1 /* 2 * Syncookies implementation for the Linux kernel 3 * 4 * Copyright (C) 1997 Andi Kleen 5 * Based on ideas by D.J.Bernstein and Eric Schenk. 6 * 7 * This program is free software; you can redistribute it and/or 8 * modify it under the terms of the GNU General Public License 9 * as published by the Free Software Foundation; either version 10 * 2 of the License, or (at your option) any later version. 11 */ 12 13 #include <linux/tcp.h> 14 #include <linux/slab.h> 15 #include <linux/random.h> 16 #include <linux/cryptohash.h> 17 #include <linux/kernel.h> 18 #include <net/tcp.h> 19 #include <net/route.h> 20 21 /* Timestamps: lowest 9 bits store TCP options */ 22 #define TSBITS 9 23 #define TSMASK (((__u32)1 << TSBITS) - 1) 24 25 extern int sysctl_tcp_syncookies; 26 27 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS]; 28 EXPORT_SYMBOL(syncookie_secret); 29 30 static __init int init_syncookies(void) 31 { 32 get_random_bytes(syncookie_secret, sizeof(syncookie_secret)); 33 return 0; 34 } 35 __initcall(init_syncookies); 36 37 #define COOKIEBITS 24 /* Upper bits store count */ 38 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) 39 40 static DEFINE_PER_CPU(__u32, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS]; 41 42 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport, 43 u32 count, int c) 44 { 45 __u32 *tmp = __get_cpu_var(cookie_scratch); 46 47 memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c])); 48 tmp[0] = (__force u32)saddr; 49 tmp[1] = (__force u32)daddr; 50 tmp[2] = ((__force u32)sport << 16) + (__force u32)dport; 51 tmp[3] = count; 52 sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5); 53 54 return tmp[17]; 55 } 56 57 58 /* 59 * when syncookies are in effect and tcp timestamps are enabled we encode 60 * tcp options in the lowest 9 bits of the timestamp value that will be 61 * sent in the syn-ack. 62 * Since subsequent timestamps use the normal tcp_time_stamp value, we 63 * must make sure that the resulting initial timestamp is <= tcp_time_stamp. 64 */ 65 __u32 cookie_init_timestamp(struct request_sock *req) 66 { 67 struct inet_request_sock *ireq; 68 u32 ts, ts_now = tcp_time_stamp; 69 u32 options = 0; 70 71 ireq = inet_rsk(req); 72 if (ireq->wscale_ok) { 73 options = ireq->snd_wscale; 74 options |= ireq->rcv_wscale << 4; 75 } 76 options |= ireq->sack_ok << 8; 77 78 ts = ts_now & ~TSMASK; 79 ts |= options; 80 if (ts > ts_now) { 81 ts >>= TSBITS; 82 ts--; 83 ts <<= TSBITS; 84 ts |= options; 85 } 86 return ts; 87 } 88 89 90 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, 91 __be16 dport, __u32 sseq, __u32 count, 92 __u32 data) 93 { 94 /* 95 * Compute the secure sequence number. 96 * The output should be: 97 * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24) 98 * + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24). 99 * Where sseq is their sequence number and count increases every 100 * minute by 1. 101 * As an extra hack, we add a small "data" value that encodes the 102 * MSS into the second hash value. 103 */ 104 105 return (cookie_hash(saddr, daddr, sport, dport, 0, 0) + 106 sseq + (count << COOKIEBITS) + 107 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data) 108 & COOKIEMASK)); 109 } 110 111 /* 112 * This retrieves the small "data" value from the syncookie. 113 * If the syncookie is bad, the data returned will be out of 114 * range. This must be checked by the caller. 115 * 116 * The count value used to generate the cookie must be within 117 * "maxdiff" if the current (passed-in) "count". The return value 118 * is (__u32)-1 if this test fails. 