syncookies.c 7.4 KB

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  1. // SPDX-License-Identifier: GPL-2.0-or-later
  2. /*
  3. * IPv6 Syncookies implementation for the Linux kernel
  4. *
  5. * Authors:
  6. * Glenn Griffin <[email protected]>
  7. *
  8. * Based on IPv4 implementation by Andi Kleen
  9. * linux/net/ipv4/syncookies.c
  10. */
  11. #include <linux/tcp.h>
  12. #include <linux/random.h>
  13. #include <linux/siphash.h>
  14. #include <linux/kernel.h>
  15. #include <net/secure_seq.h>
  16. #include <net/ipv6.h>
  17. #include <net/tcp.h>
  18. #define COOKIEBITS 24 /* Upper bits store count */
  19. #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  20. static siphash_aligned_key_t syncookie6_secret[2];
  21. /* RFC 2460, Section 8.3:
  22. * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
  23. *
  24. * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
  25. * using higher values than ipv4 tcp syncookies.
  26. * The other values are chosen based on ethernet (1500 and 9k MTU), plus
  27. * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
  28. */
  29. static __u16 const msstab[] = {
  30. 1280 - 60, /* IPV6_MIN_MTU - 60 */
  31. 1480 - 60,
  32. 1500 - 60,
  33. 9000 - 60,
  34. };
  35. static u32 cookie_hash(const struct in6_addr *saddr,
  36. const struct in6_addr *daddr,
  37. __be16 sport, __be16 dport, u32 count, int c)
  38. {
  39. const struct {
  40. struct in6_addr saddr;
  41. struct in6_addr daddr;
  42. u32 count;
  43. __be16 sport;
  44. __be16 dport;
  45. } __aligned(SIPHASH_ALIGNMENT) combined = {
  46. .saddr = *saddr,
  47. .daddr = *daddr,
  48. .count = count,
  49. .sport = sport,
  50. .dport = dport
  51. };
  52. net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
  53. return siphash(&combined, offsetofend(typeof(combined), dport),
  54. &syncookie6_secret[c]);
  55. }
  56. static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  57. const struct in6_addr *daddr,
  58. __be16 sport, __be16 dport, __u32 sseq,
  59. __u32 data)
  60. {
  61. u32 count = tcp_cookie_time();
  62. return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  63. sseq + (count << COOKIEBITS) +
  64. ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  65. & COOKIEMASK));
  66. }
  67. static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
  68. const struct in6_addr *daddr, __be16 sport,
  69. __be16 dport, __u32 sseq)
  70. {
  71. __u32 diff, count = tcp_cookie_time();
  72. cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
  73. diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
  74. if (diff >= MAX_SYNCOOKIE_AGE)
  75. return (__u32)-1;
  76. return (cookie -
  77. cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
  78. & COOKIEMASK;
  79. }
  80. u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
  81. const struct tcphdr *th, __u16 *mssp)
  82. {
  83. int mssind;
  84. const __u16 mss = *mssp;
  85. for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
  86. if (mss >= msstab[mssind])
  87. break;
  88. *mssp = msstab[mssind];
  89. return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
  90. th->dest, ntohl(th->seq), mssind);
  91. }
  92. EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
  93. __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
  94. {
  95. const struct ipv6hdr *iph = ipv6_hdr(skb);
  96. const struct tcphdr *th = tcp_hdr(skb);
  97. return __cookie_v6_init_sequence(iph, th, mssp);
  98. }
  99. int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
  100. __u32 cookie)
  101. {
  102. __u32 seq = ntohl(th->seq) - 1;
  103. __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
  104. th->source, th->dest, seq);
  105. return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
  106. }
  107. EXPORT_SYMBOL_GPL(__cookie_v6_check);
  108. struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
  109. {
  110. struct tcp_options_received tcp_opt;
  111. struct inet_request_sock *ireq;
  112. struct tcp_request_sock *treq;
  113. struct ipv6_pinfo *np = inet6_sk(sk);
  114. struct tcp_sock *tp = tcp_sk(sk);
  115. const struct tcphdr *th = tcp_hdr(skb);
  116. __u32 cookie = ntohl(th->ack_seq) - 1;
  117. struct sock *ret = sk;
  118. struct request_sock *req;
  119. int full_space, mss;
