net/tcp_fastopen: Disable active side TFO in certain scenarios

Middlebox firewall issues can potentially cause server's data being
blackholed after a successful 3WHS using TFO. Following are the related
reports from Apple:
https://www.nanog.org/sites/default/files/Paasch_Network_Support.pdf
Slide 31 identifies an issue where the client ACK to the server's data
sent during a TFO'd handshake is dropped.
C ---> syn-data ---> S
C <--- syn/ack ----- S
C (accept & write)
C <---- data ------- S
C ----- ACK -> X     S
		[retry and timeout]

https://www.ietf.org/proceedings/94/slides/slides-94-tcpm-13.pdf
Slide 5 shows a similar situation that the server's data gets dropped
after 3WHS.
C ---- syn-data ---> S
C <--- syn/ack ----- S
C ---- ack --------> S
S (accept & write)
C?  X <- data ------ S
		[retry and timeout]

This is the worst failure b/c the client can not detect such behavior to
mitigate the situation (such as disabling TFO). Failing to proceed, the
application (e.g., SSL library) may simply timeout and retry with TFO
again, and the process repeats indefinitely.

The proposed solution is to disable active TFO globally under the
following circumstances:
1. client side TFO socket detects out of order FIN
2. client side TFO socket receives out of order RST

We disable active side TFO globally for 1hr at first. Then if it
happens again, we disable it for 2h, then 4h, 8h, ...
And we reset the timeout to 1hr if a client side TFO sockets not opened
on loopback has successfully received data segs from server.
And we examine this condition during close().

The rational behind it is that when such firewall issue happens,
application running on the client should eventually close the socket as
it is not able to get the data it is expecting. Or application running
on the server should close the socket as it is not able to receive any
response from client.
In both cases, out of order FIN or RST will get received on the client
given that the firewall will not block them as no data are in those
frames.
And we want to disable active TFO globally as it helps if the middle box
is very close to the client and most of the connections are likely to
fail.

Also, add a debug sysctl:
  tcp_fastopen_blackhole_detect_timeout_sec:
    the initial timeout to use when firewall blackhole issue happens.
    This can be set and read.
    When setting it to 0, it means to disable the active disable logic.

Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 101 insertions, 0 deletions

diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
index 8ea4e9787f82..ff2d30ffc6f3 100644
--- a/net/ipv4/tcp_fastopen.c
+++ b/net/ipv4/tcp_fastopen.c
@@ -341,6 +341,13 @@ bool tcp_fastopen_cookie_check(struct sock *sk, u16 *mss,
 		cookie->len = -1;
 		return false;
 	}
+
+	/* Firewall blackhole issue check */
+	if (tcp_fastopen_active_should_disable(sk)) {
+		cookie->len = -1;
+		return false;
+	}
+
 	if (sysctl_tcp_fastopen & TFO_CLIENT_NO_COOKIE) {
 		cookie->len = -1;
 		return true;
@@ -380,3 +387,97 @@ bool tcp_fastopen_defer_connect(struct sock *sk, int *err)
 	return false;
 }
 EXPORT_SYMBOL(tcp_fastopen_defer_connect);
+
+/*
+ * The following code block is to deal with middle box issues with TFO:
+ * Middlebox firewall issues can potentially cause server's data being
+ * blackholed after a successful 3WHS using TFO.
+ * The proposed solution is to disable active TFO globally under the
+ * following circumstances:
+ *   1. client side TFO socket receives out of order FIN
+ *   2. client side TFO socket receives out of order RST
+ * We disable active side TFO globally for 1hr at first. Then if it
+ * happens again, we disable it for 2h, then 4h, 8h, ...
+ * And we reset the timeout back to 1hr when we see a successful active
+ * TFO connection with data exchanges.
+ */
+
+/* Default to 1hr */
+unsigned int sysctl_tcp_fastopen_blackhole_timeout __read_mostly = 60 * 60;
+static atomic_t tfo_active_disable_times __read_mostly = ATOMIC_INIT(0);
+static unsigned long tfo_active_disable_stamp __read_mostly;
+
+/* Disable active TFO and record current jiffies and
+ * tfo_active_disable_times
+ */
+void tcp_fastopen_active_disable(void)
+{
+	atomic_inc(&tfo_active_disable_times);
+	tfo_active_disable_stamp = jiffies;
+}
+
+/* Reset tfo_active_disable_times to 0 */
+void tcp_fastopen_active_timeout_reset(void)
+{
+	atomic_set(&tfo_active_disable_times, 0);
+}
+
+/* Calculate timeout for tfo active disable
+ * Return true if we are still in the active TFO disable period
+ * Return false if timeout already expired and we should use active TFO
+ */
+bool tcp_fastopen_active_should_disable(struct sock *sk)
+{
+	int tfo_da_times = atomic_read(&tfo_active_disable_times);
+	int multiplier;
+	unsigned long timeout;
+
+	if (!tfo_da_times)
+		return false;
+
+	/* Limit timout to max: 2^6 * initial timeout */
+	multiplier = 1 << min(tfo_da_times - 1, 6);
+	timeout = multiplier * sysctl_tcp_fastopen_blackhole_timeout * HZ;
+	if (time_before(jiffies, tfo_active_disable_stamp + timeout))
+		return true;
+
+	/* Mark check bit so we can check for successful active TFO
+	 * condition and reset tfo_active_disable_times
+	 */
+	tcp_sk(sk)->syn_fastopen_ch = 1;
+	return false;
+}
+
+/* Disable active TFO if FIN is the only packet in the ofo queue
+ * and no data is received.
+ * Also check if we can reset tfo_active_disable_times if data is
+ * received successfully on a marked active TFO sockets opened on
+ * a non-loopback interface
+ */
+void tcp_fastopen_active_disable_ofo_check(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct rb_node *p;
+	struct sk_buff *skb;
+	struct dst_entry *dst;
+
+	if (!tp->syn_fastopen)
+		return;
+
+	if (!tp->data_segs_in) {
+		p = rb_first(&tp->out_of_order_queue);
+		if (p && !rb_next(p)) {
+			skb = rb_entry(p, struct sk_buff, rbnode);
+			if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
+				tcp_fastopen_active_disable();
+				return;
+			}
+		}
+	} else if (tp->syn_fastopen_ch &&
+		   atomic_read(&tfo_active_disable_times)) {
+		dst = sk_dst_get(sk);
+		if (!(dst && dst->dev && (dst->dev->flags & IFF_LOOPBACK)))
+			tcp_fastopen_active_timeout_reset();
+		dst_release(dst);
+	}
+}