text: /*
   *  IPv6 Syncookies implementation for the Linux kernel
   *
   *  Authors:
   *  Glenn Griffin	<ggriffin.kernel@gmail.com>
   *
   *  Based on IPv4 implementation by Andi Kleen
   *  linux/net/ipv4/syncookies.c
   *
   *	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/random.h>
  #include <linux/cryptohash.h>
  #include <linux/kernel.h>
  #include <net/ipv6.h>
  #include <net/tcp.h>
  
  #define COOKIEBITS 24	/* Upper bits store count */
  #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  
  static u32 syncookie6_secret[2][16-4+SHA_DIGEST_WORDS];
  
  /* RFC 2460, Section 8.3:
   * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
   *
   * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
   * using higher values than ipv4 tcp syncookies.
   * The other values are chosen based on ethernet (1500 and 9k MTU), plus
   * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
   */
  static __u16 const msstab[] = {
  	1280 - 60, /* IPV6_MIN_MTU - 60 */
  	1480 - 60,
  	1500 - 60,
  	9000 - 60,
  };
  
  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;
  }
  
  static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  		      ipv6_cookie_scratch);
  
  static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr,
  		       __be16 sport, __be16 dport, u32 count, int c)
  {
  	__u32 *tmp;
  
  	net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
  
  	tmp  = __get_cpu_var(ipv6_cookie_scratch);
  
  	/*
  	 * we have 320 bits of information to hash, copy in the remaining
  	 * 192 bits required for sha_transform, from the syncookie6_secret
  	 * and overwrite the digest with the secret
  	 */
  	memcpy(tmp + 10, syncookie6_secret[c], 44);
  	memcpy(tmp, saddr, 16);
  	memcpy(tmp + 4, daddr, 16);
  	tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
  	tmp[9] = count;
  	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  
  	return tmp[17];
  }
  
  static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  				   const struct in6_addr *daddr,
  				   __be16 sport, __be16 dport, __u32 sseq,
  				   __u32 data)
  {
  	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));
  }
  
  static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
  				  const struct in6_addr *daddr, __be16 sport,
  				  __be16 dport, __u32 sseq)
  {
  	__u32 diff, count = tcp_cookie_time();
  
  	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
  
  	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;
  }
  
  u32 __cookie_v6_init_sequence(const struct ipv6hdr *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_v6_init_sequence);
  
  __u32 cookie_v6_init_sequence(struct sock *sk, const struct sk_buff *skb, __u16 *mssp)
  {
  	const struct ipv6hdr *iph = ipv6_hdr(skb);
  	const struct tcphdr *th = tcp_hdr(skb);
  
  	tcp_synq_overflow(sk);
  	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
  
  	return __cookie_v6_init_sequence(iph, th, mssp);
  }
  
  int __cookie_v6_check(const struct ipv6hdr *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_v6_check);
  
  struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_options_received tcp_opt;
  	struct inet_request_sock *ireq;
  	struct tcp_request_sock *treq;
  	struct ipv6_pinfo *np = inet6_sk(sk);
  	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 dst_entry *dst;
  	__u8 rcv_wscale;
  	bool ecn_ok = false;
  
  	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
  		goto out;
  
  	if (tcp_synq_no_recent_overflow(sk) ||
  		(mss = __cookie_v6_check(ipv6_hdr(skb), th, 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, 0, NULL);
  
  	if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok))
  		goto out;
  
  	ret = NULL;
  	req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
  	if (!req)
  		goto out;
  
  	ireq = inet_rsk(req);
  	treq = tcp_rsk(req);
  	treq->listener = NULL;
  
  	if (security_inet_conn_request(sk, skb, req))
  		goto out_free;
  
  	req->mss = mss;
  	ireq->ir_rmt_port = th->source;
  	ireq->ir_num = ntohs(th->dest);
  	ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
  	ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
  	if (ipv6_opt_accepted(sk, skb) ||
  	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
  	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
  		atomic_inc(&skb->users);
  		ireq->pktopts = skb;
  	}
  
  	ireq->ir_iif = sk->sk_bound_dev_if;
  	/* So that link locals have meaning */
  	if (!sk->sk_bound_dev_if &&
  	    ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
  		ireq->ir_iif = inet6_iif(skb);
  
  	ireq->ir_mark = inet_request_mark(sk, skb);
  
  	req->expires = 0UL;
  	req->num_retrans = 0;
  	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;
  	treq->rcv_isn = ntohl(th->seq) - 1;
  	treq->snt_isn = cookie;
  
  	/*
  	 * We need to lookup the dst_entry to get the correct window size.
  	 * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
  	 * me if there is a preferred way.
  	 */
  	{
  		struct in6_addr *final_p, final;
  		struct flowi6 fl6;
  		memset(&fl6, 0, sizeof(fl6));
  		fl6.flowi6_proto = IPPROTO_TCP;
  		fl6.daddr = ireq->ir_v6_rmt_addr;
  		final_p = fl6_update_dst(&fl6, np->opt, &final);
  		fl6.saddr = ireq->ir_v6_loc_addr;
  		fl6.flowi6_oif = sk->sk_bound_dev_if;
  		fl6.flowi6_mark = ireq->ir_mark;
  		fl6.fl6_dport = ireq->ir_rmt_port;
  		fl6.fl6_sport = inet_sk(sk)->inet_sport;
  		security_req_classify_flow(req, flowi6_to_flowi(&fl6));
  
  		dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
  		if (IS_ERR(dst))
  			goto out_free;
  	}
  
  	req->window_clamp = tp->window_clamp ? :dst_metric(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(dst, RTAX_INITRWND));
  
  	ireq->rcv_wscale = rcv_wscale;
  
  	ret = get_cookie_sock(sk, skb, req, dst);
  out:
  	return ret;
  out_free:
  	reqsk_free(req);
  	return NULL;
  }
  
  