1 files changed, 377 insertions, 0 deletions
diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
new file mode 100644
index 00000000..895f2157
--- /dev/null
+++ b/net/ipv4/syncookies.c
@@ -0,0 +1,377 @@
+/*
+ *  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 <net/tcp.h>
+#include <net/route.h>
+
+/* Timestamps: lowest bits store TCP options */
+#define TSBITS 6
+#define TSMASK (((__u32)1 << TSBITS) - 1)
+
+extern int sysctl_tcp_syncookies;
+
+__u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
+EXPORT_SYMBOL(syncookie_secret);
+
+static __init int init_syncookies(void)
+{
+	get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
+	return 0;
+}
+__initcall(init_syncookies);
+
+#define COOKIEBITS 24	/* Upper bits store count */
+#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 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 = __get_cpu_var(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 : 0xf;
+	options |= ireq->sack_ok << 4;
+	options |= ireq->ecn_ok << 5;
+
+	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 count,
+				   __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.
+	 */
+
+	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 within
+ * "maxdiff" if the current (passed-in) "count".  The return value
+ * is (__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 count, __u32 maxdiff)
+{
+	__u32 diff;
+
+	/* 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 >= maxdiff)
+		return (__u32)-1;
+
+	return (cookie -
+		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
+		& COOKIEMASK;	/* Leaving the data behind */
+}
+
+/*
+ * MSS Values are taken from the 2009 paper
+ * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
+ *  - values 1440 to 1460 accounted for 80% of observed mss values
+ *  - values outside the 536-1460 range are rare (<0.2%).
+ *
+ * Table must be sorted.
+ */
+static __u16 const msstab[] = {
+	64,
+	512,
+	536,
+	1024,
+	1440,
+	1460,
+	4312,
+	8960,
+};
+
+/*
+ * 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(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
+{
+	const struct iphdr *iph = ip_hdr(skb);
+	const struct tcphdr *th = tcp_hdr(skb);
+	int mssind;
+	const __u16 mss = *mssp;
+
+	tcp_synq_overflow(sk);
+
+	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
+		if (mss >= msstab[mssind])
+			break;
+	*mssp = msstab[mssind];
+
+	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
+
+	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
+				     th->source, th->dest, ntohl(th->seq),
+				     jiffies / (HZ * 60), mssind);
+}
+
+/*
+ * This (misnamed) value is the age of syncookie which is permitted.
+ * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
+ * sysctl_tcp_retries1. It's a rather complicated formula (exponential
+ * backoff) to compute at runtime so it's currently hardcoded here.
+ */
+#define COUNTER_TRIES 4
+/*
+ * Check if a ack sequence number is a valid syncookie.
+ * Return the decoded mss if it is, or 0 if not.
+ */
+static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
+{
+	const struct iphdr *iph = ip_hdr(skb);
+	const struct tcphdr *th = tcp_hdr(skb);
+	__u32 seq = ntohl(th->seq) - 1;
+	__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
+					    th->source, th->dest, seq,
+					    jiffies / (HZ * 60),
+					    COUNTER_TRIES);
+
+	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
+}
+
+static inline 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)
+		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.
+ *
+ * The lowest 4 bits store snd_wscale.
+ * next 2 bits indicate SACK and ECN support.
+ *
+ * return false if we decode an option that should not be.
+ */
+bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
+{
+	/* echoed timestamp, lowest bits contain options */
+	u32 options = tcp_opt->rcv_tsecr & TSMASK;
+
+	if (!tcp_opt->saw_tstamp)  {
+		tcp_clear_options(tcp_opt);
+		return true;
+	}
+
+	if (!sysctl_tcp_timestamps)
+		return false;
+
+	tcp_opt->sack_ok = (options >> 4) & 0x1;
+	*ecn_ok = (options >> 5) & 1;
+	if (*ecn_ok && !sysctl_tcp_ecn)
+		return false;
+
+	if (tcp_opt->sack_ok && !sysctl_tcp_sack)
+		return false;
+
+	if ((options & 0xf) == 0xf)
+		return true; /* no window scaling */
+
+	tcp_opt->wscale_ok = 1;
+	tcp_opt->snd_wscale = options & 0xf;
+	return sysctl_tcp_window_scaling != 0;
+}
+EXPORT_SYMBOL(cookie_check_timestamp);
+
+struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
+			     struct ip_options *opt)
+{
+	struct tcp_options_received tcp_opt;
+	u8 *hash_location;
+	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;
+	bool ecn_ok = false;
+	struct flowi4 fl4;
+
+	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
+		goto out;
+
+	if (tcp_synq_no_recent_overflow(sk) ||
+	    (mss = cookie_check(skb, cookie)) == 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, &hash_location, 0);
+
+	if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
+		goto out;
+
+	ret = NULL;
+	req = inet_reqsk_alloc(&tcp_request_sock_ops); /* 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->loc_port		= th->dest;
+	ireq->rmt_port		= th->source;
+	ireq->loc_addr		= ip_hdr(skb)->daddr;
+	ireq->rmt_addr		= ip_hdr(skb)->saddr;
+	ireq->ecn_ok		= ecn_ok;
+	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;
+
+	/* 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)
+	 */
+	if (opt && opt->optlen) {
+		int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
+
+		ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
+		if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
+			kfree(ireq->opt);
+			ireq->opt = NULL;
+		}
+	}
+
+	if (security_inet_conn_request(sk, skb, req)) {
+		reqsk_free(req);
+		goto out;
+	}
+
+	req->expires	= 0UL;
+	req->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, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
+			   RT_SCOPE_UNIVERSE, IPPROTO_TCP,
+			   inet_sk_flowi_flags(sk),
+			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
+			   ireq->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;
+
+	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;
+}