[PATCH 3.16.y-ckt 133/144] tcp: fix FRTO undo on cumulative ACK of SACKed range

[Luis Henriques]

3.16.7-ckt10 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Neal Cardwell <ncardwell at google.com>

commit 666b805150efd62f05810ff0db08f44a2370c937 upstream.

On processing cumulative ACKs, the FRTO code was not checking the
SACKed bit, meaning that there could be a spurious FRTO undo on a
cumulative ACK of a previously SACKed skb.

The FRTO code should only consider a cumulative ACK to indicate that
an original/unretransmitted skb is newly ACKed if the skb was not yet
SACKed.

The effect of the spurious FRTO undo would typically be to make the
connection think that all previously-sent packets were in flight when
they really weren't, leading to a stall and an RTO.

Signed-off-by: Neal Cardwell <ncardwell at google.com>
Signed-off-by: Yuchung Cheng <ycheng at google.com>
Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/ipv4/tcp_input.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a1bbebb03490..486f70050563 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3068,10 +3068,11 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
 			if (!first_ackt.v64)
 				first_ackt = last_ackt;
 
-			if (!(sacked & TCPCB_SACKED_ACKED))
+			if (!(sacked & TCPCB_SACKED_ACKED)) {
 				reord = min(pkts_acked, reord);
-			if (!after(scb->end_seq, tp->high_seq))
-				flag |= FLAG_ORIG_SACK_ACKED;
+				if (!after(scb->end_seq, tp->high_seq))
+					flag |= FLAG_ORIG_SACK_ACKED;
+			}
 		}
 
 		if (sacked & TCPCB_SACKED_ACKED)