--- /dev/null
+/*
+ * Syncookies implementation for the Linux kernel
+ *
+ * Copyright (C) 1997 Andi Kleen
+ * Based on ideas by D.J.Bernstein and Eric Schenk.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+
+#include <linux/tcp.h>
+#include <linux/slab.h>
+#include <linux/random.h>
+#include <linux/cryptohash.h>
+#include <linux/kernel.h>
+#include <linux/export.h>
+#include <net/tcp.h>
+#include <net/route.h>
+
+extern int sysctl_tcp_syncookies;
+
+static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
+
+#define COOKIEBITS 24 /* Upper bits store count */
+#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
+
+/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
+ * stores TCP options:
+ *
+ * MSB LSB
+ * | 31 ... 6 | 5 | 4 | 3 2 1 0 |
+ * | Timestamp | ECN | SACK | WScale |
+ *
+ * When we receive a valid cookie-ACK, we look at the echoed tsval (if
+ * any) to figure out which TCP options we should use for the rebuilt
+ * connection.
+ *
+ * A WScale setting of '0xf' (which is an invalid scaling value)
+ * means that original syn did not include the TCP window scaling option.
+ */
+#define TS_OPT_WSCALE_MASK 0xf
+#define TS_OPT_SACK BIT(4)
+#define TS_OPT_ECN BIT(5)
+/* There is no TS_OPT_TIMESTAMP:
+ * if ACK contains timestamp option, we already know it was
+ * requested/supported by the syn/synack exchange.
+ */
+#define TSBITS 6
+#define TSMASK (((__u32)1 << TSBITS) - 1)
+
+static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
+ ipv4_cookie_scratch);
+
+static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
+ u32 count, int c)
+{
+ __u32 *tmp;
+
+ net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
+
+ tmp = this_cpu_ptr(ipv4_cookie_scratch);
+ memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
+ tmp[0] = (__force u32)saddr;
+ tmp[1] = (__force u32)daddr;
+ tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
+ tmp[3] = count;
+ sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
+
+ return tmp[17];
+}
+
+
+/*
+ * when syncookies are in effect and tcp timestamps are enabled we encode
+ * tcp options in the lower bits of the timestamp value that will be
+ * sent in the syn-ack.
+ * Since subsequent timestamps use the normal tcp_time_stamp value, we
+ * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
+ */
+__u32 cookie_init_timestamp(struct request_sock *req)
+{
+ struct inet_request_sock *ireq;
+ u32 ts, ts_now = tcp_time_stamp;
+ u32 options = 0;
+
+ ireq = inet_rsk(req);
+
+ options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
+ if (ireq->sack_ok)
+ options |= TS_OPT_SACK;
+ if (ireq->ecn_ok)
+ options |= TS_OPT_ECN;
+
+ ts = ts_now & ~TSMASK;
+ ts |= options;
+ if (ts > ts_now) {
+ ts >>= TSBITS;
+ ts--;
+ ts <<= TSBITS;
+ ts |= options;
+ }
+ return ts;
+}
+
+
+static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
+ __be16 dport, __u32 sseq, __u32 data)
+{
+ /*
+ * Compute the secure sequence number.
+ * The output should be:
+ * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
+ * + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
+ * Where sseq is their sequence number and count increases every
+ * minute by 1.
+ * As an extra hack, we add a small "data" value that encodes the
+ * MSS into the second hash value.
+ */
+ u32 count = tcp_cookie_time();
+ return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
+ sseq + (count << COOKIEBITS) +
+ ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
+ & COOKIEMASK));
+}
+
+/*
+ * This retrieves the small "data" value from the syncookie.
+ * If the syncookie is bad, the data returned will be out of
+ * range. This must be checked by the caller.
+ *
+ * The count value used to generate the cookie must be less than
+ * MAX_SYNCOOKIE_AGE minutes in the past.
+ * The return value (__u32)-1 if this test fails.
