[ 154/184] net: fix divide by zero in tcp algorithm illinois

From: Willy Tarreau
Date: Tue Jun 04 2013 - 18:54:22 EST

2.6.32-longterm review patch. If anyone has any objections, please let me know.


From: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

commit 8f363b77ee4fbf7c3bbcf5ec2c5ca482d396d664 upstream

Reading TCP stats when using TCP Illinois congestion control algorithm
can cause a divide by zero kernel oops.

The division by zero occur in tcp_illinois_info() at:
do_div(t, ca->cnt_rtt);
where ca->cnt_rtt can become zero (when rtt_reset is called)

Steps to Reproduce:
1. Register tcp_illinois:
# sysctl -w net.ipv4.tcp_congestion_control=illinois
2. Monitor internal TCP information via command "ss -i"
# watch -d ss -i
3. Establish new TCP conn to machine

Either it fails at the initial conn, or else it needs to wait
for a loss or a reset.

This is only related to reading stats. The function avg_delay() also
performs the same divide, but is guarded with a (ca->cnt_rtt > 0) at its
calling point in update_params(). Thus, simply fix tcp_illinois_info().

Function tcp_illinois_info() / get_info() is called without
socket lock. Thus, eliminate any race condition on ca->cnt_rtt
by using a local stack variable. Simply reuse info.tcpv_rttcnt,
as its already set to ca->cnt_rtt.
Function avg_delay() is not affected by this race condition, as
its called with the socket lock.

Cc: Petr Matousek <pmatouse@xxxxxxxxxx>
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Acked-by: Stephen Hemminger <shemminger@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
net/ipv4/tcp_illinois.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_illinois.c b/net/ipv4/tcp_illinois.c
index 1eba160..c35d91f 100644
--- a/net/ipv4/tcp_illinois.c
+++ b/net/ipv4/tcp_illinois.c
@@ -313,11 +313,13 @@ static void tcp_illinois_info(struct sock *sk, u32 ext,
.tcpv_rttcnt = ca->cnt_rtt,
.tcpv_minrtt = ca->base_rtt,
- u64 t = ca->sum_rtt;

- do_div(t, ca->cnt_rtt);
- info.tcpv_rtt = t;
+ if (info.tcpv_rttcnt > 0) {
+ u64 t = ca->sum_rtt;

+ do_div(t, info.tcpv_rttcnt);
+ info.tcpv_rtt = t;
+ }
nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/