Subject: Re: [net PATCH V2] net: fix divide by zero in tcp algorithm illinois

From: Stephen Hemminger <[email-masked]>
Date: Wed, 31 Oct 2012 13:48:36 -0700

On Wed, 31 Oct 2012 13:45:32 +0100
Jesper Dangaard Brouer <[email-masked]> 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 <[email-masked]>
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> 
> ---
> 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 <[email-masked]>