linux-kernel - Re: Entropy sources

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Message-ID: <5d672be5-6234-6bb7-fc2d-ee680742628c@zytor.com>
Date:   Thu, 25 Aug 2016 17:20:57 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Sandy Harris <sandyinchina@...il.com>
Cc:     Jeffrey Walton <noloader@...il.com>, linux-crypto@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>
Subject: Re: Entropy sources

On 08/25/16 16:35, Sandy Harris wrote:
> On Thu, Aug 25, 2016 at 5:30 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 
>> The network stack is a good source of entropy, *once it is online*.
>> However, the most serious case is while the machine is still booting,
>> when the network will not have enabled yet.
>>
>>         -hpa
> 
> One possible solution is at:
> https://github.com/sandy-harris/maxwell
> 
> A small (< 700 lines) daemon that gets entropy from timer imprecision
> and variations in time for arithmetic (cache misses, interrupts, etc.)
> and pumps it into /dev/random. Make it the first userspace program
> started and all should be covered. Directory above includes a PDF doc
> with detailed rationale and some discussion of alternate solutions.
> 
> Of course if you are dealing with a system-on-a-chip or low-end
> embedded CPU & the timer is really inadequate, this will not work
> well. Conceivably well enough, but we could not know that without
> detailed analysis for each chip in question.
> 

A lot of this is exactly the same thing /dev/random already does in
kernel space.  I have already in the past expressed skepticism toward
this approach, because a lot of the analysis done has simply been bogus.

	-hpa