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

Greg Kroah-Hartman
2013-02-12T20:36:27Z

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset=UTF-8, Size: 2076 bytes --]

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

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

From: Eric Dumazet <edumazet@google.com>

[ 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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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);