  120. struct dst_entry *dst;
  121. __u8 rcv_wscale;
  122. u32 tsoff = 0;
  123. if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_syncookies) ||
  124. !th->ack || th->rst)
  125. goto out;
  126. if (tcp_synq_no_recent_overflow(sk))
  127. goto out;
  128. mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
  129. if (mss == 0) {
  130. __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
  131. goto out;
  132. }
  133. __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
  134. /* check for timestamp cookie support */
  135. memset(&tcp_opt, 0, sizeof(tcp_opt));
  136. tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
  137. if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
  138. tsoff = secure_tcpv6_ts_off(sock_net(sk),
  139. ipv6_hdr(skb)->daddr.s6_addr32,
  140. ipv6_hdr(skb)->saddr.s6_addr32);
  141. tcp_opt.rcv_tsecr -= tsoff;
  142. }
  143. if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
  144. goto out;
  145. ret = NULL;
  146. req = cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops,
  147. &tcp_request_sock_ipv6_ops, sk, skb);
  148. if (!req)
  149. goto out;
  150. ireq = inet_rsk(req);
  151. treq = tcp_rsk(req);
  152. treq->tfo_listener = false;
  153. req->mss = mss;
  154. ireq->ir_rmt_port = th->source;
  155. ireq->ir_num = ntohs(th->dest);
  156. ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
  157. ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
  158. if (security_inet_conn_request(sk, skb, req))
  159. goto out_free;
  160. if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
  161. np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
  162. np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
  163. refcount_inc(&skb->users);
  164. ireq->pktopts = skb;
  165. }
  166. ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
  167. /* So that link locals have meaning */
  168. if (!sk->sk_bound_dev_if &&
  169. ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
  170. ireq->ir_iif = tcp_v6_iif(skb);
  171. ireq->ir_mark = inet_request_mark(sk, skb);
  172. req->num_retrans = 0;
  173. ireq->snd_wscale = tcp_opt.snd_wscale;
  174. ireq->sack_ok = tcp_opt.sack_ok;
  175. ireq->wscale_ok = tcp_opt.wscale_ok;
  176. ireq->tstamp_ok = tcp_opt.saw_tstamp;
  177. req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
  178. treq->snt_synack = 0;
  179. treq->rcv_isn = ntohl(th->seq) - 1;
  180. treq->snt_isn = cookie;
  181. treq->ts_off = 0;
  182. treq->txhash = net_tx_rndhash();
  183. if (IS_ENABLED(CONFIG_SMC))
  184. ireq->smc_ok = 0;
  185. /*
  186. * We need to lookup the dst_entry to get the correct window size.
  187. * This is taken from tcp_v6_syn_recv_sock. Somebody please enlighten
  188. * me if there is a preferred way.
  189. */
  190. {
  191. struct in6_addr *final_p, final;
  192. struct flowi6 fl6;
  193. memset(&fl6, 0, sizeof(fl6));
  194. fl6.flowi6_proto = IPPROTO_TCP;
  195. fl6.daddr = ireq->ir_v6_rmt_addr;
  196. final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
  197. fl6.saddr = ireq->ir_v6_loc_addr;
  198. fl6.flowi6_oif = ireq->ir_iif;
  199. fl6.flowi6_mark = ireq->ir_mark;
  200. fl6.fl6_dport = ireq->ir_rmt_port;
  201. fl6.fl6_sport = inet_sk(sk)->inet_sport;
  202. fl6.flowi6_uid = sk->sk_uid;
  203. security_req_classify_flow(req, flowi6_to_flowi_common(&fl6));
  204. dst = ip6_dst_lookup_flow(sock_net(sk), sk, &fl6, final_p);
  205. if (IS_ERR(dst))
  206. goto out_free;
  207. }
  208. req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
  209. /* limit the window selection if the user enforce a smaller rx buffer */
  210. full_space = tcp_full_space(sk);
  211. if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
  212. (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
  213. req->rsk_window_clamp = full_space;
  214. tcp_select_initial_window(sk, full_space, req->mss,
  215. &req->rsk_rcv_wnd, &req->rsk_window_clamp,
  216. ireq->wscale_ok, &rcv_wscale,
  217. dst_metric(dst, RTAX_INITRWND));
  218. ireq->rcv_wscale = rcv_wscale;
  219. ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
  220. ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
  221. out:
  222. return ret;
  223. out_free:
  224. reqsk_free(req);
  225. return NULL;
  226. }