From e1f31f3f20ab760ebb2425e67385b2c593c35be5 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 26 May 2017 15:45:15 +0530 Subject: tcp: fix zero cwnd in tcp_cwnd_reduction [ 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. Change-Id: I6edbd82492839ca86515c64ee22828f7582900aa Fixes: 3759824da87b ("tcp: PRR uses CRB mode by default and SS mode conditionally") Reported-by: Oleksandr Natalenko Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller Signed-off-by: Akshaya --- net/ipv4/tcp_input.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index cc7f940..c0be23d 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2516,6 +2516,9 @@ static void tcp_cwnd_reduction(struct sock *sk, const int prior_unsacked, 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 (tcp_packets_in_flight(tp) > tp->snd_ssthresh) { u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered + -- 1.9.1