/* * 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. * * $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $ * * Missing: IPv6 support. */ #include <linux/tcp.h> #include <linux/slab.h> #include <linux/random.h> #include <linux/cryptohash.h> #include <linux/kernel.h> #include <net/tcp.h> extern int sysctl_tcp_syncookies; static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS]; static __init int init_syncookies(void) { get_random_bytes(syncookie_secret, sizeof(syncookie_secret)); return 0; } module_init(init_syncookies); #define COOKIEBITS 24 /* Upper bits store count */ #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport, u32 count, int c) { __u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS]; memcpy(tmp + 3, 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]; } 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 */ } /* * This table has to be sorted and terminated with (__u16)-1. * XXX generate a better table. * Unresolved Issues: HIPPI with a 64k MSS is not well supported. */ static __u16 const msstab[] = { 64 - 1, 256 - 1, 512 - 1, 536 - 1, 1024 - 1, 1440 - 1, 1460 - 1, 4312 - 1, (__u16)-1 }; /* The number doesn't include the -1 terminator */ #define NUM_MSS (ARRAY_SIZE(msstab) - 1) /* * 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) { struct tcp_sock *tp = tcp_sk(sk); const struct iphdr *iph = ip_hdr(skb); const struct tcphdr *th = tcp_hdr(skb); int mssind; const __u16 mss = *mssp; tp->last_synq_overflow = jiffies; /* XXX sort msstab[] by probability? Binary search? */ for (mssind = 0; mss > msstab[mssind + 1]; mssind++) ; *mssp = msstab[mssind] + 1; NET_INC_STATS_BH(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 < NUM_MSS ? msstab[mssind] + 1 : 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; } struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, struct ip_options *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; if (!sysctl_tcp_syncookies || !th->ack) goto out; if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) || (mss = cookie_check(skb, cookie)) == 0) { NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED); goto out; } NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV); ret = NULL; req = reqsk_alloc(&tcp_request_sock_ops); /* for safety */ if (!req) goto out; if (security_inet_conn_request(sk, skb, req)) { reqsk_free(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->rmt_port = th->source; ireq->loc_addr = ip_hdr(skb)->daddr; ireq->rmt_addr = ip_hdr(skb)->saddr; ireq->opt = NULL; /* 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) + opt->optlen; ireq->opt = kmalloc(opt_size, GFP_ATOMIC); if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) { kfree(ireq->opt); ireq->opt = NULL; } } ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0; ireq->wscale_ok = ireq->sack_ok = 0; 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. */ { struct flowi fl = { .nl_u = { .ip4_u = { .daddr = ((opt && opt->srr) ? opt->faddr : ireq->rmt_addr), .saddr = ireq->loc_addr, .tos = RT_CONN_FLAGS(sk) } }, .proto = IPPROTO_TCP, .uli_u = { .ports = { .sport = th->dest, .dport = th->source } } }; security_req_classify_flow(req, &fl); if (ip_route_output_key(&init_net, &rt, &fl)) { reqsk_free(req); goto out; } } /* Try to redo what tcp_v4_send_synack did. */ req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW); tcp_select_initial_window(tcp_full_space(sk), req->mss, &req->rcv_wnd, &req->window_clamp, 0, &rcv_wscale); /* BTW win scale with syncookies is 0 by definition */ ireq->rcv_wscale = rcv_wscale; ret = get_cookie_sock(sk, skb, req, &rt->u.dst); out: return ret; }