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net: fix divide by zero in tcp algorithm illinois 

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>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Jesper Dangaard Brouer 2012-10-31 02:45:32 +00:00 committed by David S. Miller
parent d3e9a1dc7c
commit 8f363b77ee
1 changed files with 5 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
net/ipv4/tcp_illinois.c
View File

@ -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);
}
}