| /* |
| * Syncookies implementation for the Linux kernel |
| * |
| * Copyright (C) 1997 Andi Kleen |
| * Based on ideas by D.J.Bernstein and Eric Schenk. |
| * |
| * This program is free software; you can redistribute it and/or |
| * modify it under the terms of the GNU General Public License |
| * as published by the Free Software Foundation; either version |
| * 2 of the License, or (at your option) any later version. |
| */ |
| |
| #include <linux/tcp.h> |
| #include <linux/slab.h> |
| #include <linux/random.h> |
| #include <linux/siphash.h> |
| #include <linux/kernel.h> |
| #include <linux/export.h> |
| #include <net/secure_seq.h> |
| #include <net/tcp.h> |
| #include <net/route.h> |
| |
| static siphash_key_t syncookie_secret[2] __read_mostly; |
| |
| #define COOKIEBITS 24 /* Upper bits store count */ |
| #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) |
| |
| /* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK |
| * stores TCP options: |
| * |
| * MSB LSB |
| * | 31 ... 6 | 5 | 4 | 3 2 1 0 | |
| * | Timestamp | ECN | SACK | WScale | |
| * |
| * When we receive a valid cookie-ACK, we look at the echoed tsval (if |
| * any) to figure out which TCP options we should use for the rebuilt |
| * connection. |
| * |
| * A WScale setting of '0xf' (which is an invalid scaling value) |
| * means that original syn did not include the TCP window scaling option. |
| */ |
| #define TS_OPT_WSCALE_MASK 0xf |
| #define TS_OPT_SACK BIT(4) |
| #define TS_OPT_ECN BIT(5) |
| /* There is no TS_OPT_TIMESTAMP: |
| * if ACK contains timestamp option, we already know it was |
| * requested/supported by the syn/synack exchange. |
| */ |
| #define TSBITS 6 |
| #define TSMASK (((__u32)1 << TSBITS) - 1) |
| |
| static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport, |
| u32 count, int c) |
| { |
| net_get_random_once(syncookie_secret, sizeof(syncookie_secret)); |
| return siphash_4u32((__force u32)saddr, (__force u32)daddr, |
| (__force u32)sport << 16 | (__force u32)dport, |
| count, &syncookie_secret[c]); |
| } |
| |
| |
| /* |
| * when syncookies are in effect and tcp timestamps are enabled we encode |
| * tcp options in the lower bits of the timestamp value that will be |
| * sent in the syn-ack. |
| * Since subsequent timestamps use the normal tcp_time_stamp value, we |
| * must make sure that the resulting initial timestamp is <= tcp_time_stamp. |
| */ |
| u64 cookie_init_timestamp(struct request_sock *req) |
| { |
| struct inet_request_sock *ireq; |
| u32 ts, ts_now = tcp_time_stamp_raw(); |
| u32 options = 0; |
| |
| ireq = inet_rsk(req); |
| |
| options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK; |
| if (ireq->sack_ok) |
| options |= TS_OPT_SACK; |
| if (ireq->ecn_ok) |
| options |= TS_OPT_ECN; |
| |
| ts = ts_now & ~TSMASK; |
| ts |= options; |
| if (ts > ts_now) { |
| ts >>= TSBITS; |
| ts--; |
| ts <<= TSBITS; |
| ts |= options; |
| } |
| return (u64)ts * (NSEC_PER_SEC / TCP_TS_HZ); |
| } |
| |
| |
| static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, |
| __be16 dport, __u32 sseq, __u32 data) |
| { |
| /* |
| * Compute the secure sequence number. |
| * The output should be: |
| * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24) |
| * + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24). |
| * Where sseq is their sequence number and count increases every |
| * minute by 1. |
| * As an extra hack, we add a small "data" value that encodes the |
| * MSS into the second hash value. |
| */ |
| u32 count = tcp_cookie_time(); |
| return (cookie_hash(saddr, daddr, sport, dport, 0, 0) + |
| sseq + (count << COOKIEBITS) + |
| ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data) |
| & COOKIEMASK)); |
| } |
| |
| /* |
| * This retrieves the small "data" value from the syncookie. |
| * If the syncookie is bad, the data returned will be out of |
| * range. This must be checked by the caller. |
| * |
| * The count value used to generate the cookie must be less than |
| * MAX_SYNCOOKIE_AGE minutes in the past. |
| * The return value (__u32)-1 if this test fails. |
| */ |
| static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr, |
| __be16 sport, __be16 dport, __u32 sseq) |
| { |
| u32 diff, count = tcp_cookie_time(); |
| |
| /* Strip away the layers from the cookie */ |
| cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; |
| |
| /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */ |
| diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS); |
| if (diff >= MAX_SYNCOOKIE_AGE) |
| return (__u32)-1; |
| |
| return (cookie - |
| cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) |
| & COOKIEMASK; /* Leaving the data behind */ |
| } |
| |
| /* |
| * MSS Values are chosen based on the 2011 paper |
| * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson. |
| * Values .. |
| * .. lower than 536 are rare (< 0.2%) |
| * .. between 537 and 1299 account for less than < 1.5% of observed values |
| * .. in the 1300-1349 range account for about 15 to 20% of observed mss values |
| * .. exceeding 1460 are very rare (< 0.04%) |
| * |
| * 1460 is the single most frequently announced mss value (30 to 46% depending |
| * on monitor location). Table must be sorted. |
| */ |
| static __u16 const msstab[] = { |
| 536, |
| 1300, |
| 1440, /* 1440, 1452: PPPoE */ |
| 1460, |
| }; |
| |
| /* |
| * Generate a syncookie. mssp points to the mss, which is returned |
| * rounded down to the value encoded in the cookie. |
| */ |
| u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th, |
| u16 *mssp) |
| { |
| int mssind; |
| const __u16 mss = *mssp; |
| |
| for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--) |
| if (mss >= msstab[mssind]) |
| break; |
| *mssp = msstab[mssind]; |
| |
| return secure_tcp_syn_cookie(iph->saddr, iph->daddr, |
| th->source, th->dest, ntohl(th->seq), |
| mssind); |
| } |
| EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence); |
| |
| __u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp) |
| { |
| const struct iphdr *iph = ip_hdr(skb); |
| const struct tcphdr *th = tcp_hdr(skb); |
| |
| return __cookie_v4_init_sequence(iph, th, mssp); |
| } |
| |
| /* |
| * Check if a ack sequence number is a valid syncookie. |
| * Return the decoded mss if it is, or 0 if not. |
| */ |
| int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th, |
| u32 cookie) |
| { |
| __u32 seq = ntohl(th->seq) - 1; |
| __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, |
| th->source, th->dest, seq); |
| |
| return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; |
| } |
| EXPORT_SYMBOL_GPL(__cookie_v4_check); |
| |
| struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb, |
| struct request_sock *req, |
| struct dst_entry *dst, u32 tsoff) |
| { |
| struct inet_connection_sock *icsk = inet_csk(sk); |
| struct sock *child; |
| bool own_req; |
| |
| child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst, |
| NULL, &own_req); |
| if (child) { |
| refcount_set(&req->rsk_refcnt, 1); |
| tcp_sk(child)->tsoffset = tsoff; |
| sock_rps_save_rxhash(child, skb); |
| if (inet_csk_reqsk_queue_add(sk, req, child)) |
| return child; |
| |
| bh_unlock_sock(child); |
| sock_put(child); |
| } |
| __reqsk_free(req); |
| |
| return NULL; |
| } |
| EXPORT_SYMBOL(tcp_get_cookie_sock); |
| |
| /* |
| * when syncookies are in effect and tcp timestamps are enabled we stored |
| * additional tcp options in the timestamp. |
| * This extracts these options from the timestamp echo. |
| * |
| * return false if we decode a tcp option that is disabled |
| * on the host. |
| */ |
| bool cookie_timestamp_decode(const struct net *net, |
| struct tcp_options_received *tcp_opt) |
| { |
| /* echoed timestamp, lowest bits contain options */ |
| u32 options = tcp_opt->rcv_tsecr; |
| |
| if (!tcp_opt->saw_tstamp) { |
| tcp_clear_options(tcp_opt); |
| return true; |
| } |
| |
| if (!net->ipv4.sysctl_tcp_timestamps) |
| return false; |
| |
| tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0; |
| |
| if (tcp_opt->sack_ok && !net->ipv4.sysctl_tcp_sack) |
| return false; |
| |
| if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK) |
| return true; /* no window scaling */ |
| |
| tcp_opt->wscale_ok = 1; |
| tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK; |
| |
| return net->ipv4.sysctl_tcp_window_scaling != 0; |
| } |
| EXPORT_SYMBOL(cookie_timestamp_decode); |
| |
| bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt, |
| const struct net *net, const struct dst_entry *dst) |
| { |
| bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN; |
| |
| if (!ecn_ok) |
| return false; |
| |
