net/ipv6/syncookies.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  *  IPv6 Syncookies implementation for the Linux kernel
   4  *
   5  *  Authors:
   6  *  Glenn Griffin       <ggriffin.kernel@gmail.com>
   7  *
   8  *  Based on IPv4 implementation by Andi Kleen
   9  *  linux/net/ipv4/syncookies.c
  10  */
  11
  12 #include <linux/tcp.h>
  13 #include <linux/random.h>
  14 #include <linux/siphash.h>
  15 #include <linux/kernel.h>
  16 #include <net/secure_seq.h>
  17 #include <net/ipv6.h>
  18 #include <net/tcp.h>
  19
  20 #define COOKIEBITS 24   /* Upper bits store count */
  21 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  22
  23 static siphash_key_t syncookie6_secret[2] __read_mostly;
  24
  25 /* RFC 2460, Section 8.3:
  26  * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
  27  *
  28  * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
  29  * using higher values than ipv4 tcp syncookies.
  30  * The other values are chosen based on ethernet (1500 and 9k MTU), plus
  31  * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
  32  */
  33 static __u16 const msstab[] = {
  34         1280 - 60, /* IPV6_MIN_MTU - 60 */
  35         1480 - 60,
  36         1500 - 60,
  37         9000 - 60,
  38 };
  39
  40 static u32 cookie_hash(const struct in6_addr *saddr,
  41                        const struct in6_addr *daddr,
  42                        __be16 sport, __be16 dport, u32 count, int c)
  43 {
  44         const struct {
  45                 struct in6_addr saddr;
  46                 struct in6_addr daddr;
  47                 u32 count;
  48                 __be16 sport;
  49                 __be16 dport;
  50         } __aligned(SIPHASH_ALIGNMENT) combined = {
  51                 .saddr = *saddr,
  52                 .daddr = *daddr,
  53                 .count = count,
  54                 .sport = sport,
  55                 .dport = dport
  56         };
  57
  58         net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
  59         return siphash(&combined, offsetofend(typeof(combined), dport),
  60                        &syncookie6_secret[c]);
  61 }
  62
  63 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  64                                    const struct in6_addr *daddr,
  65                                    __be16 sport, __be16 dport, __u32 sseq,
  66                                    __u32 data)
  67 {
  68         u32 count = tcp_cookie_time();
  69         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  70                 sseq + (count << COOKIEBITS) +
  71                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  72                 & COOKIEMASK));
  73 }
  74
  75 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
  76                                   const struct in6_addr *daddr, __be16 sport,
  77                                   __be16 dport, __u32 sseq)
  78 {
  79         __u32 diff, count = tcp_cookie_time();
  80
  81         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
  82
  83         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
  84         if (diff >= MAX_SYNCOOKIE_AGE)
  85                 return (__u32)-1;
  86
  87         return (cookie -
  88                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
  89                 & COOKIEMASK;
  90 }
  91
  92 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
  93                               const struct tcphdr *th, __u16 *mssp)
  94 {
  95         int mssind;
  96         const __u16 mss = *mssp;
  97
  98         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
  99                 if (mss >= msstab[mssind])
 100                         break;
 101
 102         *mssp = msstab[mssind];
 103
 104         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 105                                      th->dest, ntohl(th->seq), mssind);
 106 }
 107 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
 108
 109 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
 110 {
 111         const struct ipv6hdr *iph = ipv6_hdr(skb);
 112         const struct tcphdr *th = tcp_hdr(skb);
 113
 114         return __cookie_v6_init_sequence(iph, th, mssp);
 115 }
 116
 117 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
 118                       __u32 cookie)
 119 {
 120         __u32 seq = ntohl(th->seq) - 1;
 121         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 122                                             th->source, th->dest, seq);
 123
 124         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 125 }
 126 EXPORT_SYMBOL_GPL(__cookie_v6_check);
 127
 128 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 129 {
 130         struct tcp_options_received tcp_opt;
 131         struct inet_request_sock *ireq;
 132         struct tcp_request_sock *treq;
 133         struct ipv6_pinfo *np = inet6_sk(sk);
 134         struct tcp_sock *tp = tcp_sk(sk);
 135         const struct tcphdr *th = tcp_hdr(skb);
 136         __u32 cookie = ntohl(th->ack_seq) - 1;
