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Message-ID: <20121031134836.16b31625@s6510.linuxnetplumber.net> Date: Wed, 31 Oct 2012 13:48:36 -0700 From: Stephen Hemminger <shemminger@...tta.com> To: Jesper Dangaard Brouer <brouer@...hat.com> Cc: "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org, Petr Matousek <pmatouse@...hat.com>, Eric Dumazet <eric.dumazet@...il.com> Subject: Re: [net PATCH V2] net: fix divide by zero in tcp algorithm illinois On Wed, 31 Oct 2012 13:45:32 +0100 Jesper Dangaard Brouer <brouer@...hat.com> wrote: > 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@...hat.com> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.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); > } > } > Acked-by: Stephen Hemminger <shemminger@...tta.com> -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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