TCPCT part 1g: Responder Cookie => Initiator

Parse incoming TCP_COOKIE option(s).

Calculate <SYN,ACK> TCP_COOKIE option.

Send optional <SYN,ACK> data.

This is a significantly revised implementation of an earlier (year-old)
patch that no longer applies cleanly, with permission of the original
author (Adam Langley):

    http://thread.gmane.org/gmane.linux.network/102586

Requires:
   TCPCT part 1a: add request_values parameter for sending SYNACK
   TCPCT part 1b: generate Responder Cookie secret
   TCPCT part 1c: sysctl_tcp_cookie_size, socket option TCP_COOKIE_TRANSACTIONS
   TCPCT part 1d: define TCP cookie option, extend existing struct's
   TCPCT part 1e: implement socket option TCP_COOKIE_TRANSACTIONS
   TCPCT part 1f: Initiator Cookie => Responder

Signed-off-by: William.Allen.Simpson@gmail.com
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -407,6 +407,7 @@ extern int			tcp_recvmsg(struct kiocb *iocb, struct sock *sk,
 
 extern void			tcp_parse_options(struct sk_buff *skb,
 						  struct tcp_options_received *opt_rx,
+						  u8 **hvpp,
 						  int estab,
 						  struct dst_entry *dst);
 

net/ipv4/syncookies.c

@@ -253,6 +253,8 @@ 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);
@@ -263,7 +265,6 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 	int mss;
 	struct rtable *rt;
 	__u8 rcv_wscale;
-	struct tcp_options_received tcp_opt;
 
 	if (!sysctl_tcp_syncookies || !th->ack)
 		goto out;
@@ -341,7 +342,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 
 	/* check for timestamp cookie support */
 	memset(&tcp_opt, 0, sizeof(tcp_opt));
-	tcp_parse_options(skb, &tcp_opt, 0, &rt->u.dst);
+	tcp_parse_options(skb, &tcp_opt, &hash_location, 0, &rt->u.dst);
 
 	if (tcp_opt.saw_tstamp)
 		cookie_check_timestamp(&tcp_opt);

net/ipv4/tcp_input.c

@@ -3698,7 +3698,7 @@ static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
  * the fast version below fails.
  */
 void tcp_parse_options(struct sk_buff *skb, struct tcp_options_received *opt_rx,
-		       int estab,  struct dst_entry *dst)
+		       u8 **hvpp, int estab,  struct dst_entry *dst)
 {
 	unsigned char *ptr;
 	struct tcphdr *th = tcp_hdr(skb);
@@ -3785,7 +3785,30 @@ void tcp_parse_options(struct sk_buff *skb, struct tcp_options_received *opt_rx,
 				 */
 				break;
 #endif
-			}
+			case TCPOPT_COOKIE:
+				/* This option is variable length.
+				 */
+				switch (opsize) {
+				case TCPOLEN_COOKIE_BASE:
+					/* not yet implemented */
+					break;
+				case TCPOLEN_COOKIE_PAIR:
+					/* not yet implemented */
+					break;
+				case TCPOLEN_COOKIE_MIN+0:
+				case TCPOLEN_COOKIE_MIN+2:
+				case TCPOLEN_COOKIE_MIN+4:
+				case TCPOLEN_COOKIE_MIN+6:
+				case TCPOLEN_COOKIE_MAX:
+					/* 16-bit multiple */
+					opt_rx->cookie_plus = opsize;
+					*hvpp = ptr;
+				default:
+					/* ignore option */
+					break;
+				};
+				break;
+			};
 
 			ptr += opsize-2;
 			length -= opsize;
@@ -3813,17 +3836,20 @@ static int tcp_parse_aligned_timestamp(struct tcp_sock *tp, struct tcphdr *th)
  * If it is wrong it falls back on tcp_parse_options().
  */
 static int tcp_fast_parse_options(struct sk_buff *skb, struct tcphdr *th,
-				  struct tcp_sock *tp)
+				  struct tcp_sock *tp, u8 **hvpp)
 {
-	if (th->doff == sizeof(struct tcphdr) >> 2) {
+	/* In the spirit of fast parsing, compare doff directly to constant
+	 * values.  Because equality is used, short doff can be ignored here.
+	 */
+	if (th->doff == (sizeof(*th) / 4)) {
 		tp->rx_opt.saw_tstamp = 0;
 		return 0;
 	} else if (tp->rx_opt.tstamp_ok &&
-		   th->doff == (sizeof(struct tcphdr)>>2)+(TCPOLEN_TSTAMP_ALIGNED>>2)) {
+		   th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
 		if (tcp_parse_aligned_timestamp(tp, th))
 			return 1;
 	}
-	tcp_parse_options(skb, &tp->rx_opt, 1, NULL);
+	tcp_parse_options(skb, &tp->rx_opt, hvpp, 1, NULL);
 	return 1;
 }
 
