net/ipv4/syncookies.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-or-later
 /*
  *  Syncookies implementation for the Linux kernel
  *
  *  Copyright (C) 1997 Andi Kleen
  *  Based on ideas by D.J.Bernstein and Eric Schenk.
  */

 #include <linux/tcp.h>
 #include <linux/siphash.h>
 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <net/secure_seq.h>
 #include <net/tcp.h>
 #include <net/route.h>

 static siphash_aligned_key_t syncookie_secret[2];

 #define COOKIEBITS 24	/* Upper bits store count */
 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)

 /* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
  * stores TCP options:
  *
  * MSB                               LSB
  * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
  * |  Timestamp | ECN | SACK | WScale  |
  *
  * When we receive a valid cookie-ACK, we look at the echoed tsval (if
  * any) to figure out which TCP options we should use for the rebuilt
  * connection.
  *
  * A WScale setting of '0xf' (which is an invalid scaling value)
  * means that original syn did not include the TCP window scaling option.
  */
 #define TS_OPT_WSCALE_MASK	0xf
 #define TS_OPT_SACK		BIT(4)
 #define TS_OPT_ECN		BIT(5)
 /* There is no TS_OPT_TIMESTAMP:
  * if ACK contains timestamp option, we already know it was
  * requested/supported by the syn/synack exchange.
  */
 #define TSBITS	6
 #define TSMASK	(((__u32)1 << TSBITS) - 1)

 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
 		       u32 count, int c)
 {
 	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
 	return siphash_4u32((__force u32)saddr, (__force u32)daddr,
 			    (__force u32)sport << 16 | (__force u32)dport,
 			    count, &syncookie_secret[c]);
 }


 /*
  * 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.
  */
 u64 cookie_init_timestamp(struct request_sock *req, u64 now)
 {
 	struct inet_request_sock *ireq;
 	u32 ts, ts_now = tcp_ns_to_ts(now);
 	u32 options = 0;

 	ireq = inet_rsk(req);

 	options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
 	if (ireq->sack_ok)
 		options |= TS_OPT_SACK;
 	if (ireq->ecn_ok)
 		options |= TS_OPT_ECN;

 	ts = ts_now & ~TSMASK;
 	ts |= options;
 	if (ts > ts_now) {
 		ts >>= TSBITS;
 		ts--;
 		ts <<= TSBITS;
 		ts |= options;
 	}
 	return (u64)ts * (NSEC_PER_SEC / TCP_TS_HZ);
 }


 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(const struct sk_buff *skb, __u16 *mssp)
 {
 	const struct iphdr *iph = ip_hdr(skb);
 	const struct tcphdr *th = tcp_hdr(skb);

 	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);

 struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 				 struct request_sock *req,
 				 struct dst_entry *dst, u32 tsoff)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct sock *child;
 	bool own_req;

 	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
 						 NULL, &own_req);
 	if (child) {
 		refcount_set(&req->rsk_refcnt, 1);
 		tcp_sk(child)->tsoffset = tsoff;
 		sock_rps_save_rxhash(child, skb);

 		if (rsk_drop_req(req)) {
 			reqsk_put(req);
 			return child;
 		}

 		if (inet_csk_reqsk_queue_add(sk, req, child))
 			return child;

 		bh_unlock_sock(child);
 		sock_put(child);
 	}
 	__reqsk_free(req);

 	return NULL;
 }
 EXPORT_SYMBOL(tcp_get_cookie_sock);

