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>

1 files changed, 19 insertions, 13 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index cbbafe546c0f..3a51582bef55 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -94,12 +94,18 @@ static int tcp_v4_md5_hash_hdr(char *md5_hash, const struct tcp_md5sig_key *key,
 struct inet_hashinfo tcp_hashinfo;
 EXPORT_SYMBOL(tcp_hashinfo);
 
-static u32 tcp_v4_init_seq_and_tsoff(const struct sk_buff *skb, u32 *tsoff)
+static u32 tcp_v4_init_seq(const struct sk_buff *skb)
 {
-	return secure_tcp_seq_and_tsoff(ip_hdr(skb)->daddr,
-					ip_hdr(skb)->saddr,
-					tcp_hdr(skb)->dest,
-					tcp_hdr(skb)->source, tsoff);
+	return secure_tcp_seq(ip_hdr(skb)->daddr,
+			      ip_hdr(skb)->saddr,
+			      tcp_hdr(skb)->dest,
+			      tcp_hdr(skb)->source);
+}
+
+static u32 tcp_v4_init_ts_off(const struct sk_buff *skb)
+{
+	return secure_tcp_ts_off(ip_hdr(skb)->daddr,
+				 ip_hdr(skb)->saddr);
 }
 
 int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp)
@@ -145,7 +151,6 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 	struct flowi4 *fl4;
 	struct rtable *rt;
 	int err;
-	u32 seq;
 	struct ip_options_rcu *inet_opt;
 	struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
 
@@ -232,13 +237,13 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 	rt = NULL;
 
 	if (likely(!tp->repair)) {
-		seq = secure_tcp_seq_and_tsoff(inet->inet_saddr,
-					       inet->inet_daddr,
-					       inet->inet_sport,
-					       usin->sin_port,
-					       &tp->tsoffset);
 		if (!tp->write_seq)
-			tp->write_seq = seq;
+			tp->write_seq = secure_tcp_seq(inet->inet_saddr,
+						       inet->inet_daddr,
+						       inet->inet_sport,
+						       usin->sin_port);
+		tp->tsoffset = secure_tcp_ts_off(inet->inet_saddr,
+						 inet->inet_daddr);
 	}
 
 	inet->inet_id = tp->write_seq ^ jiffies;
@@ -1239,7 +1244,8 @@ static const struct tcp_request_sock_ops tcp_request_sock_ipv4_ops = {
 	.cookie_init_seq =	cookie_v4_init_sequence,
 #endif
 	.route_req	=	tcp_v4_route_req,
-	.init_seq_tsoff	=	tcp_v4_init_seq_and_tsoff,
+	.init_seq	=	tcp_v4_init_seq,
+	.init_ts_off	=	tcp_v4_init_ts_off,
 	.send_synack	=	tcp_v4_send_synack,
 };