linux-kernel - Re: [PATCH] random: mix all saved registers into entropy pool

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Date:	Mon, 19 May 2014 15:18:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jörn Engel <joern@...fs.org>,
	"Theodore Ts'o" <tytso@....edu>
CC:	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] random: mix all saved registers into entropy pool

On 05/19/2014 02:23 PM, Jörn Engel wrote:
> On Mon, 19 May 2014 17:17:19 -0400, Jörn Engel wrote:
>>
>> Experimentation show this to be an excellent entropy source.  Doing 1000
>> boottests with kvm and dumping a hash of the registers for the first
>> 1024 interrupts each, >40% of all hashes were unique and >80% of all
>> hashes occurred less than once 1% of the time.
> 
> And since I previously claimed the opposite, the negative result was
> caused by a kvm oddity.  When starting kvm in the background, it will
> run just fine.  But when starting kvm with "-nographic" in the
> background, the process gets stopped.  No output is generated and the
> output file is not even truncated before kvm is stopped.  Therefore
> every single run will have identical kernel output - that of the
> previous run.
> 
> With that embarrassment out of the way, I find this approach hugely
> valuable.  Even if you disagree with some detail of this patch, we
> should definitely merge something roughly related.
> 
> Jörn
> 

I think this is likely to be particularly valuable during boot.  I can
see it becoming substantially less valuable after that point, but during
boot is when the most critical.

What I do see as an issue is that the value is probably impossible to
quantify, and so I feel more than a bit queasy about giving it
/dev/random credit.  However, making sure the pool is well stirred
during boot is really way more important.

	-hpa

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