linux-kernel - Re: [PATCH v6 0/5] /dev/random

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Date:	Tue, 16 Aug 2016 15:28:45 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	herbert@...dor.apana.org.au, Ted Tso <tytso@....edu>,
	sandyinchina@...il.com, Jason Cooper <cryptography@...edaemon.net>,
	John Denker <jsd@...n.com>, Joe Perches <joe@...ches.com>,
	Pavel Machek <pavel@....cz>,
	George Spelvin <linux@...izon.com>,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 0/5] /dev/random - a new approach

On 08/15/16 22:45, Stephan Mueller wrote:
> Am Montag, 15. August 2016, 13:42:54 CEST schrieb H. Peter Anvin:
> 
> Hi H,
> 
>> On 08/11/16 05:24, Stephan Mueller wrote:
>>> * prevent fast noise sources from dominating slow noise sources
>>>
>>>   in case of /dev/random
>>
>> Can someone please explain if and why this is actually desirable, and if
>> this assessment has been passed to someone who has actual experience
>> with cryptography at the professional level?
> 
> There are two motivations for that:
> 
> - the current /dev/random is compliant to NTG.1 from AIS 20/31 which requires 
> (in brief words) that entropy comes from auditible noise sources. Currently in 
> my LRNG only RDRAND is a fast noise source which is not auditible (and it is 
> designed to cause a VM exit making it even harder to assess it). To make the 
> LRNG to comply with NTG.1, RDRAND can provide entropy but must not become the 
> sole entropy provider which is the case now with that change.
> 
> - the current /dev/random implementation follows the same concept with the 
> exception of 3.15 and 3.16 where RDRAND was not rate-limited. In later 
> versions, this was changed.
> 

I'm not saying it should be *sole*.  I am questioning the value in
limiting it, as it seems to me that it could only ever produce a worse
result.

	-hpa