 /*
  * 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.
  *
  * return false if we decode a tcp option that is disabled
  * on the host.
  */
 bool cookie_timestamp_decode(const struct net *net,
 			     struct tcp_options_received *tcp_opt)
 {
 	/* echoed timestamp, lowest bits contain options */
 	u32 options = tcp_opt->rcv_tsecr;

 	if (!tcp_opt->saw_tstamp)  {
 		tcp_clear_options(tcp_opt);
 		return true;
 	}

 	if (!net->ipv4.sysctl_tcp_timestamps)
 		return false;

 	tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;

 	if (tcp_opt->sack_ok && !net->ipv4.sysctl_tcp_sack)
 		return false;

 	if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
 		return true; /* no window scaling */

 	tcp_opt->wscale_ok = 1;
 	tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;

 	return net->ipv4.sysctl_tcp_window_scaling != 0;
 }
 EXPORT_SYMBOL(cookie_timestamp_decode);

 bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
 		   const struct net *net, const struct dst_entry *dst)
 {
 	bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;

 	if (!ecn_ok)
 		return false;

 	if (net->ipv4.sysctl_tcp_ecn)
 		return true;

 	return dst_feature(dst, RTAX_FEATURE_ECN);
 }
 EXPORT_SYMBOL(cookie_ecn_ok);

 struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
 					    const struct tcp_request_sock_ops *af_ops,
 					    struct sock *sk,
 					    struct sk_buff *skb)
 {
 	struct tcp_request_sock *treq;
 	struct request_sock *req;

 #ifdef CONFIG_MPTCP
 	if (sk_is_mptcp(sk))
 		ops = &mptcp_subflow_request_sock_ops;
 #endif

 	req = inet_reqsk_alloc(ops, sk, false);
 	if (!req)
 		return NULL;

 	treq = tcp_rsk(req);

 	/* treq->af_specific might be used to perform TCP_MD5 lookup */
 	treq->af_specific = af_ops;

 	treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
 #if IS_ENABLED(CONFIG_MPTCP)
 	treq->is_mptcp = sk_is_mptcp(sk);
 	if (treq->is_mptcp) {
 		int err = mptcp_subflow_init_cookie_req(req, sk, skb);

 		if (err) {
 			reqsk_free(req);
 			return NULL;
 		}
 	}
 #endif

 	return req;
 }
 EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);

 /* On input, sk is a listener.
  * Output is listener if incoming packet would not create a child
  *           NULL if memory could not be allocated.
  */
 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
 {
 	struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.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 full_space, mss;
 	struct rtable *rt;
 	__u8 rcv_wscale;
 	struct flowi4 fl4;
 	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_v4_check(ip_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_tcp_ts_off(sock_net(sk),
 					  ip_hdr(skb)->daddr,
 					  ip_hdr(skb)->saddr);
 		tcp_opt.rcv_tsecr -= tsoff;
 	}

 	if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
 		goto out;

 	ret = NULL;
 	req = cookie_tcp_reqsk_alloc(&tcp_request_sock_ops,
 				     &tcp_request_sock_ipv4_ops, sk, skb);
 	if (!req)
 		goto out;

 	ireq = inet_rsk(req);
 	treq = tcp_rsk(req);
 	treq->rcv_isn		= ntohl(th->seq) - 1;
 	treq->snt_isn		= cookie;
 	treq->ts_off		= 0;
 	treq->txhash		= net_tx_rndhash();
 	req->mss		= mss;
 	ireq->ir_num		= ntohs(th->dest);
 	ireq->ir_rmt_port	= th->source;
 	sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
 	sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
 	ireq->ir_mark		= inet_request_mark(sk, skb);
 	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->tfo_listener	= false;

 	if (IS_ENABLED(CONFIG_SMC))
 		ireq->smc_ok = 0;

 	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);

 	/* 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)
 	 */
 	RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb));

 	if (security_inet_conn_request(sk, skb, req)) {
 		reqsk_free(req);
 		goto out;
 	}

 	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, ireq->ir_iif, ireq->ir_mark,
 			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 			   inet_sk_flowi_flags(sk),
 			   opt->srr ? opt->faddr : ireq->ir_rmt_addr,
 			   ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
 	security_req_classify_flow(req, flowi4_to_flowi_common(&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->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->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(&rt->dst, RTAX_INITRWND));

 	ireq->rcv_wscale  = rcv_wscale;
 	ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);

