tcp: fix FRTO undo on cumulative ACK of SACKed range

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@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index fb4cf8b..f501ac0 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3105,10 +3105,11 @@ 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)