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 const struct iphdr *iph = ip_hdr(skb); 165 const struct tcphdr *th = tcp_hdr(skb); 166 int mssind; 167 const __u16 mss = *mssp; 168 169 tcp_synq_overflow(sk); 170 171 /* XXX sort msstab[] by probability? Binary search? */ 172 for (mssind = 0; mss > msstab[mssind + 1]; mssind++) 173 ; 174 *mssp = msstab[mssind] + 1; 175 176 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT); 177 178 return secure_tcp_syn_cookie(iph->saddr, iph->daddr, 179 th->source, th->dest, ntohl(th->seq), 180 jiffies / (HZ * 60), mssind); 181 } 182 183 /* 184 * This (misnamed) value is the age of syncookie which is permitted. 185 * Its ideal value should be dependent on TCP_TIMEOUT_INIT and 186 * sysctl_tcp_retries1. It's a rather complicated formula (exponential 187 * backoff) to compute at runtime so it's currently hardcoded here. 188 */ 189 #define COUNTER_TRIES 4 190 /* 191 * Check if a ack sequence number is a valid syncookie. 192 * Return the decoded mss if it is, or 0 if not. 193 */ 194 static inline int cookie_check(struct sk_buff *skb, __u32 cookie) 195 { 196 const struct iphdr *iph = ip_hdr(skb); 197 const struct tcphdr *th = tcp_hdr(skb); 198 __u32 seq = ntohl(th->seq) - 1; 199 __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, 200 th->source, th->dest, seq, 201 jiffies / (HZ * 60), 202 COUNTER_TRIES); 203 204 return mssind < NUM_MSS ? msstab[mssind] + 1 : 0; 205 } 206 207 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, 208 struct request_sock *req, 209 struct dst_entry *dst) 210 { 211 struct inet_connection_sock *icsk = inet_csk(sk); 212 struct sock *child; 213 214 child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst); 215 if (child) 216 inet_csk_reqsk_queue_add(sk, req, child); 217 else 218 reqsk_free(req); 219 220 return child; 221 } 222 223 224 /* 225 * when syncookies are in effect and tcp timestamps are enabled we stored 226 * additional tcp options in the timestamp. 227 * This extracts these options from the timestamp echo. 228 * 229 * The lowest 4 bits are for snd_wscale 230 * The next 4 lsb are for rcv_wscale 231 * The next lsb is for sack_ok 232 */ 233 void cookie_check_timestamp(struct tcp_options_received *tcp_opt) 234 { 235 /* echoed timestamp, 9 lowest bits contain options */ 236 u32 options = tcp_opt->rcv_tsecr & TSMASK; 237 238 tcp_opt->snd_wscale = options & 0xf; 239 options >>= 4; 240 tcp_opt->rcv_wscale = options & 0xf; 241 242 tcp_opt->sack_ok = (options >> 4) & 0x1; 243 244 if (tcp_opt->sack_ok) 245 tcp_sack_reset(tcp_opt); 246 247 if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale) 248 tcp_opt->wscale_ok = 1; 249 } 250 EXPORT_SYMBOL(cookie_check_timestamp); 251 252 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, 253 struct ip_options *opt) 254 { 255 struct inet_request_sock *ireq; 256 struct tcp_request_sock *treq; 257 struct tcp_sock *tp = tcp_sk(sk); 258 const struct tcphdr *th = tcp_hdr(skb); 259 __u32 cookie = ntohl(th->ack_seq) - 1; 260 struct sock *ret = sk; 261 struct request_sock *req; 262 int mss; 263 struct rtable *rt; 264 __u8 rcv_wscale; 265 struct tcp_options_received tcp_opt; 266 267 if (!sysctl_tcp_syncookies || !th->ack) 268 goto out; 269 270 if (tcp_synq_no_recent_overflow(sk) || 271 (mss = cookie_check(skb, cookie)) == 0) { 272 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); 273 goto out; 274 } 275 276 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); 277 278 /* check for timestamp cookie support */ 279 memset(&tcp_opt, 0, sizeof(tcp_opt)); 280 tcp_parse_options(skb, &tcp_opt, 0); 281 282 if (tcp_opt.saw_tstamp) 283 cookie_check_timestamp(&tcp_opt); 284 285 ret = NULL; 286 req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */ 287 if (!req) 288 goto out; 289 290 ireq = inet_rsk(req); 291 treq = tcp_rsk(req); 292 treq->rcv_isn = ntohl(th->seq) - 1; 293 treq->snt_isn = cookie; 294 req->mss = mss; 295 ireq->loc_port = th->dest; 296 ireq->rmt_port = th->source; 297 ireq->loc_addr = ip_hdr(skb)->daddr; 298 ireq->rmt_addr = ip_hdr(skb)->saddr; 299 ireq->ecn_ok = 0; 300 ireq->snd_wscale = tcp_opt.snd_wscale; 301 ireq->rcv_wscale = tcp_opt.rcv_wscale; 302 ireq->sack_ok = tcp_opt.sack_ok; 303 ireq->wscale_ok = tcp_opt.wscale_ok; 304 ireq->tstamp_ok = tcp_opt.saw_tstamp; 305 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0; 306 307 /* We throwed the options of the initial SYN away, so we hope 308 * the ACK carries the same options again (see RFC1122 4.2.3.8) 309 */ 310 if (opt && opt->optlen) { 311 int opt_size = sizeof(struct ip_options) + opt->optlen; 312 313 ireq->opt = kmalloc(opt_size, GFP_ATOMIC); 314 if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) { 315 kfree(ireq->opt); 316 ireq->opt = NULL; 317 } 318 } 319 320 if (security_inet_conn_request(sk, skb, req)) { 321 reqsk_free(req); 322 goto out; 323 } 324 325 req->expires = 0UL; 326 req->retrans = 0; 327 328 /* 329 * We need to lookup the route here to get at the correct 330 * window size. We should better make sure that the window size 331 * hasn't changed since we received the original syn, but I see 332 * no easy way to do this. 333 */ 334 { 335 struct flowi fl = { .nl_u = { .ip4_u = 336 { .daddr = ((opt && opt->srr) ? 337 opt->faddr : 338 ireq->rmt_addr), 339 .saddr = ireq->loc_addr, 340 .tos = RT_CONN_FLAGS(sk) } }, 341 .proto = IPPROTO_TCP, 342 .flags = inet_sk_flowi_flags(sk), 343 .uli_u = { .ports = 344 { .sport = th->dest, 345 .dport = th->source } } }; 346 security_req_classify_flow(req, &fl); 347 if (ip_route_output_key(&init_net, &rt, &fl)) { 348 reqsk_free(req); 349 goto out; 350 } 351 } 352 353 /* Try to redo what tcp_v4_send_synack did. */ 354 req->window_clamp = tp->window_clamp ? :dst_metric(&rt->u.dst, RTAX_WINDOW); 355 356 tcp_select_initial_window(tcp_full_space(sk), req->mss, 357 &req->rcv_wnd, &req->window_clamp, 358 ireq->wscale_ok, &rcv_wscale); 359 360 ireq->rcv_wscale = rcv_wscale; 361 362 ret = get_cookie_sock(sk, skb, req, &rt->u.dst); 363 out: return ret; 364 } 365