text: // SPDX-License-Identifier: GPL-2.0-or-later
  /*
   *  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
   */
  
  #include <linux/tcp.h>
  #include <linux/random.h>
  #include <linux/siphash.h>
  #include <linux/kernel.h>
  #include <net/secure_seq.h>
  #include <net/ipv6.h>
  #include <net/tcp.h>
  
  #define COOKIEBITS 24	/* Upper bits store count */
  #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  
  static siphash_aligned_key_t syncookie6_secret[2];
  
  /* 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 u32 cookie_hash(const struct in6_addr *saddr,
  		       const struct in6_addr *daddr,
  		       __be16 sport, __be16 dport, u32 count, int c)
  {
  	const struct {
  		struct in6_addr saddr;
  		struct in6_addr daddr;
  		u32 count;
  		__be16 sport;
  		__be16 dport;
  	} __aligned(SIPHASH_ALIGNMENT) combined = {
  		.saddr = *saddr,
  		.daddr = *daddr,
  		.count = count,
  		.sport = sport,
  		.dport = dport
  	};
  
  	net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
  	return siphash(&combined, offsetofend(typeof(combined), dport),
  		       &syncookie6_secret[c]);
  }
  
  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(const struct sk_buff *skb, __u16 *mssp)
  {
  	const struct ipv6hdr *iph = ipv6_hdr(skb);
  	const struct tcphdr *th = tcp_hdr(skb);
  
  	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 full_space, mss;
  	struct dst_entry *dst;
  	__u8 rcv_wscale;
  	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_v6_check(ipv6_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_tcpv6_ts_off(sock_net(sk),
  					    ipv6_hdr(skb)->daddr.s6_addr32,
  					    ipv6_hdr(skb)->saddr.s6_addr32);
  		tcp_opt.rcv_tsecr -= tsoff;
  	}
  
  	if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
  		goto out;
  
  	ret = NULL;
  	req = cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb);
  	if (!req)
  		goto out;
  
  	ireq = inet_rsk(req);
  	treq = tcp_rsk(req);
  	treq->tfo_listener = false;
  
  	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, &TCP_SKB_CB(skb)->header.h6) ||
  	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
  	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
  		refcount_inc(&skb->users);
  		ireq->pktopts = skb;
  	}
  
  	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
  	/* 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 = tcp_v6_iif(skb);
  
  	ireq->ir_mark = inet_request_mark(sk, skb);
  
  	req->num_retrans = 0;
  	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->rcv_isn = ntohl(th->seq) - 1;
  	treq->snt_isn = cookie;
  	treq->ts_off = 0;
  	treq->txhash = net_tx_rndhash();
  	if (IS_ENABLED(CONFIG_SMC))
  		ireq->smc_ok = 0;
  
  	/*
  	 * 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, rcu_dereference(np->opt), &final);
  		fl6.saddr = ireq->ir_v6_loc_addr;
  		fl6.flowi6_oif = ireq->ir_iif;
  		fl6.flowi6_mark = ireq->ir_mark;
  		fl6.fl6_dport = ireq->ir_rmt_port;
  		fl6.fl6_sport = inet_sk(sk)->inet_sport;
  		fl6.flowi6_uid = sk->sk_uid;
  		security_req_classify_flow(req, flowi6_to_flowi_common(&fl6));
  
  		dst = ip6_dst_lookup_flow(sock_net(sk), sk, &fl6, final_p);
  		if (IS_ERR(dst))
  			goto out_free;
  	}
  
  	req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
  	/* limit the window selection if the user enforce a smaller rx buffer */
  	full_space = tcp_full_space(sk);
  	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
  	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
  		req->rsk_window_clamp = full_space;
  
  	tcp_select_initial_window(sk, full_space, req->mss,
  				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
  				  ireq->wscale_ok, &rcv_wscale,
  				  dst_metric(dst, RTAX_INITRWND));
  
  	ireq->rcv_wscale = rcv_wscale;
  	ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
  
  	ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
  out:
  	return ret;
  out_free:
  	reqsk_free(req);
  	return NULL;
  }
  