tcp: fix an infinite loop in tcp_slow_start()

Since commit 9dc27415 (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@iki.fi>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_cong.c

@@ -310,6 +310,12 @@ 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
@@ -324,7 +330,7 @@ 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
@@ -334,11 +340,11 @@ 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);