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/cryptohash.h>
  21 #include <linux/kernel.h>
  22 #include <net/tcp.h>
  23
  24 extern int sysctl_tcp_syncookies;
  25
  26 static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
  27
  28 static __init int init_syncookies(void)
  29 {
  30         get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
  31         return 0;
  32 }
  33 module_init(init_syncookies);
  34
  35 #define COOKIEBITS 24   /* Upper bits store count */
  36 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  37
  38 static u32 cookie_hash(u32 saddr, u32 daddr, u32 sport, u32 dport,
  39                        u32 count, int c)
  40 {
  41         __u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS];
  42
  43         memcpy(tmp + 3, syncookie_secret[c], sizeof(syncookie_secret[c]));
  44         tmp[0] = saddr;
  45         tmp[1] = daddr;
  46         tmp[2] = (sport << 16) + dport;
  47         tmp[3] = count;
  48         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  49
  50         return tmp[17];
  51 }
  52
  53 static __u32 secure_tcp_syn_cookie(__u32 saddr, __u32 daddr, __u16 sport,
  54                                    __u16 dport, __u32 sseq, __u32 count,
  55                                    __u32 data)
  56 {
  57         /*
  58          * Compute the secure sequence number.
  59          * The output should be:
  60          *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
  61          *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
  62          * Where sseq is their sequence number and count increases every
  63          * minute by 1.
  64          * As an extra hack, we add a small "data" value that encodes the
  65          * MSS into the second hash value.
  66          */
  67
  68         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  69                 sseq + (count << COOKIEBITS) +
  70                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  71                  & COOKIEMASK));
  72 }
  73
  74 /*
  75  * This retrieves the small "data" value from the syncookie.
  76  * If the syncookie is bad, the data returned will be out of
  77  * range.  This must be checked by the caller.
  78  *
  79  * The count value used to generate the cookie must be within
  80  * "maxdiff" if the current (passed-in) "count".  The return value
  81  * is (__u32)-1 if this test fails.
  82  */
  83 static __u32 check_tcp_syn_cookie(__u32 cookie, __u32 saddr, __u32 daddr,
  84                                   __u16 sport, __u16 dport, __u32 sseq,
  85                                   __u32 count, __u32 maxdiff)
  86 {
  87         __u32 diff;
  88
  89         /* Strip away the layers from the cookie */
  90         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
  91
  92         /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
  93         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
  94         if (diff >= maxdiff)
  95                 return (__u32)-1;
  96
  97         return (cookie -
  98                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
  99                 & COOKIEMASK;   /* Leaving the data behind */
 100 }
 101
 102 /*
 103  * This table has to be sorted and terminated with (__u16)-1.
 104  * XXX generate a better table.
 105  * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
 106  */
 107 static __u16 const msstab[] = {
 108         64 - 1,
 109         256 - 1,
 110         512 - 1,
 111         536 - 1,
 112         1024 - 1,
 113         1440 - 1,
 114         1460 - 1,
 115         4312 - 1,
 116         (__u16)-1
 117 };
 118 /* The number doesn't include the -1 terminator */
 119 #define NUM_MSS (ARRAY_SIZE(msstab) - 1)
 120
 121 /*
 122  * Generate a syncookie.  mssp points to the mss, which is returned
 123  * rounded down to the value encoded in the cookie.
 124  */
 125 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 126 {
 127         struct tcp_sock *tp = tcp_sk(sk);
 128         int mssind;
 129         const __u16 mss = *mssp;
 130
 131
 132         tp->last_synq_overflow = jiffies;
 133
 134         /* XXX sort msstab[] by probability?  Binary search? */
 135         for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
 136                 ;
 137         *mssp = msstab[mssind] + 1;
 138
 139         NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESSENT);
 140
 141         return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr,
 142                                      skb->h.th->source, skb->h.th->dest,
 143                                      ntohl(skb->h.th->seq),
 144                                      jiffies / (HZ * 60), mssind);
 145 }
 146
 147 /*
 148  * This (misnamed) value is the age of syncookie which is permitted.
 149  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
 150  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
 151  * backoff) to compute at runtime so it's currently hardcoded here.
 152  */
 153 #define COUNTER_TRIES 4
 154 /*
 155  * Check if a ack sequence number is a valid syncookie.
