From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: [net PATCH V2] net: fix divide by zero in tcp algorithm illinois Date: Wed, 31 Oct 2012 13:48:36 -0700 Message-ID: <20121031134836.16b31625@s6510.linuxnetplumber.net> References: <20121031124318.30915.32293.stgit@dragon> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: "David S. Miller" , netdev@vger.kernel.org, Petr Matousek , Eric Dumazet To: Jesper Dangaard Brouer Return-path: Received: from mail.vyatta.com ([76.74.103.46]:33687 "EHLO mail.vyatta.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760217Ab2JaUsp (ORCPT ); Wed, 31 Oct 2012 16:48:45 -0400 In-Reply-To: <20121031124318.30915.32293.stgit@dragon> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 31 Oct 2012 13:45:32 +0100 Jesper Dangaard Brouer 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 > Signed-off-by: Jesper Dangaard Brouer > > --- > 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