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![Eric Dumazet](/assets/img/avatar_default.png)
For passive TCP connections using TCP_DEFER_ACCEPT facility, we incorrectly increment req->retrans each time timeout triggers while no SYNACK is sent. SYNACK are not sent for TCP_DEFER_ACCEPT that were established (for which we received the ACK from client). Only the last SYNACK is sent so that we can receive again an ACK from client, to move the req into accept queue. We plan to change this later to avoid the useless retransmit (and potential problem as this SYNACK could be lost) TCP_INFO later gives wrong information to user, claiming imaginary retransmits. Decouple req->retrans field into two independent fields : num_retrans : number of retransmit num_timeout : number of timeouts num_timeout is the counter that is incremented at each timeout, regardless of actual SYNACK being sent or not, and used to compute the exponential timeout. Introduce inet_rtx_syn_ack() helper to increment num_retrans only if ->rtx_syn_ack() succeeded. Use inet_rtx_syn_ack() from tcp_check_req() to increment num_retrans when we re-send a SYNACK in answer to a (retransmitted) SYN. Prior to this patch, we were not counting these retransmits. Change tcp_v[46]_rtx_synack() to increment TCP_MIB_RETRANSSEGS only if a synack packet was successfully queued. Reported-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: Julian Anastasov <ja@ssi.bg> Cc: Vijay Subramanian <subramanian.vijay@gmail.com> Cc: Elliott Hughes <enh@google.com> Cc: Neal Cardwell <ncardwell@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
380 lines
10 KiB
C
380 lines
10 KiB
C
/*
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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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#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 <linux/export.h>
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#include <net/tcp.h>
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#include <net/route.h>
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/* Timestamps: lowest bits store TCP options */
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#define TSBITS 6
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#define TSMASK (((__u32)1 << TSBITS) - 1)
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extern int sysctl_tcp_syncookies;
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__u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
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EXPORT_SYMBOL(syncookie_secret);
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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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__initcall(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 DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
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ipv4_cookie_scratch);
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static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
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u32 count, int c)
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{
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__u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
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memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
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tmp[0] = (__force u32)saddr;
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tmp[1] = (__force u32)daddr;
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tmp[2] = ((__force u32)sport << 16) + (__force u32)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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/*
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* when syncookies are in effect and tcp timestamps are enabled we encode
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* tcp options in the lower bits of the timestamp value that will be
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* sent in the syn-ack.
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* Since subsequent timestamps use the normal tcp_time_stamp value, we
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* must make sure that the resulting initial timestamp is <= tcp_time_stamp.
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*/
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__u32 cookie_init_timestamp(struct request_sock *req)
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{
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struct inet_request_sock *ireq;
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u32 ts, ts_now = tcp_time_stamp;
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u32 options = 0;
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ireq = inet_rsk(req);
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options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
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options |= ireq->sack_ok << 4;
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options |= ireq->ecn_ok << 5;
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ts = ts_now & ~TSMASK;
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ts |= options;
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if (ts > ts_now) {
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ts >>= TSBITS;
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ts--;
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ts <<= TSBITS;
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ts |= options;
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}
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return ts;
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}
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static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
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__be16 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, __be32 saddr, __be32 daddr,
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__be16 sport, __be16 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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* MSS Values are taken from the 2009 paper
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* 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
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* - values 1440 to 1460 accounted for 80% of observed mss values
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* - values outside the 536-1460 range are rare (<0.2%).
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*
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* Table must be sorted.
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*/
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static __u16 const msstab[] = {
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64,
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512,
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536,
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1024,
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1440,
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1460,
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4312,
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8960,
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};
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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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const struct iphdr *iph = ip_hdr(skb);
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const struct tcphdr *th = tcp_hdr(skb);
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int mssind;
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const __u16 mss = *mssp;
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tcp_synq_overflow(sk);
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for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
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if (mss >= msstab[mssind])
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break;
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*mssp = msstab[mssind];
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NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
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return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
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th->source, th->dest, ntohl(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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const struct iphdr *iph = ip_hdr(skb);
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const struct tcphdr *th = tcp_hdr(skb);
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__u32 seq = ntohl(th->seq) - 1;
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__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
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th->source, th->dest, seq,
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jiffies / (HZ * 60),
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COUNTER_TRIES);
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return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
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}
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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 inet_connection_sock *icsk = inet_csk(sk);
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struct sock *child;
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child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
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if (child)
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inet_csk_reqsk_queue_add(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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/*
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* when syncookies are in effect and tcp timestamps are enabled we stored
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* additional tcp options in the timestamp.
