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From: Jesper Dangaard Brouer <brouer@redhat.com>
Newsgroups: gmane.linux.network
Subject: [net PATCH V2] net: fix divide by zero in tcp algorithm illinois
Date: Wed, 31 Oct 2012 13:45:32 +0100
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Cc: Jesper Dangaard Brouer <brouer@redhat.com>, netdev@vger.kernel.org,
Petr Matousek <pmatouse@redhat.com>,
Stephen Hemminger <shemminger@vyatta.com>,
Eric Dumazet <eric.dumazet@gmail.com>
To: "David S. Miller" <davem@davemloft.net>
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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@redhat.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
V2:
Address Eric Dumazets input:
- Save 2 bytes of stack, by using info.tcpv_rttcnt.
- Help compiler, and define "u64 t" inside if() lexical scope.
net/ipv4/tcp_illinois.c | 8 +++++---
1 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/net/ipv4/tcp_illinois.c b/net/ipv4/tcp_illinois.c
index 813b43a..834857f 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);
}
}