 137         struct sock *ret = sk;
 138         struct request_sock *req;
 139         int mss;
 140         struct dst_entry *dst;
 141         __u8 rcv_wscale;
 142         u32 tsoff = 0;
 143
 144         if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
 145                 goto out;
 146
 147         if (tcp_synq_no_recent_overflow(sk))
 148                 goto out;
 149
 150         mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
 151         if (mss == 0) {
 152                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 153                 goto out;
 154         }
 155
 156         __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 157
 158         /* check for timestamp cookie support */
 159         memset(&tcp_opt, 0, sizeof(tcp_opt));
 160         tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
 161
 162         if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
 163                 tsoff = secure_tcpv6_ts_off(sock_net(sk),
 164                                             ipv6_hdr(skb)->daddr.s6_addr32,
 165                                             ipv6_hdr(skb)->saddr.s6_addr32);
 166                 tcp_opt.rcv_tsecr -= tsoff;
 167         }
 168
 169         if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
 170                 goto out;
 171
 172         ret = NULL;
 173         req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false);
 174         if (!req)
 175                 goto out;
 176
 177         ireq = inet_rsk(req);
 178         treq = tcp_rsk(req);
 179         treq->tfo_listener = false;
 180
 181         if (security_inet_conn_request(sk, skb, req))
 182                 goto out_free;
 183
 184         req->mss = mss;
 185         ireq->ir_rmt_port = th->source;
 186         ireq->ir_num = ntohs(th->dest);
 187         ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
 188         ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
 189         if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
 190             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 191             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 192                 refcount_inc(&skb->users);
 193                 ireq->pktopts = skb;
 194         }
 195
 196         ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
 197         /* So that link locals have meaning */
 198         if (!sk->sk_bound_dev_if &&
 199             ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
 200                 ireq->ir_iif = tcp_v6_iif(skb);
 201
 202         ireq->ir_mark = inet_request_mark(sk, skb);
 203
 204         req->num_retrans = 0;
 205         ireq->snd_wscale        = tcp_opt.snd_wscale;
 206         ireq->sack_ok           = tcp_opt.sack_ok;
 207         ireq->wscale_ok         = tcp_opt.wscale_ok;
 208         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 209         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 210         treq->snt_synack        = 0;
 211         treq->rcv_isn = ntohl(th->seq) - 1;
 212         treq->snt_isn = cookie;
 213         treq->ts_off = 0;
 214         treq->txhash = net_tx_rndhash();
 215         if (IS_ENABLED(CONFIG_SMC))
 216                 ireq->smc_ok = 0;
 217
 218         /*
 219          * We need to lookup the dst_entry to get the correct window size.
 220          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 221          * me if there is a preferred way.
 222          */
 223         {
 224                 struct in6_addr *final_p, final;
 225                 struct flowi6 fl6;
 226                 memset(&fl6, 0, sizeof(fl6));
 227                 fl6.flowi6_proto = IPPROTO_TCP;
 228                 fl6.daddr = ireq->ir_v6_rmt_addr;
 229                 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
 230                 fl6.saddr = ireq->ir_v6_loc_addr;
 231                 fl6.flowi6_oif = ireq->ir_iif;
 232                 fl6.flowi6_mark = ireq->ir_mark;
 233                 fl6.fl6_dport = ireq->ir_rmt_port;
 234                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 235                 fl6.flowi6_uid = sk->sk_uid;
 236                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 237
 238                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
 239                 if (IS_ERR(dst))
 240                         goto out_free;
 241         }
 242
 243         req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 244         tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
 245                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
 246                                   ireq->wscale_ok, &rcv_wscale,
 247                                   dst_metric(dst, RTAX_INITRWND));
 248
 249         ireq->rcv_wscale = rcv_wscale;
 250         ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
 251
 252         ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
 253 out:
 254         return ret;
 255 out_free:
 256         reqsk_free(req);
 257         return NULL;
 258 }