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Linux/net/ipv6/syncookies.c

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  1 /*
  2  *  IPv6 Syncookies implementation for the Linux kernel
  3  *
  4  *  Authors:
  5  *  Glenn Griffin       <ggriffin.kernel@gmail.com>
  6  *
  7  *  Based on IPv4 implementation by Andi Kleen
  8  *  linux/net/ipv4/syncookies.c
  9  *
 10  *      This program is free software; you can redistribute it and/or
 11  *      modify it under the terms of the GNU General Public License
 12  *      as published by the Free Software Foundation; either version
 13  *      2 of the License, or (at your option) any later version.
 14  *
 15  */
 16 
 17 #include <linux/tcp.h>
 18 #include <linux/random.h>
 19 #include <linux/cryptohash.h>
 20 #include <linux/kernel.h>
 21 #include <net/ipv6.h>
 22 #include <net/tcp.h>
 23 
 24 #define COOKIEBITS 24   /* Upper bits store count */
 25 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
 26 
 27 static u32 syncookie6_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
 28 
 29 /* RFC 2460, Section 8.3:
 30  * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
 31  *
 32  * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
 33  * using higher values than ipv4 tcp syncookies.
 34  * The other values are chosen based on ethernet (1500 and 9k MTU), plus
 35  * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
 36  */
 37 static __u16 const msstab[] = {
 38         1280 - 60, /* IPV6_MIN_MTU - 60 */
 39         1480 - 60,
 40         1500 - 60,
 41         9000 - 60,
 42 };
 43 
 44 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS], ipv6_cookie_scratch);
 45 
 46 static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr,
 47                        __be16 sport, __be16 dport, u32 count, int c)
 48 {
 49         __u32 *tmp;
 50 
 51         net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
 52 
 53         tmp  = this_cpu_ptr(ipv6_cookie_scratch);
 54 
 55         /*
 56          * we have 320 bits of information to hash, copy in the remaining
 57          * 192 bits required for sha_transform, from the syncookie6_secret
 58          * and overwrite the digest with the secret
 59          */
 60         memcpy(tmp + 10, syncookie6_secret[c], 44);
 61         memcpy(tmp, saddr, 16);
 62         memcpy(tmp + 4, daddr, 16);
 63         tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
 64         tmp[9] = count;
 65         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
 66 
 67         return tmp[17];
 68 }
 69 
 70 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
 71                                    const struct in6_addr *daddr,
 72                                    __be16 sport, __be16 dport, __u32 sseq,
 73                                    __u32 data)
 74 {
 75         u32 count = tcp_cookie_time();
 76         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
 77                 sseq + (count << COOKIEBITS) +
 78                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
 79                 & COOKIEMASK));
 80 }
 81 
 82 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
 83                                   const struct in6_addr *daddr, __be16 sport,
 84                                   __be16 dport, __u32 sseq)
 85 {
 86         __u32 diff, count = tcp_cookie_time();
 87 
 88         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 89 
 90         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 91         if (diff >= MAX_SYNCOOKIE_AGE)
 92                 return (__u32)-1;
 93 
 94         return (cookie -
 95                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 96                 & COOKIEMASK;
 97 }
 98 
 99 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
100                               const struct tcphdr *th, __u16 *mssp)
101 {
102         int mssind;
103         const __u16 mss = *mssp;
104 
105         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
106                 if (mss >= msstab[mssind])
107                         break;
108 
109         *mssp = msstab[mssind];
110 
111         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
112                                      th->dest, ntohl(th->seq), mssind);
113 }
114 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
115 
116 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
117 {
118         const struct ipv6hdr *iph = ipv6_hdr(skb);
119         const struct tcphdr *th = tcp_hdr(skb);
120 
121         return __cookie_v6_init_sequence(iph, th, mssp);
122 }
123 
124 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
125                       __u32 cookie)
126 {
127         __u32 seq = ntohl(th->seq) - 1;
128         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
129                                             th->source, th->dest, seq);
130 
