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280 lines
7.7 KiB
280 lines
7.7 KiB
/*
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* Syncookies implementation for the Linux kernel
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*
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* Copyright (C) 1997 Andi Kleen
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* Based on ideas by D.J.Bernstein and Eric Schenk.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*
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* $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $
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*
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* Missing: IPv6 support.
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*/
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#include <linux/tcp.h>
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#include <linux/slab.h>
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#include <linux/random.h>
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#include <linux/cryptohash.h>
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#include <linux/kernel.h>
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#include <net/tcp.h>
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extern int sysctl_tcp_syncookies;
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static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
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static __init int init_syncookies(void)
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{
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get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
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return 0;
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}
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module_init(init_syncookies);
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#define COOKIEBITS 24 /* Upper bits store count */
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#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
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static u32 cookie_hash(u32 saddr, u32 daddr, u32 sport, u32 dport,
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u32 count, int c)
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{
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__u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS];
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memcpy(tmp + 3, syncookie_secret[c], sizeof(syncookie_secret[c]));
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tmp[0] = saddr;
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tmp[1] = daddr;
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tmp[2] = (sport << 16) + dport;
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tmp[3] = count;
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sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
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return tmp[17];
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}
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static __u32 secure_tcp_syn_cookie(__u32 saddr, __u32 daddr, __u16 sport,
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__u16 dport, __u32 sseq, __u32 count,
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__u32 data)
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{
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/*
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* Compute the secure sequence number.
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* The output should be:
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* HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
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* + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
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* Where sseq is their sequence number and count increases every
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* minute by 1.
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* As an extra hack, we add a small "data" value that encodes the
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* MSS into the second hash value.
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*/
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return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
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sseq + (count << COOKIEBITS) +
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((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
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& COOKIEMASK));
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}
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/*
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* This retrieves the small "data" value from the syncookie.
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* If the syncookie is bad, the data returned will be out of
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* range. This must be checked by the caller.
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*
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* The count value used to generate the cookie must be within
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* "maxdiff" if the current (passed-in) "count". The return value
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* is (__u32)-1 if this test fails.
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*/
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static __u32 check_tcp_syn_cookie(__u32 cookie, __u32 saddr, __u32 daddr,
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__u16 sport, __u16 dport, __u32 sseq,
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__u32 count, __u32 maxdiff)
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{
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__u32 diff;
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/* Strip away the layers from the cookie */
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cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
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/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
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diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
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if (diff >= maxdiff)
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return (__u32)-1;
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return (cookie -
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cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
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& COOKIEMASK; /* Leaving the data behind */
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}
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/*
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* This table has to be sorted and terminated with (__u16)-1.
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* XXX generate a better table.
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* Unresolved Issues: HIPPI with a 64k MSS is not well supported.
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*/
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static __u16 const msstab[] = {
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64 - 1,
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256 - 1,
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512 - 1,
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536 - 1,
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1024 - 1,
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1440 - 1,
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1460 - 1,
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4312 - 1,
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(__u16)-1
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};
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/* The number doesn't include the -1 terminator */
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#define NUM_MSS (ARRAY_SIZE(msstab) - 1)
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/*
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* Generate a syncookie. mssp points to the mss, which is returned
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* rounded down to the value encoded in the cookie.
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*/
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__u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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int mssind;
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const __u16 mss = *mssp;
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tp->last_synq_overflow = jiffies;
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/* XXX sort msstab[] by probability? Binary search? */
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for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
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;
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*mssp = msstab[mssind] + 1;
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NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESSENT);
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return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr,
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skb->h.th->source, skb->h.th->dest,
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ntohl(skb->h.th->seq),
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jiffies / (HZ * 60), mssind);
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}
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/*
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* This (misnamed) value is the age of syncookie which is permitted.
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* Its ideal value should be dependent on TCP_TIMEOUT_INIT and
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* sysctl_tcp_retries1. It's a rather complicated formula (exponential
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* backoff) to compute at runtime so it's currently hardcoded here.
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*/
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#define COUNTER_TRIES 4
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/*
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* Check if a ack sequence number is a valid syncookie.
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* Return the decoded mss if it is, or 0 if not.
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*/
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static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
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{
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__u32 seq;
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__u32 mssind;
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seq = ntohl(skb->h.th->seq)-1;
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mssind = check_tcp_syn_cookie(cookie,
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skb->nh.iph->saddr, skb->nh.iph->daddr,
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skb->h.th->source, skb->h.th->dest,
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seq, jiffies / (HZ * 60), COUNTER_TRIES);
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return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
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}
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extern struct request_sock_ops tcp_request_sock_ops;
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static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
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struct request_sock *req,
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struct dst_entry *dst)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct sock *child;
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child = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
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if (child)
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tcp_acceptq_queue(sk, req, child);
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else
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reqsk_free(req);
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return child;
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}
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struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
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struct ip_options *opt)
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{
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struct inet_request_sock *ireq;
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struct tcp_request_sock *treq;
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struct tcp_sock *tp = tcp_sk(sk);
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__u32 cookie = ntohl(skb->h.th->ack_seq) - 1;
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struct sock *ret = sk;
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struct request_sock *req;
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int mss;
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struct rtable *rt;
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__u8 rcv_wscale;
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if (!sysctl_tcp_syncookies || !skb->h.th->ack)
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goto out;
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if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
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(mss = cookie_check(skb, cookie)) == 0) {
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NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED);
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goto out;
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}
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NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV);
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ret = NULL;
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req = reqsk_alloc(&tcp_request_sock_ops); /* for safety */
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if (!req)
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goto out;
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ireq = inet_rsk(req);
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treq = tcp_rsk(req);
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treq->rcv_isn = htonl(skb->h.th->seq) - 1;
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treq->snt_isn = cookie;
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req->mss = mss;
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ireq->rmt_port = skb->h.th->source;
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ireq->loc_addr = skb->nh.iph->daddr;
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ireq->rmt_addr = skb->nh.iph->saddr;
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ireq->opt = NULL;
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/* We throwed the options of the initial SYN away, so we hope
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* the ACK carries the same options again (see RFC1122 4.2.3.8)
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*/
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if (opt && opt->optlen) {
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int opt_size = sizeof(struct ip_options) + opt->optlen;
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ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
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if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
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kfree(ireq->opt);
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ireq->opt = NULL;
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}
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}
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ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0;
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ireq->wscale_ok = ireq->sack_ok = 0;
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req->expires = 0UL;
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req->retrans = 0;
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/*
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* We need to lookup the route here to get at the correct
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* window size. We should better make sure that the window size
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* hasn't changed since we received the original syn, but I see
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* no easy way to do this.
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*/
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{
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struct flowi fl = { .nl_u = { .ip4_u =
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{ .daddr = ((opt && opt->srr) ?
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opt->faddr :
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ireq->rmt_addr),
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.saddr = ireq->loc_addr,
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.tos = RT_CONN_FLAGS(sk) } },
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.proto = IPPROTO_TCP,
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.uli_u = { .ports =
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{ .sport = skb->h.th->dest,
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.dport = skb->h.th->source } } };
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if (ip_route_output_key(&rt, &fl)) {
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reqsk_free(req);
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goto out;
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}
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}
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/* Try to redo what tcp_v4_send_synack did. */
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req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW);
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tcp_select_initial_window(tcp_full_space(sk), req->mss,
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&req->rcv_wnd, &req->window_clamp,
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0, &rcv_wscale);
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/* BTW win scale with syncookies is 0 by definition */
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ireq->rcv_wscale = rcv_wscale;
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ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
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out: return ret;
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}
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