[PATCH] tcp: fix TCP_USER_TIMEOUT with zero window

[Enke Chen <enkechen2020@gmail.com>]

From: Enke Chen <enchen@paloaltonetworks.com>

The TCP session does not terminate with TCP_USER_TIMEOUT when data
remain untransmitted due to zero window.

The number of unanswered zero-window probes (tcp_probes_out) is
reset to zero with incoming acks irrespective of the window size,
as described in tcp_probe_timer():

    RFC 1122 4.2.2.17 requires the sender to stay open indefinitely
    as long as the receiver continues to respond probes. We support
    this by default and reset icsk_probes_out with incoming ACKs.

This counter, however, is the wrong one to be used in calculating the
duration that the window remains closed and data remain untransmitted.
Thanks to Jonathan Maxwell <jmaxwell37@gmail.com> for diagnosing the
actual issue.

In this patch a separate counter is introduced to track the number of
zero-window probes that are not answered with any non-zero window ack.
This new counter is used in determining when to abort the session with
TCP_USER_TIMEOUT.

Cc: stable@vger.kernel.org
Fixes: 9721e709fa68 ("tcp: simplify window probe aborting on USER_TIMEOUT")
Reported-by: William McCall <william.mccall@gmail.com>
Signed-off-by: Enke Chen <enchen@paloaltonetworks.com>
---
 include/linux/tcp.h   | 5 +++++
 net/ipv4/tcp.c        | 1 +
 net/ipv4/tcp_input.c  | 3 ++-
 net/ipv4/tcp_output.c | 2 ++
 net/ipv4/tcp_timer.c  | 5 +++--
 5 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 2f87377e9af7..c9415b30fa67 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -352,6 +352,11 @@ struct tcp_sock {
 
 	int			linger2;
 
+	/* While icsk_probes_out is for unanswered 0 window probes, this
+	 * counter is for 0-window probes that are not answered with any
+	 * non-zero window (nzw) acks.
+	 */
+	u8	probes_nzw;
 
 /* Sock_ops bpf program related variables */
 #ifdef CONFIG_BPF
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index ed42d2193c5c..af6a41a5a5ac 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2940,6 +2940,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	icsk->icsk_rto = TCP_TIMEOUT_INIT;
 	icsk->icsk_rto_min = TCP_RTO_MIN;
 	icsk->icsk_delack_max = TCP_DELACK_MAX;
+	tp->probes_nzw = 0;
 	tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 	tp->snd_cwnd = TCP_INIT_CWND;
 	tp->snd_cwnd_cnt = 0;
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c7e16b0ed791..4812a969c18a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3377,13 +3377,14 @@ static void tcp_ack_probe(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct sk_buff *head = tcp_send_head(sk);
-	const struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
 
 	/* Was it a usable window open? */
 	if (!head)
 		return;
 	if (!after(TCP_SKB_CB(head)->end_seq, tcp_wnd_end(tp))) {
 		icsk->icsk_backoff = 0;
+		tp->probes_nzw = 0;
 		inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
 		/* Socket must be waked up by subsequent tcp_data_snd_check().
 		 * This function is not for random using!
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f322e798a351..1b64cdabc299 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -4084,10 +4084,12 @@ void tcp_send_probe0(struct sock *sk)
 		/* Cancel probe timer, if it is not required. */
 		icsk->icsk_probes_out = 0;
 		icsk->icsk_backoff = 0;
+		tp->probes_nzw = 0;
 		return;
 	}
 
 	icsk->icsk_probes_out++;
+	tp->probes_nzw++;
 	if (err <= 0) {
 		if (icsk->icsk_backoff < net->ipv4.sysctl_tcp_retries2)
 			icsk->icsk_backoff++;
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 6c62b9ea1320..87e9f5998b8e 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -349,6 +349,7 @@ static void tcp_probe_timer(struct sock *sk)
 
 	if (tp->packets_out || !skb) {
 		icsk->icsk_probes_out = 0;
+		tp->probes_nzw = 0;
 		return;
 	}
 
@@ -360,8 +361,8 @@ static void tcp_probe_timer(struct sock *sk)
 	 * corresponding system limit. We also implement similar policy when
 	 * we use RTO to probe window in tcp_retransmit_timer().
 	 */
-	if (icsk->icsk_user_timeout) {
-		u32 elapsed = tcp_model_timeout(sk, icsk->icsk_probes_out,
+	if (icsk->icsk_user_timeout && tp->probes_nzw) {
+		u32 elapsed = tcp_model_timeout(sk, tp->probes_nzw,
 						tcp_probe0_base(sk));
 
 		if (elapsed >= icsk->icsk_user_timeout)
-- 
2.29.2