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

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Date:   Fri, 19 Aug 2016 10:20:18 -0700
From:   "H. Peter Anvin" <hpa@...ux.intel.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        Theodore Ts'o <tytso@....edu>, Pavel Machek <pavel@....cz>,
        Stephan Mueller <smueller@...onox.de>, sandyinchina@...il.com,
        Jason Cooper <cryptography@...edaemon.net>,
        John Denker <jsd@...n.com>, Joe Perches <joe@...ches.com>,
        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 08/18/16 22:56, Herbert Xu wrote:
> On Thu, Aug 18, 2016 at 10:49:47PM -0400, Theodore Ts'o wrote:
>>
>> That really depends on the system.  We can't assume that people are
>> using systems with a 100Hz clock interrupt.  More often than not
>> people are using tickless kernels these days.  That's actually the
>> problem with changing /dev/urandom to block until things are
>> initialized.
> 
> Couldn't we disable tickless until urandom has been seeded? In fact
> perhaps we should accelerate the timer interrupt rate until it has
> been seeded?
> 

The biggest problem there is that the timer interrupt adds *no* entropy
unless there is a source of asynchronicity in the system.  On PCs,
traditionally the timer has been run from a completely different crystal
(14.31818 MHz) than the CPU, which is the ideal situation, but if they
are run off the same crystal and run in lockstep, there is very little
if anything there.  On some systems, the timer may even *be* the only
source of time, and the entropy truly is zero.

	-hpa