Patch "tcp: fix zero cwnd in tcp_cwnd_reduction" has been added to the 4.3-stable tree

This is a note to let you know that I've just added the patch titled

tcp: fix zero cwnd in tcp_cwnd_reduction

to the 4.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tcp-fix-zero-cwnd-in-tcp_cwnd_reduction.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Yuchung Cheng <[email protected]>
Date: Wed, 6 Jan 2016 12:42:38 -0800
Subject: tcp: fix zero cwnd in tcp_cwnd_reduction

From: Yuchung Cheng <[email protected]>

[ Upstream commit 8b8a321ff72c785ed5e8b4cf6eda20b35d427390 ]

Patch 3759824da87b ("tcp: PRR uses CRB mode by default and SS mode conditionally") introduced a bug that cwnd may become 0 when both inflight and sndcnt are 0 (cwnd = inflight + sndcnt). This may lead to a div-by-zero if the connection starts another cwnd reduction phase by setting tp->prior_cwnd to the current cwnd (0) in tcp_init_cwnd_reduction().

To prevent this we skip PRR operation when nothing is acked or sacked. Then cwnd must be positive in all cases as long as ssthresh is positive:

1) The proportional reduction mode
inflight > ssthresh > 0

2) The reduction bound mode
a) inflight == ssthresh > 0

b) inflight < ssthresh
sndcnt > 0 since newly_acked_sacked > 0 and inflight < ssthresh

Therefore in all cases inflight and sndcnt can not both be 0. We check invalid tp->prior_cwnd to avoid potential div0 bugs.

In reality this bug is triggered only with a sequence of less common events. For example, the connection is terminating an ECN-triggered cwnd reduction with an inflight 0, then it receives reordered/old ACKs or DSACKs from prior transmission (which acks nothing). Or the connection is in fast recovery stage that marks everything lost, but fails to retransmit due to local issues, then receives data packets from other end which acks nothing.

---
 net/ipv4/tcp_input.c |    3 +++
 1 file changed, 3 insertions(+)

@@ -2525,6 +2525,9 @@ static void tcp_cwnd_reduction(struct so
 	int newly_acked_sacked = prior_unsacked -
 				 (tp->packets_out - tp->sacked_out);
 
+	if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
+		return;
+
 	tp->prr_delivered += newly_acked_sacked;
 	if (delta < 0) {
 		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/tcp-fix-zero-cwnd-in-tcp_cwnd_reduction.patch queue-4.3/tcp_yeah-don-t-set-ssthresh-below-2.patch