@@ -5077,10 +5103,12 @@ static int tcp_dma_try_early_copy(struct sock *sk, struct sk_buff *skb,
 static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 			      struct tcphdr *th, int syn_inerr)
 {
+	u8 *hash_location;
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	/* RFC1323: H1. Apply PAWS check first. */
-	if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp &&
+	if (tcp_fast_parse_options(skb, th, tp, &hash_location) &&
+	    tp->rx_opt.saw_tstamp &&
 	    tcp_paws_discard(sk, skb)) {
 		if (!th->rst) {
 			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
@@ -5368,12 +5396,14 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
 static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 					 struct tcphdr *th, unsigned len)
 {
-	struct tcp_sock *tp = tcp_sk(sk);
+	u8 *hash_location;
 	struct inet_connection_sock *icsk = inet_csk(sk);
-	int saved_clamp = tp->rx_opt.mss_clamp;
+	struct tcp_sock *tp = tcp_sk(sk);
 	struct dst_entry *dst = __sk_dst_get(sk);
+	struct tcp_cookie_values *cvp = tp->cookie_values;
+	int saved_clamp = tp->rx_opt.mss_clamp;
 
-	tcp_parse_options(skb, &tp->rx_opt, 0, dst);
+	tcp_parse_options(skb, &tp->rx_opt, &hash_location, 0, dst);
 
 	if (th->ack) {
 		/* rfc793:
@@ -5470,6 +5500,31 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 		 * Change state from SYN-SENT only after copied_seq
 		 * is initialized. */
 		tp->copied_seq = tp->rcv_nxt;
+
+		if (cvp != NULL &&
+		    cvp->cookie_pair_size > 0 &&
+		    tp->rx_opt.cookie_plus > 0) {
+			int cookie_size = tp->rx_opt.cookie_plus
+					- TCPOLEN_COOKIE_BASE;
+			int cookie_pair_size = cookie_size
+					     + cvp->cookie_desired;
+
+			/* A cookie extension option was sent and returned.
+			 * Note that each incoming SYNACK replaces the
+			 * Responder cookie.  The initial exchange is most
+			 * fragile, as protection against spoofing relies
+			 * entirely upon the sequence and timestamp (above).
+			 * This replacement strategy allows the correct pair to
+			 * pass through, while any others will be filtered via
+			 * Responder verification later.
+			 */
+			if (sizeof(cvp->cookie_pair) >= cookie_pair_size) {
+				memcpy(&cvp->cookie_pair[cvp->cookie_desired],
+				       hash_location, cookie_size);
+				cvp->cookie_pair_size = cookie_pair_size;
+			}
+		}
+
 		smp_mb();
 		tcp_set_state(sk, TCP_ESTABLISHED);
 