+ */
+static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
+ __be16 sport, __be16 dport, __u32 sseq)
+{
+ u32 diff, count = tcp_cookie_time();
+
+ /* Strip away the layers from the cookie */
+ cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
+
+ /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
+ diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
+ if (diff >= MAX_SYNCOOKIE_AGE)
+ return (__u32)-1;
+
+ return (cookie -
+ cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
+ & COOKIEMASK; /* Leaving the data behind */
+}
+
+/*
+ * MSS Values are chosen based on the 2011 paper
+ * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
+ * Values ..
+ * .. lower than 536 are rare (< 0.2%)
+ * .. between 537 and 1299 account for less than < 1.5% of observed values
+ * .. in the 1300-1349 range account for about 15 to 20% of observed mss values
+ * .. exceeding 1460 are very rare (< 0.04%)
+ *
+ * 1460 is the single most frequently announced mss value (30 to 46% depending
+ * on monitor location). Table must be sorted.
+ */
+static __u16 const msstab[] = {
+ 536,
+ 1300,
+ 1440, /* 1440, 1452: PPPoE */
+ 1460,
+};
+
+/*
+ * Generate a syncookie. mssp points to the mss, which is returned
+ * rounded down to the value encoded in the cookie.
+ */
+u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
+ u16 *mssp)
+{
+ int mssind;
+ const __u16 mss = *mssp;
+
+ for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
+ if (mss >= msstab[mssind])
+ break;
+ *mssp = msstab[mssind];
+
+ return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
+ th->source, th->dest, ntohl(th->seq),
+ mssind);
+}
+EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
+
+__u32 cookie_v4_init_sequence(struct sock *sk, const struct sk_buff *skb,
+ __u16 *mssp)
+{
+ const struct iphdr *iph = ip_hdr(skb);
+ const struct tcphdr *th = tcp_hdr(skb);
+
+ tcp_synq_overflow(sk);
+ NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
+
+ return __cookie_v4_init_sequence(iph, th, mssp);
+}
+
+/*
+ * Check if a ack sequence number is a valid syncookie.
+ * Return the decoded mss if it is, or 0 if not.
+ */
+int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
+ u32 cookie)
+{
+ __u32 seq = ntohl(th->seq) - 1;
+ __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
+ th->source, th->dest, seq);
+
+ return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
+}
+EXPORT_SYMBOL_GPL(__cookie_v4_check);
+
+static struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
+ struct request_sock *req,
+ struct dst_entry *dst)
+{
+ struct inet_connection_sock *icsk = inet_csk(sk);
+ struct sock *child;
+
+ child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
+ if (child) {
+ atomic_set(&req->rsk_refcnt, 1);
+ inet_csk_reqsk_queue_add(sk, req, child);
+ } else {
+ reqsk_free(req);
+ }
+ return child;
+}
+
+
+/*
+ * when syncookies are in effect and tcp timestamps are enabled we stored
+ * additional tcp options in the timestamp.
+ * This extracts these options from the timestamp echo.
+ *
+ * return false if we decode a tcp option that is disabled
+ * on the host.
