tcp: randomize timestamps on syncookies

Whole point of randomization was to hide server uptime, but an attacker
can simply start a syn flood and TCP generates 'old style' timestamps,
directly revealing server jiffies value.

Also, TSval sent by the server to a particular remote address vary
depending on syncookies being sent or not, potentially triggering PAWS
drops for innocent clients.

Lets implement proper randomization, including for SYNcookies.

Also we do not need to export sysctl_tcp_timestamps, since it is not
used from a module.

In v2, I added Florian feedback and contribution, adding tsoff to
tcp_get_cookie_sock().

v3 removed one unused variable in tcp_v4_connect() as Florian spotted.

Fixes: 95a22caee396c ("tcp: randomize tcp timestamp offsets for each connection")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Tested-by: Florian Westphal <fw@strlen.de>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 28 insertions, 14 deletions

diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
index 895ff650db43..5abc3692b901 100644
--- a/net/ipv6/syncookies.c
+++ b/net/ipv6/syncookies.c
@@ -18,6 +18,7 @@
 #include <linux/random.h>
 #include <linux/siphash.h>
 #include <linux/kernel.h>
+#include <net/secure_seq.h>
 #include <net/ipv6.h>
 #include <net/tcp.h>
 
@@ -143,6 +144,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 	int mss;
 	struct dst_entry *dst;
 	__u8 rcv_wscale;
+	u32 tsoff = 0;
 
 	if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
 		goto out;
@@ -162,6 +164,12 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 	memset(&tcp_opt, 0, sizeof(tcp_opt));
 	tcp_parse_options(skb, &tcp_opt, 0, NULL);
 
+	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
+		tsoff = secure_tcpv6_ts_off(ipv6_hdr(skb)->daddr.s6_addr32,
+					    ipv6_hdr(skb)->saddr.s6_addr32);
+		tcp_opt.rcv_tsecr -= tsoff;
+	}
+
 	if (!cookie_timestamp_decode(&tcp_opt))
 		goto out;
 
@@ -242,7 +250,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 	ireq->rcv_wscale = rcv_wscale;
 	ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
 
-	ret = tcp_get_cookie_sock(sk, skb, req, dst);
+	ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
 out:
 	return ret;
 out_free:
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 8e42e8f54b70..aeb9497b5bb7 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -101,12 +101,18 @@ static void inet6_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
 	}
 }
 
-static u32 tcp_v6_init_seq_and_tsoff(const struct sk_buff *skb, u32 *tsoff)
+static u32 tcp_v6_init_seq(const struct sk_buff *skb)
 {
-	return secure_tcpv6_seq_and_tsoff(ipv6_hdr(skb)->daddr.s6_addr32,
-					  ipv6_hdr(skb)->saddr.s6_addr32,
-					  tcp_hdr(skb)->dest,
-					  tcp_hdr(skb)->source, tsoff);
+	return secure_tcpv6_seq(ipv6_hdr(skb)->daddr.s6_addr32,
+				ipv6_hdr(skb)->saddr.s6_addr32,
+				tcp_hdr(skb)->dest,
+				tcp_hdr(skb)->source);
+}
+
+static u32 tcp_v6_init_ts_off(const struct sk_buff *skb)
+{
+	return secure_tcpv6_ts_off(ipv6_hdr(skb)->daddr.s6_addr32,
+				   ipv6_hdr(skb)->saddr.s6_addr32);
 }
 
 static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
@@ -122,7 +128,6 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
 	struct flowi6 fl6;
 	struct dst_entry *dst;
 	int addr_type;
-	u32 seq;
 	int err;
 	struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
 
@@ -282,13 +287,13 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
 	sk_set_txhash(sk);
 
 	if (likely(!tp->repair)) {
-		seq = secure_tcpv6_seq_and_tsoff(np->saddr.s6_addr32,
-						 sk->sk_v6_daddr.s6_addr32,
-						 inet->inet_sport,
-						 inet->inet_dport,
-						 &tp->tsoffset);
 		if (!tp->write_seq)
-			tp->write_seq = seq;
+			tp->write_seq = secure_tcpv6_seq(np->saddr.s6_addr32,
+							 sk->sk_v6_daddr.s6_addr32,
+							 inet->inet_sport,
+							 inet->inet_dport);
+		tp->tsoffset = secure_tcpv6_ts_off(np->saddr.s6_addr32,
+						   sk->sk_v6_daddr.s6_addr32);
 	}
 
 	if (tcp_fastopen_defer_connect(sk, &err))
@@ -749,7 +754,8 @@ static const struct tcp_request_sock_ops tcp_request_sock_ipv6_ops = {
 	.cookie_init_seq =	cookie_v6_init_sequence,
 #endif
 	.route_req	=	tcp_v6_route_req,
-	.init_seq_tsoff	=	tcp_v6_init_seq_and_tsoff,
+	.init_seq	=	tcp_v6_init_seq,
+	.init_ts_off	=	tcp_v6_init_ts_off,
 	.send_synack	=	tcp_v6_send_synack,
 };