/*
   *  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;
  
  static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
  
  #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;
  
  	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
  
  	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 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(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
  {
  	const struct iphdr *iph = ip_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_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);
  
  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,
  			struct net *net, 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 && !net->ipv4.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;
  	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_v4_check(ip_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 = 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->ir_num		= ntohs(th->dest);
  	ireq->ir_rmt_port	= th->source;
  	ireq->ir_loc_addr	= ip_hdr(skb)->daddr;
  	ireq->ir_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;
  	treq->listener		= 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_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->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, sk->sk_bound_dev_if, sk->sk_mark,
  			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
  			   inet_sk_flowi_flags(sk),
  			   (opt && opt->srr) ? opt->faddr : ireq->ir_rmt_addr,
  			   ireq->ir_loc_addr, th->source, th->dest);
  	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;
  }