119 */ 120 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr, 121 __be16 sport, __be16 dport, __u32 sseq, 122 __u32 count, __u32 maxdiff) 123 { 124 __u32 diff; 125 126 /* Strip away the layers from the cookie */ 127 cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; 128 129 /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */ 130 diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS); 131 if (diff >= maxdiff) 132 return (__u32)-1; 133 134 return (cookie - 135 cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) 136 & COOKIEMASK; /* Leaving the data behind */ 137 } 138 139 /* 140 * This table has to be sorted and terminated with (__u16)-1. 141 * XXX generate a better table. 142 * Unresolved Issues: HIPPI with a 64k MSS is not well supported. 143 */ 144 static __u16 const msstab[] = { 145 64 - 1, 146 256 - 1, 147 512 - 1, 148 536 - 1, 149 1024 - 1, 150 1440 - 1, 151 1460 - 1, 152 4312 - 1, 153 (__u16)-1 154 }; 155 /* The number doesn't include the -1 terminator */ 156 #define NUM_MSS (ARRAY_SIZE(msstab) - 1) 157 158 /* 159 * Generate a syncookie. mssp points to the mss, which is returned 160 * rounded down to the value encoded in the cookie. 161 */ 162 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp) 163 { 164 struct tcp_sock *tp = tcp_sk(sk); 165 const struct iphdr *iph = ip_hdr(skb); 166 const struct tcphdr *th = tcp_hdr(skb); 167 int mssind; 168 const __u16 mss = *mssp; 169 170 tp->last_synq_overflow = jiffies; 171 172 /* XXX sort msstab[] by probability? Binary search? */ 173 for (mssind = 0; mss > msstab[mssind + 1]; mssind++) 174 ; 175 *mssp = msstab[mssind] + 1; 176 177 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT); 178 179 return secure_tcp_syn_cookie(iph->saddr, iph->daddr, 180 th->source, th->dest, ntohl(th->seq), 181 jiffies / (HZ * 60), mssind); 182 } 183 184 /* 185 * This (misnamed) value is the age of syncookie which is permitted. 186 * Its ideal value should be dependent on TCP_TIMEOUT_INIT and 187 * sysctl_tcp_retries1. It's a rather complicated formula (exponential 188 * backoff) to compute at runtime so it's currently hardcoded here. 189 */ 190 #define COUNTER_TRIES 4 191 /* 192 * Check if a ack sequence number is a valid syncookie. 193 * Return the decoded mss if it is, or 0 if not. 194 */ 195 static inline int cookie_check(struct sk_buff *skb, __u32 cookie) 196 { 197 const struct iphdr *iph = ip_hdr(skb); 198 const struct tcphdr *th = tcp_hdr(skb); 199 __u32 seq = ntohl(th->seq) - 1; 200 __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, 201 th->source, th->dest, seq, 202 jiffies / (HZ * 60), 203 COUNTER_TRIES); 204 205 return mssind < NUM_MSS ? msstab[mssind] + 1 : 0; 206 } 207 208 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, 209 struct request_sock *req, 210 struct dst_entry *dst) 211 { 212 struct inet_connection_sock *icsk = inet_csk(sk); 213 struct sock *child; 214 215 child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst); 216 if (child) 217 inet_csk_reqsk_queue_add(sk, req, child); 218 else 219 reqsk_free(req); 220 221 return child; 222 } 223 224 225 /* 226 * when syncookies are in effect and tcp timestamps are enabled we stored 227 * additional tcp options in the timestamp. 