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

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Message-ID: <3213943.8fHWQDGiN1@tauon.atsec.com>
Date:	Tue, 21 Jun 2016 07:17:43 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Theodore Ts'o <tytso@....edu>
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

Am Dienstag, 21. Juni 2016, 01:12:55 schrieb Theodore Ts'o:

Hi Theodore,

> 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.

My concern is that interrupts have *much* more entropy than 1/64th. With a 
revaluation of the assumed entropy in interrupts, we will serve *all* systems 
much better and not just systems with HID.

As said, I think we heavily penalize server type and VM environments against 
desktop systems by crediting entropy in large scale to HID and conversely to a 
much lesser degree to interrupts.
> 
> 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.

I am not saying that there is an active attack vector. All I want is to 
revalue the entropy in one interrupt which can only be done if we drop the HID 
time stamp collection.

Ciao
Stephan