 156  * Return the decoded mss if it is, or 0 if not.
 157  */
 158 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
 159 {
 160         __u32 seq;
 161         __u32 mssind;
 162
 163         seq = ntohl(skb->h.th->seq)-1;
 164         mssind = check_tcp_syn_cookie(cookie,
 165                                       skb->nh.iph->saddr, skb->nh.iph->daddr,
 166                                       skb->h.th->source, skb->h.th->dest,
 167                                       seq, jiffies / (HZ * 60), COUNTER_TRIES);
 168
 169         return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
 170 }
 171
 172 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 173                                            struct request_sock *req,
 174                                            struct dst_entry *dst)
 175 {
 176         struct tcp_sock *tp = tcp_sk(sk);
 177         struct sock *child;
 178
 179         child = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
 180         if (child)
 181                 inet_csk_reqsk_queue_add(sk, req, child);
 182         else
 183                 reqsk_free(req);
 184
 185         return child;
 186 }
 187
 188 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 189                              struct ip_options *opt)
 190 {
 191         struct inet_request_sock *ireq;
 192         struct tcp_request_sock *treq;
 193         struct tcp_sock *tp = tcp_sk(sk);
 194         __u32 cookie = ntohl(skb->h.th->ack_seq) - 1;
 195         struct sock *ret = sk;
 196         struct request_sock *req;
 197         int mss;
 198         struct rtable *rt;
 199         __u8 rcv_wscale;
 200
 201         if (!sysctl_tcp_syncookies || !skb->h.th->ack)
 202                 goto out;
 203
 204         if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
 205             (mss = cookie_check(skb, cookie)) == 0) {
 206                 NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED);
 207                 goto out;
 208         }
 209
 210         NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV);
 211
 212         ret = NULL;
 213         req = reqsk_alloc(&tcp_request_sock_ops); /* for safety */
 214         if (!req)
 215                 goto out;
 216
 217         ireq = inet_rsk(req);
 218         treq = tcp_rsk(req);
 219         treq->rcv_isn           = htonl(skb->h.th->seq) - 1;
 220         treq->snt_isn           = cookie;
 221         req->mss                = mss;
 222         ireq->rmt_port          = skb->h.th->source;
 223         ireq->loc_addr          = skb->nh.iph->daddr;
 224         ireq->rmt_addr          = skb->nh.iph->saddr;
 225         ireq->opt               = NULL;
 226
 227         /* We throwed the options of the initial SYN away, so we hope
 228          * the ACK carries the same options again (see RFC1122 4.2.3.8)
 229          */
 230         if (opt && opt->optlen) {
 231                 int opt_size = sizeof(struct ip_options) + opt->optlen;
 232
 233                 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
 234                 if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
 235                         kfree(ireq->opt);
 236                         ireq->opt = NULL;
 237                 }
 238         }
 239
 240         ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0;
 241         ireq->wscale_ok  = ireq->sack_ok = 0;
 242         req->expires    = 0UL;
 243         req->retrans    = 0;
 244
 245         /*
 246          * We need to lookup the route here to get at the correct
 247          * window size. We should better make sure that the window size
 248          * hasn't changed since we received the original syn, but I see
 249          * no easy way to do this.
 250          */
 251         {
 252                 struct flowi fl = { .nl_u = { .ip4_u =
 253                                               { .daddr = ((opt && opt->srr) ?
 254                                                           opt->faddr :
 255                                                           ireq->rmt_addr),
 256                                                 .saddr = ireq->loc_addr,
 257                                                 .tos = RT_CONN_FLAGS(sk) } },
 258                                     .proto = IPPROTO_TCP,
 259                                     .uli_u = { .ports =
 260                                                { .sport = skb->h.th->dest,
 261                                                  .dport = skb->h.th->source } } };
 262                 if (ip_route_output_key(&rt, &fl)) {
 263                         reqsk_free(req);
 264                         goto out;
 265                 }
 266         }
 267
 268         /* Try to redo what tcp_v4_send_synack did. */
 269         req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW);
 270         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 271                                   &req->rcv_wnd, &req->window_clamp,
 272                                   0, &rcv_wscale);
 273         /* BTW win scale with syncookies is 0 by definition */
 274         ireq->rcv_wscale  = rcv_wscale;
 275
 276         ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
 277 out:    return ret;
 278 }