[ 53/61] tcp: fix an infinite loop in tcp_slow_start()

[Greg Kroah-Hartman]

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

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


From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ Upstream commit 973ec449bb4f2b8c514bacbcb4d9506fc31c8ce3 ]

Since commit 9dc274151a548 (tcp: fix ABC in tcp_slow_start()),
a nul snd_cwnd triggers an infinite loop in tcp_slow_start()

Avoid this infinite loop and log a one time error for further
analysis. FRTO code is suspected to cause this bug.

Reported-by: Pasi Kärkkäinen <pasik@xxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Neal Cardwell <ncardwell@xxxxxxxxxx>
Cc: Yuchung Cheng <ycheng@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ipv4/tcp_cong.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -309,6 +309,12 @@ void tcp_slow_start(struct tcp_sock *tp)
 {
 	int cnt; /* increase in packets */
 	unsigned int delta = 0;
+	u32 snd_cwnd = tp->snd_cwnd;
+
+	if (unlikely(!snd_cwnd)) {
+		pr_err_once("snd_cwnd is nul, please report this bug.\n");
+		snd_cwnd = 1U;
+	}
 
 	/* RFC3465: ABC Slow start
 	 * Increase only after a full MSS of bytes is acked
@@ -323,7 +329,7 @@ void tcp_slow_start(struct tcp_sock *tp)
 	if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
 		cnt = sysctl_tcp_max_ssthresh >> 1;	/* limited slow start */
 	else
-		cnt = tp->snd_cwnd;			/* exponential increase */
+		cnt = snd_cwnd;				/* exponential increase */
 
 	/* RFC3465: ABC
 	 * We MAY increase by 2 if discovered delayed ack
@@ -333,11 +339,11 @@ void tcp_slow_start(struct tcp_sock *tp)
 	tp->bytes_acked = 0;
 
 	tp->snd_cwnd_cnt += cnt;
-	while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
-		tp->snd_cwnd_cnt -= tp->snd_cwnd;
+	while (tp->snd_cwnd_cnt >= snd_cwnd) {
+		tp->snd_cwnd_cnt -= snd_cwnd;
 		delta++;
 	}
-	tp->snd_cwnd = min(tp->snd_cwnd + delta, tp->snd_cwnd_clamp);
+	tp->snd_cwnd = min(snd_cwnd + delta, tp->snd_cwnd_clamp);
 }
 EXPORT_SYMBOL_GPL(tcp_slow_start);
 


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