linux/net/ipv6/syncookies.c
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   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
  23static 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 */
  33static __u16 const msstab[] = {
  34        1280 - 60, /* IPV6_MIN_MTU - 60 */
  35        1480 - 60,
  36        1500 - 60,
  37        9000 - 60,
  38};
  39
  40static 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
  63static __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
  75static __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
  92u32 __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}
 107EXPORT_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
 117int __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}
 126EXPORT_SYMBOL_GPL(__cookie_v6_check);
 127
 128struct 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 full_space, 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 = cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb);
 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_common(&fl6));
 237
 238                dst = ip6_dst_lookup_flow(sock_net(sk), 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        /* limit the window selection if the user enforce a smaller rx buffer */
 245        full_space = tcp_full_space(sk);
 246        if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
 247            (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
 248                req->rsk_window_clamp = full_space;
 249
 250        tcp_select_initial_window(sk, full_space, req->mss,
 251                                  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
 252                                  ireq->wscale_ok, &rcv_wscale,
 253                                  dst_metric(dst, RTAX_INITRWND));
 254
 255        ireq->rcv_wscale = rcv_wscale;
 256        ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
 257
 258        ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
 259out:
 260        return ret;
 261out_free:
 262        reqsk_free(req);
 263        return NULL;
 264}
 265
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