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* This extracts these options from the timestamp echo.
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*
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* The lowest 4 bits store snd_wscale.
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* next 2 bits indicate SACK and ECN support.
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*
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* return false if we decode an option that should not be.
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*/
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bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
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{
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/* echoed timestamp, lowest bits contain options */
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u32 options = tcp_opt->rcv_tsecr & TSMASK;
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if (!tcp_opt->saw_tstamp) {
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tcp_clear_options(tcp_opt);
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return true;
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}
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if (!sysctl_tcp_timestamps)
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return false;
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tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
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*ecn_ok = (options >> 5) & 1;
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if (*ecn_ok && !sysctl_tcp_ecn)
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return false;
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if (tcp_opt->sack_ok && !sysctl_tcp_sack)
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return false;
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if ((options & 0xf) == 0xf)
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return true; /* no window scaling */
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tcp_opt->wscale_ok = 1;
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tcp_opt->snd_wscale = options & 0xf;
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return sysctl_tcp_window_scaling != 0;
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}
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EXPORT_SYMBOL(cookie_check_timestamp);
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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 tcp_options_received tcp_opt;
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const u8 *hash_location;
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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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const struct tcphdr *th = tcp_hdr(skb);
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__u32 cookie = ntohl(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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bool ecn_ok = false;
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struct flowi4 fl4;
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if (!sysctl_tcp_syncookies || !th->ack || th->rst)
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goto out;
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if (tcp_synq_no_recent_overflow(sk) ||
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(mss = cookie_check(skb, cookie)) == 0) {
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NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
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goto out;
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}
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NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
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/* check for timestamp cookie support */
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memset(&tcp_opt, 0, sizeof(tcp_opt));
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tcp_parse_options(skb, &tcp_opt, &hash_location, 0, NULL);
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if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
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goto out;
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ret = NULL;
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req = inet_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 = ntohl(th->seq) - 1;
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treq->snt_isn = cookie;
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req->mss = mss;
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ireq->loc_port = th->dest;
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ireq->rmt_port = th->source;
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ireq->loc_addr = ip_hdr(skb)->daddr;
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ireq->rmt_addr = ip_hdr(skb)->saddr;
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ireq->ecn_ok = ecn_ok;
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ireq->snd_wscale = tcp_opt.snd_wscale;
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ireq->sack_ok = tcp_opt.sack_ok;
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ireq->wscale_ok = tcp_opt.wscale_ok;
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ireq->tstamp_ok = tcp_opt.saw_tstamp;
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req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
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treq->snt_synack = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
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treq->listener = 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_rcu) + 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->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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if (security_inet_conn_request(sk, skb, req)) {
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reqsk_free(req);
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goto out;
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}
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req->expires = 0UL;
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req->num_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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flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
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RT_SCOPE_UNIVERSE, IPPROTO_TCP,
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inet_sk_flowi_flags(sk),
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(opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
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ireq->loc_addr, th->source, th->dest);
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security_req_classify_flow(req, flowi4_to_flowi(&fl4));
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rt = ip_route_output_key(sock_net(sk), &fl4);
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if (IS_ERR(rt)) {
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reqsk_free(req);
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goto out;
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}
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/* Try to redo what tcp_v4_send_synack did. */
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req->window_clamp = tp->window_clamp ? :dst_metric(&rt->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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ireq->wscale_ok, &rcv_wscale,
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dst_metric(&rt->dst, RTAX_INITRWND));
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ireq->rcv_wscale = rcv_wscale;
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ret = get_cookie_sock(sk, skb, req, &rt->dst);
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/* ip_queue_xmit() depends on our flow being setup
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* Normal sockets get it right from inet_csk_route_child_sock()
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*/
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if (ret)
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inet_sk(ret)->cork.fl.u.ip4 = fl4;
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out: return ret;
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}
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