| if (net->ipv4.sysctl_tcp_ecn) |
| return true; |
| |
| return dst_feature(dst, RTAX_FEATURE_ECN); |
| } |
| EXPORT_SYMBOL(cookie_ecn_ok); |
| |
| /* On input, sk is a listener. |
| * Output is listener if incoming packet would not create a child |
| * NULL if memory could not be allocated. |
| */ |
| struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb) |
| { |
| struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt; |
| struct tcp_options_received tcp_opt; |
| struct inet_request_sock *ireq; |
| struct tcp_request_sock *treq; |
| struct tcp_sock *tp = tcp_sk(sk); |
| const struct tcphdr *th = tcp_hdr(skb); |
| __u32 cookie = ntohl(th->ack_seq) - 1; |
| struct sock *ret = sk; |
| struct request_sock *req; |
| int mss; |
| struct rtable *rt; |
| __u8 rcv_wscale; |
| struct flowi4 fl4; |
| u32 tsoff = 0; |
| |
| if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst) |
| goto out; |
| |
| if (tcp_synq_no_recent_overflow(sk)) |
| goto out; |
| |
| mss = __cookie_v4_check(ip_hdr(skb), th, cookie); |
| if (mss == 0) { |
| __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); |
| goto out; |
| } |
| |
| __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); |
| |
| /* check for timestamp cookie support */ |
| memset(&tcp_opt, 0, sizeof(tcp_opt)); |
| tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL); |
| |
| if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) { |
| tsoff = secure_tcp_ts_off(sock_net(sk), |
| ip_hdr(skb)->daddr, |
| ip_hdr(skb)->saddr); |
| tcp_opt.rcv_tsecr -= tsoff; |
| } |
| |
| if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt)) |
| goto out; |
| |
| ret = NULL; |
| req = inet_reqsk_alloc(&tcp_request_sock_ops, sk, false); /* for safety */ |
| if (!req) |
| goto out; |
| |
| ireq = inet_rsk(req); |
| treq = tcp_rsk(req); |
| treq->rcv_isn = ntohl(th->seq) - 1; |
| treq->snt_isn = cookie; |
| treq->ts_off = 0; |
| treq->txhash = net_tx_rndhash(); |
| req->mss = mss; |
| ireq->ir_num = ntohs(th->dest); |
| ireq->ir_rmt_port = th->source; |
| sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr); |
| sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr); |
| ireq->ir_mark = inet_request_mark(sk, skb); |
| ireq->snd_wscale = tcp_opt.snd_wscale; |
| ireq->sack_ok = tcp_opt.sack_ok; |
| ireq->wscale_ok = tcp_opt.wscale_ok; |
| ireq->tstamp_ok = tcp_opt.saw_tstamp; |
| req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0; |
| treq->snt_synack = 0; |
| treq->tfo_listener = false; |
| if (IS_ENABLED(CONFIG_SMC)) |
| ireq->smc_ok = 0; |
| |
| ireq->ir_iif = inet_request_bound_dev_if(sk, skb); |
| |
| /* We throwed the options of the initial SYN away, so we hope |
| * the ACK carries the same options again (see RFC1122 4.2.3.8) |
| */ |
| RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb)); |
| |
| if (security_inet_conn_request(sk, skb, req)) { |
| reqsk_free(req); |
| goto out; |
| } |
| |
| req->num_retrans = 0; |
| |
| /* |
| * We need to lookup the route here to get at the correct |
| * window size. We should better make sure that the window size |
| * hasn't changed since we received the original syn, but I see |
| * no easy way to do this. |
| */ |
| flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark, |
| RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP, |
| inet_sk_flowi_flags(sk), |
| opt->srr ? opt->faddr : ireq->ir_rmt_addr, |
| ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid); |
| security_req_classify_flow(req, flowi4_to_flowi(&fl4)); |
| rt = ip_route_output_key(sock_net(sk), &fl4); |
| if (IS_ERR(rt)) { |
| reqsk_free(req); |
| goto out; |
| } |
| |
| /* Try to redo what tcp_v4_send_synack did. */ |
| req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW); |
| |
| tcp_select_initial_window(sk, tcp_full_space(sk), req->mss, |
| &req->rsk_rcv_wnd, &req->rsk_window_clamp, |
| ireq->wscale_ok, &rcv_wscale, |
| dst_metric(&rt->dst, RTAX_INITRWND)); |
| |
| ireq->rcv_wscale = rcv_wscale; |
| ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst); |
| |
| ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff); |
| /* ip_queue_xmit() depends on our flow being setup |
| * Normal sockets get it right from inet_csk_route_child_sock() |
| */ |
| if (ret) |
| inet_sk(ret)->cork.fl.u.ip4 = fl4; |
| out: return ret; |
| } |