linux-kernel - Re: [PATCH v5 0/7] /dev/random

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Message-ID: <20160621051255.GG9848@thunk.org>
Date:	Tue, 21 Jun 2016 01:12:55 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	George Spelvin <linux@...encehorizons.net>, andi@...stfloor.org,
	cryptography@...edaemon.net, herbert@...dor.apana.org.au,
	hpa@...ux.intel.com, joe@...ches.com, jsd@...n.com,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux@...izon.com, pavel@....cz, sandyinchina@...il.com
Subject: Re: [PATCH v5 0/7] /dev/random - a new approach

On Mon, Jun 20, 2016 at 09:00:49PM +0200, Stephan Mueller wrote:
> 
> The time stamp maintenance is the exact cause for the correlation: one HID 
> event triggers:
> 
> - add_interrupt_randomness which takes high-res time stamp, Jiffies and some 
> pointers
> 
> - add_input_randomness which takes high-res time stamp, Jiffies and HID event 
> value
> 
> The same applies to disk events. My suggestion is to get rid of the double 
> counting of time stamps for one event.
> 
> And I guess I do not need to stress that correlation of data that is supposed 
> to be entropic is not good :-)

What is your concern, specifically?  If it is in the entropy
accounting, there is more entropy in HID event interrupts, so I don't
think adding the extra 1/64th bit of entropy is going to be problematic.

If it is that there are two timestamps that are closely correleated
being added into the pool, the add_interrupt_randomness() path is
going to mix that timestamp with the interrupt timings from 63 other
interrupts before it is mixed into the input pool, while the
add_input_randomness() mixes it directly into the pool.  So if you
think there is a way this could be leveraged into attack, please give
specifics --- but I think we're on pretty solid ground here.

Cheers,

						- Ted