+ */
+bool cookie_timestamp_decode(struct tcp_options_received *tcp_opt)
+{
+ /* echoed timestamp, lowest bits contain options */
+ u32 options = tcp_opt->rcv_tsecr;
+
+ if (!tcp_opt->saw_tstamp) {
+ tcp_clear_options(tcp_opt);
+ return true;
+ }
+
+ if (!sysctl_tcp_timestamps)
+ return false;
+
+ tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
+
+ if (tcp_opt->sack_ok && !sysctl_tcp_sack)
+ return false;
+
+ if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
+ return true; /* no window scaling */
+
+ tcp_opt->wscale_ok = 1;
+ tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
+
+ return sysctl_tcp_window_scaling != 0;
+}
+EXPORT_SYMBOL(cookie_timestamp_decode);
+
+bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
+ const struct net *net, const struct dst_entry *dst)
+{
+ bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
+
+ if (!ecn_ok)
+ return false;
+
+ if (net->ipv4.sysctl_tcp_ecn)
+ return true;
+
+ return dst_feature(dst, RTAX_FEATURE_ECN);
+}
+EXPORT_SYMBOL(cookie_ecn_ok);
+
+struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
+{
+ struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
+ struct tcp_options_received tcp_opt;
+ struct inet_request_sock *ireq;
+ struct tcp_request_sock *treq;
+ struct tcp_sock *tp = tcp_sk(sk);
+ const struct tcphdr *th = tcp_hdr(skb);
+ __u32 cookie = ntohl(th->ack_seq) - 1;
+ struct sock *ret = sk;
+ struct request_sock *req;
+ int mss;
+ struct rtable *rt;
+ __u8 rcv_wscale;
+ struct flowi4 fl4;
+
+ if (!sysctl_tcp_syncookies || !th->ack || th->rst)
+ goto out;
+
+ if (tcp_synq_no_recent_overflow(sk))
+ goto out;
+
+ mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
+ if (mss == 0) {
+ NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
+ goto out;
+ }
+
+ NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
+
+ /* check for timestamp cookie support */
+ memset(&tcp_opt, 0, sizeof(tcp_opt));
+ tcp_parse_options(skb, &tcp_opt, 0, NULL);
+
+ if (!cookie_timestamp_decode(&tcp_opt))
+ goto out;
+
+ ret = NULL;
+ req = inet_reqsk_alloc(&tcp_request_sock_ops, sk); /* for safety */
+ if (!req)
+ goto out;
+
+ ireq = inet_rsk(req);
+ treq = tcp_rsk(req);
+ treq->rcv_isn = ntohl(th->seq) - 1;
+ treq->snt_isn = cookie;
+ req->mss = mss;
+ ireq->ir_num = ntohs(th->dest);
+ ireq->ir_rmt_port = th->source;
+ sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
+ sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
+ ireq->ir_mark = inet_request_mark(sk, skb);
+ ireq->snd_wscale = tcp_opt.snd_wscale;
+ ireq->sack_ok = tcp_opt.sack_ok;
+ ireq->wscale_ok = tcp_opt.wscale_ok;
+ ireq->tstamp_ok = tcp_opt.saw_tstamp;
+ req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
+ treq->snt_synack = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
+ treq->tfo_listener = false;
+
+ ireq->ir_iif = sk->sk_bound_dev_if;
+
+ /* We throwed the options of the initial SYN away, so we hope
+ * the ACK carries the same options again (see RFC1122 4.2.3.8)
+ */
+ ireq->opt = tcp_v4_save_options(skb);
+
+ if (security_inet_conn_request(sk, skb, req)) {
+ reqsk_free(req);
+ goto out;
+ }
+
+ req->num_retrans = 0;
+
+ /*
+ * We need to lookup the route here to get at the correct
+ * window size. We should better make sure that the window size
+ * hasn't changed since we received the original syn, but I see
+ * no easy way to do this.
+ */
+ flowi4_init_output(&fl4, sk->sk_bound_dev_if, ireq->ir_mark,
+ RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
+ inet_sk_flowi_flags(sk),
+ opt->srr ? opt->faddr : ireq->ir_rmt_addr,
+ ireq->ir_loc_addr, th->source, th->dest);
+ security_req_classify_flow(req, flowi4_to_flowi(&fl4));
+ rt = ip_route_output_key(sock_net(sk), &fl4);
+ if (IS_ERR(rt)) {
+ reqsk_free(req);
+ goto out;
+ }
+
+ /* Try to redo what tcp_v4_send_synack did. */
+ req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
+
+ tcp_select_initial_window(tcp_full_space(sk), req->mss,
+ &req->rcv_wnd, &req->window_clamp,
+ ireq->wscale_ok, &rcv_wscale,
+ dst_metric(&rt->dst, RTAX_INITRWND));
+
+ ireq->rcv_wscale = rcv_wscale;
+ ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
+
+ ret = get_cookie_sock(sk, skb, req, &rt->dst);
+ /* ip_queue_xmit() depends on our flow being setup
+ * Normal sockets get it right from inet_csk_route_child_sock()
+ */
+ if (ret)
+ inet_sk(ret)->cork.fl.u.ip4 = fl4;
+out: return ret;
+}
Code Review / kvmfornfv.git / blobdiff