228 * This extracts these options from the timestamp echo. 229 * 230 * The lowest 4 bits are for snd_wscale 231 * The next 4 lsb are for rcv_wscale 232 * The next lsb is for sack_ok 233 */ 234 void cookie_check_timestamp(struct tcp_options_received *tcp_opt) 235 { 236 /* echoed timestamp, 9 lowest bits contain options */ 237 u32 options = tcp_opt->rcv_tsecr & TSMASK; 238 239 tcp_opt->snd_wscale = options & 0xf; 240 options >>= 4; 241 tcp_opt->rcv_wscale = options & 0xf; 242 243 tcp_opt->sack_ok = (options >> 4) & 0x1; 244 245 if (tcp_opt->sack_ok) 246 tcp_sack_reset(tcp_opt); 247 248 if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale) 249 tcp_opt->wscale_ok = 1; 250 } 251 EXPORT_SYMBOL(cookie_check_timestamp); 252 253 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, 254 struct ip_options *opt) 255 { 256 struct inet_request_sock *ireq; 257 struct tcp_request_sock *treq; 258 struct tcp_sock *tp = tcp_sk(sk); 259 const struct tcphdr *th = tcp_hdr(skb); 260 __u32 cookie = ntohl(th->ack_seq) - 1; 261 struct sock *ret = sk; 262 struct request_sock *req; 263 int mss; 264 struct rtable *rt; 265 __u8 rcv_wscale; 266 struct tcp_options_received tcp_opt; 267 268 if (!sysctl_tcp_syncookies || !th->ack) 269 goto out; 270 271 if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) || 272 (mss = cookie_check(skb, cookie)) == 0) { 273 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); 274 goto out; 275 } 276 277 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); 278 279 /* check for timestamp cookie support */ 280 memset(&tcp_opt, 0, sizeof(tcp_opt)); 281 tcp_parse_options(skb, &tcp_opt, 0); 282 283 if (tcp_opt.saw_tstamp) 284 cookie_check_timestamp(&tcp_opt); 285 286 ret = NULL; 287 req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */ 288 if (!req) 289 goto out; 290 291 ireq = inet_rsk(req); 292 treq = tcp_rsk(req); 293 treq->rcv_isn = ntohl(th->seq) - 1; 294 treq->snt_isn = cookie; 295 req->mss = mss; 296 ireq->loc_port = th->dest; 297 ireq->rmt_port = th->source; 298 ireq->loc_addr = ip_hdr(skb)->daddr; 299 ireq->rmt_addr = ip_hdr(skb)->saddr; 300 ireq->ecn_ok = 0; 301 ireq->snd_wscale = tcp_opt.snd_wscale; 302 ireq->rcv_wscale = tcp_opt.rcv_wscale; 303 ireq->sack_ok = tcp_opt.sack_ok; 304 ireq->wscale_ok = tcp_opt.wscale_ok; 305 ireq->tstamp_ok = tcp_opt.saw_tstamp; 306 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0; 307 308 /* We throwed the options of the initial SYN away, so we hope 309 * the ACK carries the same options again (see RFC1122 4.2.3.8) 310 */ 311 if (opt && opt->optlen) { 312 int opt_size = sizeof(struct ip_options) + opt->optlen; 313 314 ireq->opt = kmalloc(opt_size, GFP_ATOMIC); 315 if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) { 316 kfree(ireq->opt); 317 ireq->opt = NULL; 318 } 319 } 320 321 if (security_inet_conn_request(sk, skb, req)) { 322 reqsk_free(req); 323 goto out; 324 } 325 326 req->expires = 0UL; 327 req->retrans = 0; 328 329 /* 330 * We need to lookup the route here to get at the correct 331 * window size. We should better make sure that the window size 332 * hasn't changed since we received the original syn, but I see 333 * no easy way to do this. 334 */ 335 { 336 struct flowi fl = { .nl_u = { .ip4_u = 337 { .daddr = ((opt && opt->srr) ? 338 opt->faddr : 339 ireq->rmt_addr), 340 .saddr = ireq->loc_addr, 341 .tos = RT_CONN_FLAGS(sk) } }, 342 .proto = IPPROTO_TCP, 343 .flags = inet_sk_flowi_flags(sk), 344 .uli_u = { .ports = 345 { .sport = th->dest, 346 .dport = th->source } } }; 347 security_req_classify_flow(req, &fl); 348 if (ip_route_output_key(&init_net, &rt, &fl)) { 349 reqsk_free(req); 350 goto out; 351 } 352 } 353 354 /* Try to redo what tcp_v4_send_synack did. */ 355 req->window_clamp = tp->window_clamp ? :dst_metric(&rt->u.dst, RTAX_WINDOW); 356 357 tcp_select_initial_window(tcp_full_space(sk), req->mss, 358 &req->rcv_wnd, &req->window_clamp, 359 ireq->wscale_ok, &rcv_wscale); 360 361 ireq->rcv_wscale = rcv_wscale; 362 363 ret = get_cookie_sock(sk, skb, req, &rt->u.dst); 364 out: return ret; 365 } 366