net/ipv4/syncookies.c - kernel/msm-4.9 - Gitiles

 /*
  *  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;

 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
 EXPORT_SYMBOL(syncookie_secret);

 static __init int init_syncookies(void)
 {
 	get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
 	return 0;
 }
 __initcall(init_syncookies);

 #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 = __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 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 */
 }

 /*
  * MSS Values are taken from the 2009 paper
  * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
  *  - values 1440 to 1460 accounted for 80% of observed mss values
  *  - values outside the 536-1460 range are rare (<0.2%).
  *
  * Table must be sorted.
  */
 static __u16 const msstab[] = {
 	64,
 	512,
 	536,
 	1024,
 	1440,
 	1460,
 	4312,
 	8960,
 };

 /*
  * 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)
 {
 	const struct iphdr *iph = ip_hdr(skb);
 	const struct tcphdr *th = tcp_hdr(skb);
 	int mssind;
 	const __u16 mss = *mssp;

 	tcp_synq_overflow(sk);

 	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 		if (mss >= msstab[mssind])
 			break;
 	*mssp = msstab[mssind];

 	NET_INC_STATS_BH(sock_net(sk), 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 < ARRAY_SIZE(msstab) ? msstab[mssind] : 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;
 }


 /*
  * 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, 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 && !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;
 	const u8 *hash_location;
 	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_check(skb, 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, &hash_location, 0, NULL);

 	if (!cookie_check_timestamp(&tcp_opt, &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->loc_port		= th->dest;
 	ireq->rmt_port		= th->source;
 	ireq->loc_addr		= ip_hdr(skb)->daddr;
 	ireq->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;

 	/* 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->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, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
 			   RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 			   inet_sk_flowi_flags(sk),
 			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
 			   ireq->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;
 }
	/*
	* 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;

	__u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
	EXPORT_SYMBOL(syncookie_secret);

	static __init int init_syncookies(void)
	{
	get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
	return 0;
	}
	__initcall(init_syncookies);

	#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 = __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 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 */
	}

	/*
	* MSS Values are taken from the 2009 paper
	* 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
	* - values 1440 to 1460 accounted for 80% of observed mss values
	* - values outside the 536-1460 range are rare (<0.2%).
	*
	* Table must be sorted.
	*/
	static __u16 const msstab[] = {
	64,
	512,
	536,
	1024,
	1440,
	1460,
	4312,
	8960,
	};

	/*
	* 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)
	{
	const struct iphdr *iph = ip_hdr(skb);
	const struct tcphdr *th = tcp_hdr(skb);
	int mssind;
	const __u16 mss = *mssp;

	tcp_synq_overflow(sk);

	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
	if (mss >= msstab[mssind])
	break;
	*mssp = msstab[mssind];

	NET_INC_STATS_BH(sock_net(sk), 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 < ARRAY_SIZE(msstab) ? msstab[mssind] : 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;
	}


	/*
	* 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, 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 && !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;
	const u8 *hash_location;
	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_check(skb, 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, &hash_location, 0, NULL);

	if (!cookie_check_timestamp(&tcp_opt, &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->loc_port = th->dest;
	ireq->rmt_port = th->source;
	ireq->loc_addr = ip_hdr(skb)->daddr;
	ireq->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;

	/* 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->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, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
	RT_SCOPE_UNIVERSE, IPPROTO_TCP,
	inet_sk_flowi_flags(sk),
	(opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
	ireq->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;
	}