Re: [PATCH] udp: port starting location not random

[David Miller <davem@davemloft.net>]

From: Stephen Hemminger <shemminger@vyatta.com>
Date: Thu, 4 Oct 2012 14:08:28 -0700

> While working on VXLAN, noticed a bug in UDP introduced by:
> 
> commit 9088c5609584684149f3fb5b065aa7f18dcb03ff
> Author: Eric Dumazet <dada1@cosmosbay.com>
> Date:   Wed Oct 8 11:44:17 2008 -0700
> 
>     udp: Improve port randomization
>     
> 
> The logic for choosing where to start for port randomization incorrectly
> calculates the starting port number. It is always ends up using
> the low end of the range independent of the value of random.
> This causes all UDP port searches to start at the same port.
> 
> Doing the following fixes it but at the cost of doing a real divide.
> 
> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> 
> ---
> Resend, previous send was not going to netdev.
> 
> Not sure if worth fixing for stable, because only has performance impact
> and some application might be depending on current broken behaviour.
> 
> 
> 
> --- a/net/ipv4/udp.c	2012-10-01 17:06:53.107427436 -0700
> +++ b/net/ipv4/udp.c	2012-10-04 13:43:21.278960379 -0700
> @@ -216,7 +216,7 @@ int udp_lib_get_port(struct sock *sk, un
>  		remaining = (high - low) + 1;
>  
>  		rand = net_random();
> -		first = (((u64)rand * remaining) >> 32) + low;
> +		first = rand % remaining + low;

Try replacing "remaining" with "(remaining << (64 - 16))" in
the expression instead.