| /* |
| * 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/cryptohash.h> |
| #include <linux/kernel.h> |
| #include <linux/export.h> |
| #include <net/tcp.h> |
| #include <net/route.h> |
| |
| /* Timestamps: lowest bits store TCP options */ |
| #define TSBITS 6 |
| #define TSMASK (((__u32)1 << TSBITS) - 1) |
| |
| extern int sysctl_tcp_syncookies; |
| |
| __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS]; |
| EXPORT_SYMBOL(syncookie_secret); |
| |
| static __init int init_syncookies(void) |
| { |
| get_random_bytes(syncookie_secret, sizeof(syncookie_secret)); |
| return 0; |
| } |
| __initcall(init_syncookies); |
| |
| #define COOKIEBITS 24 /* Upper bits store count */ |
| #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) |
| |
| static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS], |
| ipv4_cookie_scratch); |
| |
| static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport, |
| u32 count, int c) |
| { |
| __u32 *tmp = __get_cpu_var(ipv4_cookie_scratch); |
| |
| memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c])); |
| tmp[0] = (__force u32)saddr; |
| tmp[1] = (__force u32)daddr; |
| tmp[2] = ((__force u32)sport << 16) + (__force u32)dport; |
| tmp[3] = count; |
| sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5); |
| |
| return tmp[17]; |
| } |
| |
| |
| /* |
| * 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. |
| */ |
| __u32 cookie_init_timestamp(struct request_sock *req) |
| { |
| struct inet_request_sock *ireq; |
| u32 ts, ts_now = tcp_time_stamp; |
| u32 options = 0; |
| |
| ireq = inet_rsk(req); |
| |
| options = ireq->wscale_ok ? ireq->snd_wscale : 0xf; |
| options |= ireq->sack_ok << 4; |
| options |= ireq->ecn_ok << 5; |
| |
| ts = ts_now & ~TSMASK; |
| ts |= options; |
| if (ts > ts_now) { |
| ts >>= TSBITS; |
| ts--; |
| ts <<= TSBITS; |
| ts |= options; |
| } |
| return ts; |
| } |
| |
| |
| static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, |
| __be16 dport, __u32 sseq, __u32 count, |
| __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. |
| */ |
| |
| 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 within |
| * "maxdiff" if the current (passed-in) "count". The return value |
| * is (__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 count, __u32 maxdiff) |
| { |
| __u32 diff; |
| |
| /* 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 >= maxdiff) |
| return (__u32)-1; |
| |
| return (cookie - |
| cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) |
| & COOKIEMASK; /* Leaving the data behind */ |
| } |
| |
| /* |
| * MSS Values are taken from the 2009 paper |
| * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson: |
| * - values 1440 to 1460 accounted for 80% of observed mss values |
| * - values outside the 536-1460 range are rare (<0.2%). |
| * |
| * Table must be sorted. |
| */ |
| static __u16 const msstab[] = { |
| 64, |
| 512, |
| 536, |
| 1024, |
| 1440, |
| 1460, |
| 4312, |
| 8960, |
| }; |
| |
| /* |
| * 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(struct sock *sk, struct sk_buff *skb, __u16 *mssp) |
| { |
| const struct iphdr *iph = ip_hdr(skb); |
| const struct tcphdr *th = tcp_hdr(skb); |
| int mssind; |
| const __u16 mss = *mssp; |
| |
| tcp_synq_overflow(sk); |
| |
| for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--) |
| if (mss >= msstab[mssind]) |
| break; |
| *mssp = msstab[mssind]; |
| |
| NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT); |
| |
| return secure_tcp_syn_cookie(iph->saddr, iph->daddr, |
| th->source, th->dest, ntohl(th->seq), |
| jiffies / (HZ * 60), mssind); |
| } |
| |
| /* |
| * This (misnamed) value is the age of syncookie which is permitted. |
| * Its ideal value should be dependent on TCP_TIMEOUT_INIT and |
| * sysctl_tcp_retries1. It's a rather complicated formula (exponential |
| * backoff) to compute at runtime so it's currently hardcoded here. |
| */ |
| #define COUNTER_TRIES 4 |
| /* |
| * Check if a ack sequence number is a valid syncookie. |
| * Return the decoded mss if it is, or 0 if not. |
| */ |
| static inline int cookie_check(struct sk_buff *skb, __u32 cookie) |
| { |
| const struct iphdr *iph = ip_hdr(skb); |
