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_aligned_key_t syncookie6_secret[2];
  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 {
 119         __u32 cookie = ntohl(th->ack_seq) - 1;
 120         __u32 seq = ntohl(th->seq) - 1;
 121         __u32 mssind;
 122
 123         mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 124                                       th->source, th->dest, seq);
 125
 126         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 127 }
 128 EXPORT_SYMBOL_GPL(__cookie_v6_check);
 129
 130 static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
 131                                              struct sk_buff *skb)
 132 {
 133         struct tcp_options_received tcp_opt;
 134         u32 tsoff = 0;
 135         int mss;
 136
 137         if (tcp_synq_no_recent_overflow(sk))
 138                 goto out;
 139
 140         mss = __cookie_v6_check(ipv6_hdr(skb), tcp_hdr(skb));
 141         if (!mss) {
 142                 __NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
 143                 goto out;
 144         }
 145
 146         __NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
 147
 148         /* check for timestamp cookie support */
 149         memset(&tcp_opt, 0, sizeof(tcp_opt));
 150         tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
 151
 152         if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
 153                 tsoff = secure_tcpv6_ts_off(net,
 154                                             ipv6_hdr(skb)->daddr.s6_addr32,
 155                                             ipv6_hdr(skb)->saddr.s6_addr32);
 156                 tcp_opt.rcv_tsecr -= tsoff;
 157         }
 158
 159         if (!cookie_timestamp_decode(net, &tcp_opt))
 160                 goto out;
 161
 162         return cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb,
 163                                       &tcp_opt, mss, tsoff);
 164 out:
 165         return ERR_PTR(-EINVAL);
 166 }
 167
 168 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 169 {
 170         const struct tcphdr *th = tcp_hdr(skb);
 171         struct ipv6_pinfo *np = inet6_sk(sk);
 172         struct tcp_sock *tp = tcp_sk(sk);
 173         struct inet_request_sock *ireq;
 174         struct net *net = sock_net(sk);
 175         struct request_sock *req;
 176         struct dst_entry *dst;
 177         struct sock *ret = sk;
 178         __u8 rcv_wscale;
 179         int full_space;
 180         SKB_DR(reason);
 181
 182         if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
 183             !th->ack || th->rst)
 184                 goto out;
 185
 186         if (cookie_bpf_ok(skb)) {
 187                 req = cookie_bpf_check(sk, skb);
 188         } else {
 189                 req = cookie_tcp_check(net, sk, skb);
 190                 if (IS_ERR(req))
 191                         goto out;
 192         }
 193         if (!req) {
 194                 SKB_DR_SET(reason, NO_SOCKET);
 195                 goto out_drop;
 196         }
 197
 198         ireq = inet_rsk(req);
 199
 200         ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
 201         ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
 202
 203         if (security_inet_conn_request(sk, skb, req)) {
 204                 SKB_DR_SET(reason, SECURITY_HOOK);
 205                 goto out_free;
 206         }
 207
 208         if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
 209             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 210             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 211                 refcount_inc(&skb->users);
 212                 ireq->pktopts = skb;
 213         }
 214
 215         /* So that link locals have meaning */
 216         if (!sk->sk_bound_dev_if &&
 217             ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
 218                 ireq->ir_iif = tcp_v6_iif(skb);
 219
 220         tcp_ao_syncookie(sk, skb, req, AF_INET6);
 221
 222         /*
 223          * We need to lookup the dst_entry to get the correct window size.
 224          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 225          * me if there is a preferred way.
 226          */
 227         {
 228                 struct in6_addr *final_p, final;
 229                 struct flowi6 fl6;
 230                 memset(&fl6, 0, sizeof(fl6));
 231                 fl6.flowi6_proto = IPPROTO_TCP;
 232                 fl6.daddr = ireq->ir_v6_rmt_addr;
 233                 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
 234                 fl6.saddr = ireq->ir_v6_loc_addr;
 235                 fl6.flowi6_oif = ireq->ir_iif;
 236                 fl6.flowi6_mark = ireq->ir_mark;
 237                 fl6.fl6_dport = ireq->ir_rmt_port;
 238                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 239                 fl6.flowi6_uid = sk->sk_uid;
 240                 security_req_classify_flow(req, flowi6_to_flowi_common(&fl6));
 241
 242                 dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p);
 243                 if (IS_ERR(dst)) {
 244                         SKB_DR_SET(reason, IP_OUTNOROUTES);
 245                         goto out_free;
 246                 }
 247         }
 248
 249         req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :dst_metric(dst, RTAX_WINDOW);
 250         /* limit the window selection if the user enforce a smaller rx buffer */
 251         full_space = tcp_full_space(sk);
 252         if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
 253             (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
 254                 req->rsk_window_clamp = full_space;
 255
 256         tcp_select_initial_window(sk, full_space, req->mss,
 257                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
 258                                   ireq->wscale_ok, &rcv_wscale,
 259                                   dst_metric(dst, RTAX_INITRWND));
 260
 261         /* req->syncookie is set true only if ACK is validated
 262          * by BPF kfunc, then, rcv_wscale is already configured.
 263          */
 264         if (!req->syncookie)
 265                 ireq->rcv_wscale = rcv_wscale;
 266         ireq->ecn_ok &= cookie_ecn_ok(net, dst);
 267
 268         ret = tcp_get_cookie_sock(sk, skb, req, dst);
 269         if (!ret) {
 270                 SKB_DR_SET(reason, NO_SOCKET);
 271                 goto out_drop;
 272         }
 273 out:
 274         return ret;
 275 out_free:
 276         reqsk_free(req);
 277 out_drop:
 278         sk_skb_reason_drop(sk, skb, reason);
 279         return NULL;
 280 }