 	ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff);
 	/* 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;
 }
	// SPDX-License-Identifier: GPL-2.0-or-later
	/*
	* Syncookies implementation for the Linux kernel
	*
	* Copyright (C) 1997 Andi Kleen
	* Based on ideas by D.J.Bernstein and Eric Schenk.
	*/

	#include <linux/tcp.h>
	#include <linux/siphash.h>
	#include <linux/kernel.h>
	#include <linux/export.h>
	#include <net/secure_seq.h>
	#include <net/tcp.h>
	#include <net/route.h>

	static siphash_aligned_key_t syncookie_secret[2];

	#define COOKIEBITS 24 /* Upper bits store count */
	#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)

	/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
	* stores TCP options:
	*
	* MSB LSB
	* \| 31 ... 6 \| 5 \| 4 \| 3 2 1 0 \|
	* \| Timestamp \| ECN \| SACK \| WScale \|
	*
	* When we receive a valid cookie-ACK, we look at the echoed tsval (if
	* any) to figure out which TCP options we should use for the rebuilt
	* connection.
	*
	* A WScale setting of '0xf' (which is an invalid scaling value)
	* means that original syn did not include the TCP window scaling option.
	*/
	#define TS_OPT_WSCALE_MASK 0xf
	#define TS_OPT_SACK BIT(4)
	#define TS_OPT_ECN BIT(5)
	/* There is no TS_OPT_TIMESTAMP:
	* if ACK contains timestamp option, we already know it was
	* requested/supported by the syn/synack exchange.
	*/
	#define TSBITS 6
	#define TSMASK (((__u32)1 << TSBITS) - 1)

	static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
	u32 count, int c)
	{
	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
	return siphash_4u32((__force u32)saddr, (__force u32)daddr,
	(__force u32)sport << 16 \| (__force u32)dport,
	count, &syncookie_secret[c]);
	}


	/*
	* 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.
	*/
	u64 cookie_init_timestamp(struct request_sock *req, u64 now)
	{
	struct inet_request_sock *ireq;
	u32 ts, ts_now = tcp_ns_to_ts(now);
	u32 options = 0;

	ireq = inet_rsk(req);

	options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
	if (ireq->sack_ok)
	options \|= TS_OPT_SACK;
	if (ireq->ecn_ok)
	options \|= TS_OPT_ECN;

	ts = ts_now & ~TSMASK;
	ts \|= options;
	if (ts > ts_now) {
	ts >>= TSBITS;
	ts--;
	ts <<= TSBITS;
	ts \|= options;
	}
	return (u64)ts * (NSEC_PER_SEC / TCP_TS_HZ);
	}


	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(const struct sk_buff skb, __u16 mssp)
	{
	const struct iphdr *iph = ip_hdr(skb);
	const struct tcphdr *th = tcp_hdr(skb);

	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);

	struct sock tcp_get_cookie_sock(struct sock sk, struct sk_buff *skb,
	struct request_sock *req,
	struct dst_entry *dst, u32 tsoff)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct sock *child;
	bool own_req;

	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
	NULL, &own_req);
	if (child) {
	refcount_set(&req->rsk_refcnt, 1);
	tcp_sk(child)->tsoffset = tsoff;
	sock_rps_save_rxhash(child, skb);

	if (rsk_drop_req(req)) {
	reqsk_put(req);
	return child;
	}

	if (inet_csk_reqsk_queue_add(sk, req, child))
	return child;

	bh_unlock_sock(child);
	sock_put(child);
	}
	__reqsk_free(req);

	return NULL;
	}
	EXPORT_SYMBOL(tcp_get_cookie_sock);

