linux-kernel - Re: Why does reading from /dev/urandom deplete entropy so much?

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Message-ID: <20071204222811.GI19691@waste.org>
Date:	Tue, 4 Dec 2007 16:28:12 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Mike McGrath <mmcgrath@...hat.com>
Cc:	Theodore Tso <tytso@....edu>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ray Lee <ray@...rabbit.org>, Adrian Bunk <bunk@...nel.org>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?

On Tue, Dec 04, 2007 at 04:12:51PM -0600, Mike McGrath wrote:
> Theodore Tso wrote:
> >On Tue, Dec 04, 2007 at 02:48:12PM -0600, Mike McGrath wrote:
> >  
> >>Alan Cox wrote:
> >>    
> >>>>Here's the top 5:
> >>>>
> >>>>   266 28caf2c3-9766-4fe1-9e4c-d6b0ba8a0132
> >>>>   336 810e7126-1c69-4aff-b8b1-9db0fa8aa15a
> >>>>   402 c8dbb9d3-a9bd-4ba6-b92e-4a294ba5a95f
> >>>>   884 06e84493-e024-44b1-9b32-32d78af04039
> >>>>   931 e2b67e1d-e325-4740-b938-795addb45280
> >>>>
> >>>>The left number is times this month someone has submitted a profile with
> >>>>that UUID.  If we take the last one as an example has come from over 800
> >>>>IP's in the last 20 days.  It seems very unlikely that one person would
> >>>>find his way to 800 different IP's this month.  Let me know if you'd
> >>>>like more.
> >>>>    
> >>>>        
> >>Background - Smolt runs this during its install:
> >>
> >>/bin/cat /proc/sys/kernel/random/uuid > /etc/sysconfig/hw-uuid
> >>
> >>For most users this would be run by the RPM %post scripts during install 
> >>from anaconda. For some reason there are some UUID's (like those listed 
> >>above) that come up more often then it seems they should if they are 
> >>truly random.
> >>    
> >
> >Would this be by any chance using kickstart where there is no user
> >interaction, and no way of gathering entropy during the install process?
> >The random number generator isn't *magic* you know....
> >  
> 
> This is certainly possible but I think its unlikely. Not many people 
> know about smolt. Most of our profiles come from prompting people during 
> firstboot which gets skipped when people are running kickstart. I'll 
> make sure to follow up with people that report a duplicate.

Please describe the process from boot to writing the UUID. Are we
typically booting from R/O media? Is a random reseed done in the
startup scripts? What is the source of the seed? How much user
interaction is there?

If we have entropy input below the reseed threshold, we can get
duplicates. That means we could have like a dozen or more keystrokes.
Serial consoles and the like will also be problematic.

-- 
Mathematics is the supreme nostalgia of our time.
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