net/ipv4/tcp_ipv4.c

@@ -1213,9 +1213,12 @@ static struct timewait_sock_ops tcp_timewait_sock_ops = {
 
 int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 {
+	struct tcp_extend_values tmp_ext;
 	struct tcp_options_received tmp_opt;
+	u8 *hash_location;
 	struct request_sock *req;
 	struct inet_request_sock *ireq;
+	struct tcp_sock *tp = tcp_sk(sk);
 	struct dst_entry *dst = NULL;
 	__be32 saddr = ip_hdr(skb)->saddr;
 	__be32 daddr = ip_hdr(skb)->daddr;
@@ -1271,15 +1274,49 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 
 	tcp_clear_options(&tmp_opt);
 	tmp_opt.mss_clamp = TCP_MSS_DEFAULT;
-	tmp_opt.user_mss  = tcp_sk(sk)->rx_opt.user_mss;
+	tmp_opt.user_mss  = tp->rx_opt.user_mss;
+	tcp_parse_options(skb, &tmp_opt, &hash_location, 0, dst);
+
+	if (tmp_opt.cookie_plus > 0 &&
+	    tmp_opt.saw_tstamp &&
+	    !tp->rx_opt.cookie_out_never &&
+	    (sysctl_tcp_cookie_size > 0 ||
+	     (tp->cookie_values != NULL &&
+	      tp->cookie_values->cookie_desired > 0))) {
+		u8 *c;
+		u32 *mess = &tmp_ext.cookie_bakery[COOKIE_DIGEST_WORDS];
+		int l = tmp_opt.cookie_plus - TCPOLEN_COOKIE_BASE;
+
+		if (tcp_cookie_generator(&tmp_ext.cookie_bakery[0]) != 0)
+			goto drop_and_release;
+
+		/* Secret recipe starts with IP addresses */
+		*mess++ ^= daddr;
+		*mess++ ^= saddr;
 
-	tcp_parse_options(skb, &tmp_opt, 0, dst);
+		/* plus variable length Initiator Cookie */
+		c = (u8 *)mess;
+		while (l-- > 0)
+			*c++ ^= *hash_location++;
+
+#ifdef CONFIG_SYN_COOKIES
+		want_cookie = 0;	/* not our kind of cookie */
+#endif
+		tmp_ext.cookie_out_never = 0; /* false */
+		tmp_ext.cookie_plus = tmp_opt.cookie_plus;
+	} else if (!tp->rx_opt.cookie_in_always) {
+		/* redundant indications, but ensure initialization. */
+		tmp_ext.cookie_out_never = 1; /* true */
+		tmp_ext.cookie_plus = 0;
+	} else {
+		goto drop_and_release;
+	}
+	tmp_ext.cookie_in_always = tp->rx_opt.cookie_in_always;
 
 	if (want_cookie && !tmp_opt.saw_tstamp)
 		tcp_clear_options(&tmp_opt);
 
 	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
-
 	tcp_openreq_init(req, &tmp_opt, skb);
 
 	if (security_inet_conn_request(sk, skb, req))
@@ -1339,7 +1376,9 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 	}
 	tcp_rsk(req)->snt_isn = isn;
 
-	if (__tcp_v4_send_synack(sk, dst, req, NULL) || want_cookie)
+	if (__tcp_v4_send_synack(sk, dst, req,
+				 (struct request_values *)&tmp_ext) ||
+	    want_cookie)
 		goto drop_and_free;
 
 	inet_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);

net/ipv4/tcp_minisocks.c

@@ -90,13 +90,14 @@ enum tcp_tw_status
 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 			   const struct tcphdr *th)
 {
-	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 	struct tcp_options_received tmp_opt;
+	u8 *hash_location;
+	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 	int paws_reject = 0;
 
 	if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
 		tmp_opt.tstamp_ok = 1;
-		tcp_parse_options(skb, &tmp_opt, 1, NULL);
+		tcp_parse_options(skb, &tmp_opt, &hash_location, 1, NULL);
 
 		if (tmp_opt.saw_tstamp) {
 			tmp_opt.ts_recent	= tcptw->tw_ts_recent;
@@ -518,15 +519,16 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 			   struct request_sock *req,
 			   struct request_sock **prev)
 {
+	struct tcp_options_received tmp_opt;
+	u8 *hash_location;
+	struct sock *child;
 	const struct tcphdr *th = tcp_hdr(skb);
 	__be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
 	int paws_reject = 0;
-	struct tcp_options_received tmp_opt;
-	struct sock *child;
 
-	if ((th->doff > (sizeof(struct tcphdr)>>2)) && (req->ts_recent)) {
+	if ((th->doff > (sizeof(*th) >> 2)) && (req->ts_recent)) {
 		tmp_opt.tstamp_ok = 1;
-		tcp_parse_options(skb, &tmp_opt, 1, NULL);
+		tcp_parse_options(skb, &tmp_opt, &hash_location, 1, NULL);
 
 		if (tmp_opt.saw_tstamp) {
 			tmp_opt.ts_recent = req->ts_recent;