	/*
	* 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.
	*
	* return false if we decode a tcp option that is disabled
	* on the host.
	*/
	bool cookie_timestamp_decode(const struct net *net,
	struct tcp_options_received *tcp_opt)
	{
	/* echoed timestamp, lowest bits contain options */
	u32 options = tcp_opt->rcv_tsecr;

	if (!tcp_opt->saw_tstamp) {
	tcp_clear_options(tcp_opt);
	return true;
	}

	if (!net->ipv4.sysctl_tcp_timestamps)
	return false;

	tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;

	if (tcp_opt->sack_ok && !net->ipv4.sysctl_tcp_sack)
	return false;

	if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
	return true; /* no window scaling */

	tcp_opt->wscale_ok = 1;
	tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;

	return net->ipv4.sysctl_tcp_window_scaling != 0;
	}
	EXPORT_SYMBOL(cookie_timestamp_decode);

	bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
	const struct net net, const struct dst_entry dst)
	{
	bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;

	if (!ecn_ok)
	return false;

	if (net->ipv4.sysctl_tcp_ecn)
	return true;

	return dst_feature(dst, RTAX_FEATURE_ECN);
	}
	EXPORT_SYMBOL(cookie_ecn_ok);

	struct request_sock cookie_tcp_reqsk_alloc(const struct request_sock_ops ops,
	const struct tcp_request_sock_ops *af_ops,
	struct sock *sk,
	struct sk_buff *skb)
	{
	struct tcp_request_sock *treq;
	struct request_sock *req;

	#ifdef CONFIG_MPTCP
	if (sk_is_mptcp(sk))
	ops = &mptcp_subflow_request_sock_ops;
	#endif

	req = inet_reqsk_alloc(ops, sk, false);
	if (!req)
	return NULL;

	treq = tcp_rsk(req);

	/* treq->af_specific might be used to perform TCP_MD5 lookup */
	treq->af_specific = af_ops;

	treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
	#if IS_ENABLED(CONFIG_MPTCP)
	treq->is_mptcp = sk_is_mptcp(sk);
	if (treq->is_mptcp) {
	int err = mptcp_subflow_init_cookie_req(req, sk, skb);

	if (err) {
	reqsk_free(req);
	return NULL;
	}
	}
	#endif

	return req;
	}
	EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);

	/* On input, sk is a listener.
	* Output is listener if incoming packet would not create a child
	* NULL if memory could not be allocated.
	*/
	struct sock cookie_v4_check(struct sock sk, struct sk_buff *skb)
	{
	struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.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 full_space, mss;
	struct rtable *rt;
	__u8 rcv_wscale;
	struct flowi4 fl4;
	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_v4_check(ip_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_tcp_ts_off(sock_net(sk),
	ip_hdr(skb)->daddr,
	ip_hdr(skb)->saddr);
	tcp_opt.rcv_tsecr -= tsoff;
	}

	if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
	goto out;

	ret = NULL;
	req = cookie_tcp_reqsk_alloc(&tcp_request_sock_ops,
	&tcp_request_sock_ipv4_ops, sk, skb);
	if (!req)
	goto out;

	ireq = inet_rsk(req);
	treq = tcp_rsk(req);
	treq->rcv_isn = ntohl(th->seq) - 1;
	treq->snt_isn = cookie;
	treq->ts_off = 0;
	treq->txhash = net_tx_rndhash();
	req->mss = mss;
	ireq->ir_num = ntohs(th->dest);
	ireq->ir_rmt_port = th->source;
	sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
	sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
	ireq->ir_mark = inet_request_mark(sk, skb);
	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->tfo_listener = false;

	if (IS_ENABLED(CONFIG_SMC))
	ireq->smc_ok = 0;

	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);

	/* 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)
	*/
	RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb));

	if (security_inet_conn_request(sk, skb, req)) {
	reqsk_free(req);
	goto out;
	}

	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, ireq->ir_iif, ireq->ir_mark,
	RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
	inet_sk_flowi_flags(sk),
	opt->srr ? opt->faddr : ireq->ir_rmt_addr,
	ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
	security_req_classify_flow(req, flowi4_to_flowi_common(&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->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->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(&rt->dst, RTAX_INITRWND));

	ireq->rcv_wscale = rcv_wscale;
	ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);

	ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff);
	/* 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;
	}