net/netfilter/xt_TCPMSS.c

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  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);
346 

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