| const struct tcphdr *th = tcp_hdr(skb); |
| __u32 seq = ntohl(th->seq) - 1; |
| __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, |
| th->source, th->dest, seq, |
| jiffies / (HZ * 60), |
| COUNTER_TRIES); |
| |
| return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; |
| } |
| |
| static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, |
| struct request_sock *req, |
| struct dst_entry *dst) |
| { |
| struct inet_connection_sock *icsk = inet_csk(sk); |
| struct sock *child; |
| |
| child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst); |
| if (child) |
| inet_csk_reqsk_queue_add(sk, req, child); |
| else |
| reqsk_free(req); |
| |
| return child; |
| } |
| |
| |
| /* |
| * 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. |
| * |
| * The lowest 4 bits store snd_wscale. |
| * next 2 bits indicate SACK and ECN support. |
| * |
| * return false if we decode an option that should not be. |
| */ |
| bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok) |
| { |
| /* echoed timestamp, lowest bits contain options */ |
| u32 options = tcp_opt->rcv_tsecr & TSMASK; |
| |
| if (!tcp_opt->saw_tstamp) { |
| tcp_clear_options(tcp_opt); |
| return true; |
| } |
| |
| if (!sysctl_tcp_timestamps) |
| return false; |
| |
| tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0; |
| *ecn_ok = (options >> 5) & 1; |
| if (*ecn_ok && !sysctl_tcp_ecn) |
| return false; |
| |
| if (tcp_opt->sack_ok && !sysctl_tcp_sack) |
| return false; |
| |
| if ((options & 0xf) == 0xf) |
| return true; /* no window scaling */ |
| |
| tcp_opt->wscale_ok = 1; |
| tcp_opt->snd_wscale = options & 0xf; |
| return sysctl_tcp_window_scaling != 0; |
| } |
| EXPORT_SYMBOL(cookie_check_timestamp); |
| |
| struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, |
| struct ip_options *opt) |
| { |
| struct tcp_options_received tcp_opt; |
| const u8 *hash_location; |
| 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; |
| bool ecn_ok = false; |
| struct flowi4 fl4; |
| |
| if (!sysctl_tcp_syncookies || !th->ack || th->rst) |
| goto out; |
| |
| if (tcp_synq_no_recent_overflow(sk) || |
| (mss = cookie_check(skb, cookie)) == 0) { |
| NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); |
| goto out; |
| } |
| |
| NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); |
| |
| /* check for timestamp cookie support */ |
| memset(&tcp_opt, 0, sizeof(tcp_opt)); |
| tcp_parse_options(skb, &tcp_opt, &hash_location, 0); |
| |
| if (!cookie_check_timestamp(&tcp_opt, &ecn_ok)) |
| goto out; |
| |
| ret = NULL; |
| req = inet_reqsk_alloc(&tcp_request_sock_ops); /* 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; |
| req->mss = mss; |
| ireq->loc_port = th->dest; |
| ireq->rmt_port = th->source; |
| ireq->loc_addr = ip_hdr(skb)->daddr; |
| ireq->rmt_addr = ip_hdr(skb)->saddr; |
| ireq->ecn_ok = ecn_ok; |
| 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 = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0; |
| |
| /* 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) |
| */ |
| if (opt && opt->optlen) { |
| int opt_size = sizeof(struct ip_options_rcu) + opt->optlen; |
| |
| ireq->opt = kmalloc(opt_size, GFP_ATOMIC); |
| if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) { |
| kfree(ireq->opt); |
| ireq->opt = NULL; |
| } |
| } |
| |
| if (security_inet_conn_request(sk, skb, req)) { |
| reqsk_free(req); |
| goto out; |
| } |
| |
| req->expires = 0UL; |
| req->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, 0, sk->sk_mark, RT_CONN_FLAGS(sk), |
| RT_SCOPE_UNIVERSE, IPPROTO_TCP, |
| inet_sk_flowi_flags(sk), |
| (opt && opt->srr) ? opt->faddr : ireq->rmt_addr, |
| ireq->loc_addr, th->source, th->dest, |
| sock_i_uid(sk)); |
| 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->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW); |
| |
| tcp_select_initial_window(tcp_full_space(sk), req->mss, |
| &req->rcv_wnd, &req->window_clamp, |
| ireq->wscale_ok, &rcv_wscale, |
| dst_metric(&rt->dst, RTAX_INITRWND)); |
| |
| ireq->rcv_wscale = rcv_wscale; |
| |
| ret = get_cookie_sock(sk, skb, req, &rt->dst); |
| /* 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; |
| } |