131         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
132 }
133 EXPORT_SYMBOL_GPL(__cookie_v6_check);
134 
135 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
136 {
137         struct tcp_options_received tcp_opt;
138         struct inet_request_sock *ireq;
139         struct tcp_request_sock *treq;
140         struct ipv6_pinfo *np = inet6_sk(sk);
141         struct tcp_sock *tp = tcp_sk(sk);
142         const struct tcphdr *th = tcp_hdr(skb);
143         __u32 cookie = ntohl(th->ack_seq) - 1;
144         struct sock *ret = sk;
145         struct request_sock *req;
146         int mss;
147         struct dst_entry *dst;
148         __u8 rcv_wscale;
149 
150         if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
151                 goto out;
152 
153         if (tcp_synq_no_recent_overflow(sk))
154                 goto out;
155 
156         mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
157         if (mss == 0) {
158                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
159                 goto out;
160         }
161 
162         __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
163 
164         /* check for timestamp cookie support */
165         memset(&tcp_opt, 0, sizeof(tcp_opt));
166         tcp_parse_options(skb, &tcp_opt, 0, NULL);
167 
168         if (!cookie_timestamp_decode(&tcp_opt))
169                 goto out;
170 
171         ret = NULL;
172         req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false);
173         if (!req)
174                 goto out;
175 
176         ireq = inet_rsk(req);
177         treq = tcp_rsk(req);
178         treq->tfo_listener = false;
179 
180         if (security_inet_conn_request(sk, skb, req))
181                 goto out_free;
182 
183         req->mss = mss;
184         ireq->ir_rmt_port = th->source;
185         ireq->ir_num = ntohs(th->dest);
186         ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
187         ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
188         if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
189             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
190             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
191                 atomic_inc(&skb->users);
192                 ireq->pktopts = skb;
193         }
194 
195         ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
196         /* So that link locals have meaning */
197         if (!sk->sk_bound_dev_if &&
198             ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
199                 ireq->ir_iif = tcp_v6_iif(skb);
200 
201         ireq->ir_mark = inet_request_mark(sk, skb);
202 
203         req->num_retrans = 0;
204         ireq->snd_wscale        = tcp_opt.snd_wscale;
205         ireq->sack_ok           = tcp_opt.sack_ok;
206         ireq->wscale_ok         = tcp_opt.wscale_ok;
207         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
208         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
209         treq->snt_synack.v64    = 0;
210         treq->rcv_isn = ntohl(th->seq) - 1;
211         treq->snt_isn = cookie;
212 
213         /*
214          * We need to lookup the dst_entry to get the correct window size.
215          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
216          * me if there is a preferred way.
217          */
218         {
219                 struct in6_addr *final_p, final;
220                 struct flowi6 fl6;
221                 memset(&fl6, 0, sizeof(fl6));
222                 fl6.flowi6_proto = IPPROTO_TCP;
223                 fl6.daddr = ireq->ir_v6_rmt_addr;
224                 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
225                 fl6.saddr = ireq->ir_v6_loc_addr;
226                 fl6.flowi6_oif = ireq->ir_iif;
227                 fl6.flowi6_mark = ireq->ir_mark;
228                 fl6.fl6_dport = ireq->ir_rmt_port;
229                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
230                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
231 
232                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
233                 if (IS_ERR(dst))
234                         goto out_free;
235         }
236 
237         req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
238         tcp_select_initial_window(tcp_full_space(sk), req->mss,
239                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
240                                   ireq->wscale_ok, &rcv_wscale,
241                                   dst_metric(dst, RTAX_INITRWND));
242 
243         ireq->rcv_wscale = rcv_wscale;
244         ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
245 
246         ret = tcp_get_cookie_sock(sk, skb, req, dst);
247 out:
248         return ret;
249 out_free:
250         reqsk_free(req);
251         return NULL;
252 }
253 

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