tcp: fix false undo corner cases

[ Upstream commit 6e08d5e3c8236e7484229e46fdf92006e1dd4c49 ]

The undo code assumes that, upon entering loss recovery, TCP
1) always retransmit something
2) the retransmission never fails locally (e.g., qdisc drop)

so undo_marker is set in tcp_enter_recovery() and undo_retrans is
incremented only when tcp_retransmit_skb() is successful.

When the assumption is broken because TCP's cwnd is too small to
retransmit or the retransmit fails locally. The next (DUP)ACK
would incorrectly revert the cwnd and the congestion state in
tcp_try_undo_dsack() or tcp_may_undo(). Subsequent (DUP)ACKs
may enter the recovery state. The sender repeatedly enter and
(incorrectly) exit recovery states if the retransmits continue to
fail locally while receiving (DUP)ACKs.

The fix is to initialize undo_retrans to -1 and start counting on
the first retransmission. Always increment undo_retrans even if the
retransmissions fail locally because they couldn't cause DSACKs to
undo the cwnd reduction.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2d7e6259e7e8daee8f2600441521a43744a1a0d4..09b85cdda16544de80f57c65c07528919eab5547 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1106,7 +1106,7 @@ static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
        }
 
        /* D-SACK for already forgotten data... Do dumb counting. */
-       if (dup_sack && tp->undo_marker && tp->undo_retrans &&
+       if (dup_sack && tp->undo_marker && tp->undo_retrans > 0 &&
            !after(end_seq_0, prior_snd_una) &&
            after(end_seq_0, tp->undo_marker))
                tp->undo_retrans--;
@@ -1187,7 +1187,7 @@ static u8 tcp_sacktag_one(struct sock *sk,
 
        /* Account D-SACK for retransmitted packet. */
        if (dup_sack && (sacked & TCPCB_RETRANS)) {
-               if (tp->undo_marker && tp->undo_retrans &&
+               if (tp->undo_marker && tp->undo_retrans > 0 &&
                    after(end_seq, tp->undo_marker))
                        tp->undo_retrans--;
                if (sacked & TCPCB_SACKED_ACKED)
@@ -1893,7 +1893,7 @@ static void tcp_clear_retrans_partial(struct tcp_sock *tp)
        tp->lost_out = 0;
 
        tp->undo_marker = 0;
-       tp->undo_retrans = 0;
+       tp->undo_retrans = -1;
 }
 
 void tcp_clear_retrans(struct tcp_sock *tp)
@@ -2664,7 +2664,7 @@ static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
 
        tp->prior_ssthresh = 0;
        tp->undo_marker = tp->snd_una;
-       tp->undo_retrans = tp->retrans_out;
+       tp->undo_retrans = tp->retrans_out ? : -1;
 
        if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
                if (!ece_ack)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 12d6016bdd9a675aa39fb24fd91fb8c80c741ed3..589b5ac1339eeeaf7421f86ca5ce3e2e7614c497 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2472,8 +2472,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
                if (!tp->retrans_stamp)
                        tp->retrans_stamp = TCP_SKB_CB(skb)->when;
 
-               tp->undo_retrans += tcp_skb_pcount(skb);
-
                /* snd_nxt is stored to detect loss of retransmitted segment,
                 * see tcp_input.c tcp_sacktag_write_queue().
                 */
@@ -2481,6 +2479,10 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
        } else {
                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
        }
+
+       if (tp->undo_retrans < 0)
+               tp->undo_retrans = 0;
+       tp->undo_retrans += tcp_skb_pcount(skb);
        return err;
 }