LXR linux-bk/net/ipv4/syncookies.c

   1/*
   2 *  Syncookies implementation for the Linux kernel
   3 *
   4 *  Copyright (C) 1997 Andi Kleen
   5 *  Based on ideas by D.J.Bernstein and Eric Schenk. 
   6 *
   7 *      This program is free software; you can redistribute it and/or
   8 *      modify it under the terms of the GNU General Public License
   9 *      as published by the Free Software Foundation; either version
  10 *      2 of the License, or (at your option) any later version.
  11 * 
  12 *  $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $
  13 *
  14 *  Missing: IPv6 support. 
  15 */
  16
  17#include <linux/tcp.h>
  18#include <linux/slab.h>
  19#include <linux/random.h>
  20#include <linux/kernel.h>
  21#include <net/tcp.h>
  22
  23extern int sysctl_tcp_syncookies;
  24
  25/* 
  26 * This table has to be sorted and terminated with (__u16)-1.
  27 * XXX generate a better table.
  28 * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
  29 */
  30static __u16 const msstab[] = {
  31        64 - 1,
  32        256 - 1,        
  33        512 - 1,
  34        536 - 1,
  35        1024 - 1,       
  36        1440 - 1,
  37        1460 - 1,
  38        4312 - 1,
  39        (__u16)-1
  40};
  41/* The number doesn't include the -1 terminator */
  42#define NUM_MSS (ARRAY_SIZE(msstab) - 1)
  43
  44/*
  45 * Generate a syncookie.  mssp points to the mss, which is returned
  46 * rounded down to the value encoded in the cookie.
  47 */
  48__u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
  49{
  50        struct tcp_opt *tp = tcp_sk(sk);
  51        int mssind;
  52        const __u16 mss = *mssp;
  53
  54        
  55        tp->last_synq_overflow = jiffies;
  56
  57        /* XXX sort msstab[] by probability?  Binary search? */
  58        for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
  59                ;
  60        *mssp = msstab[mssind] + 1;
  61
  62        NET_INC_STATS_BH(SyncookiesSent);
  63
  64        return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr,
  65                                     skb->h.th->source, skb->h.th->dest,
  66                                     ntohl(skb->h.th->seq),
  67                                     jiffies / (HZ * 60), mssind);
  68}
  69
  70/* 
  71 * This (misnamed) value is the age of syncookie which is permitted.
  72 * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
  73 * sysctl_tcp_retries1. It's a rather complicated formula (exponential
  74 * backoff) to compute at runtime so it's currently hardcoded here.
  75 */
  76#define COUNTER_TRIES 4
  77/*  
  78 * Check if a ack sequence number is a valid syncookie. 
  79 * Return the decoded mss if it is, or 0 if not.
  80 */
  81static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
  82{
  83        __u32 seq; 
  84        __u32 mssind;
  85
  86        seq = ntohl(skb->h.th->seq)-1; 
  87        mssind = check_tcp_syn_cookie(cookie,
  88                                      skb->nh.iph->saddr, skb->nh.iph->daddr,
  89                                      skb->h.th->source, skb->h.th->dest,
  90                                      seq, jiffies / (HZ * 60), COUNTER_TRIES);
  91
  92        return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
  93}
  94
  95extern struct or_calltable or_ipv4;
  96
  97static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  98                                           struct open_request *req,
  99                                           struct dst_entry *dst)
 100{
 101        struct tcp_opt *tp = tcp_sk(sk);
 102        struct sock *child;
 103
 104        child = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
 105        if (child) {
 106                sk_set_owner(child, sk->sk_owner);
 107                tcp_acceptq_queue(sk, req, child);
 108        } else
 109                tcp_openreq_free(req);
 110
 111        return child;
 112}
 113
 114struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 115                             struct ip_options *opt)
 116{
 117        struct tcp_opt *tp = tcp_sk(sk);
 118        __u32 cookie = ntohl(skb->h.th->ack_seq) - 1; 
 119        struct sock *ret = sk;
 120        struct open_request *req; 
 121        int mss; 
 122        struct rtable *rt; 
 123        __u8 rcv_wscale;
 124
 125        if (!sysctl_tcp_syncookies || !skb->h.th->ack)
 126                goto out;
 127
 128        if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
 129            (mss = cookie_check(skb, cookie)) == 0) {
 130                NET_INC_STATS_BH(SyncookiesFailed);
 131                goto out;
 132        }
 133
 134        NET_INC_STATS_BH(SyncookiesRecv);
 135
 136        req = tcp_openreq_alloc();
 137        ret = NULL;
 138        if (!req)
 139                goto out;
 140
 141        req->rcv_isn            = htonl(skb->h.th->seq) - 1;
 142        req->snt_isn            = cookie; 
 143        req->mss                = mss;
 144        req->rmt_port           = skb->h.th->source;
 145        req->af.v4_req.loc_addr = skb->nh.iph->daddr;
 146        req->af.v4_req.rmt_addr = skb->nh.iph->saddr;
 147        req->class              = &or_ipv4; /* for savety */
 148        req->af.v4_req.opt      = NULL;
 149
 150        /* We throwed the options of the initial SYN away, so we hope
 151         * the ACK carries the same options again (see RFC1122 4.2.3.8)
 152         */
 153        if (opt && opt->optlen) {
 154                int opt_size = sizeof(struct ip_options) + opt->optlen;
 155
 156                req->af.v4_req.opt = kmalloc(opt_size, GFP_ATOMIC);
 157                if (req->af.v4_req.opt) {
 158                        if (ip_options_echo(req->af.v4_req.opt, skb)) {
 159                                kfree(req->af.v4_req.opt);
 160                                req->af.v4_req.opt = NULL;
 161                        }
 162                }
 163        }
 164
 165        req->snd_wscale = req->rcv_wscale = req->tstamp_ok = 0;
 166        req->wscale_ok  = req->sack_ok = 0; 
 167        req->expires    = 0UL; 
 168        req->retrans    = 0; 
 169        
 170        /*
 171         * We need to lookup the route here to get at the correct
 172         * window size. We should better make sure that the window size
 173         * hasn't changed since we received the original syn, but I see
 174         * no easy way to do this. 
 175         */
 176        {
 177                struct flowi fl = { .nl_u = { .ip4_u =
 178                                              { .daddr = ((opt && opt->srr) ?
 179                                                          opt->faddr :
 180                                                          req->af.v4_req.rmt_addr),
 181                                                .saddr = req->af.v4_req.loc_addr,
 182                                                .tos = RT_CONN_FLAGS(sk) } },
 183                                    .proto = IPPROTO_TCP,
 184                                    .uli_u = { .ports =
 185                                               { .sport = skb->h.th->dest,
 186                                                 .dport = skb->h.th->source } } };
 187                if (ip_route_output_key(&rt, &fl)) {
 188                        tcp_openreq_free(req);
 189                        goto out; 
 190                }
 191        }
 192
 193        /* Try to redo what tcp_v4_send_synack did. */
 194        req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW);
 195        tcp_select_initial_window(tcp_full_space(sk), req->mss,
 196                                  &req->rcv_wnd, &req->window_clamp, 
 197                                  0, &rcv_wscale);
 198        /* BTW win scale with syncookies is 0 by definition */
 199        req->rcv_wscale   = rcv_wscale; 
 200
 201        ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
 202out:    return ret;
 203}
 204