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Date: Mon, 25 Jan 2016 09:02:07 +0100
From: Florian Weimer <fw@...eb.enyo.de>
To: oss-security@...ts.openwall.com
Subject: Linux potential division by zero in TCP code
While looking for something else entirely, I came across this commit,
initially reported at <https://lkml.org/lkml/2015/12/21/435>:
commit 8b8a321ff72c785ed5e8b4cf6eda20b35d427390
Author: Yuchung Cheng <ycheng@...gle.com>
Date: Wed Jan 6 12:42:38 2016 -0800
tcp: fix zero cwnd in tcp_cwnd_reduction
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.
I haven't analyzed this, but it looks potentially security-relvant
(although the last paragraph above suggests it's not entirely
straightforward to trigger).
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