linux-kernel - Re: [PATCH] nvmem: sunxi-sid: SID content is not a valid source of randomness

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Message-ID: <20161025132648.txeo3rw6yz5wutrg@lukather>
Date:   Tue, 25 Oct 2016 15:26:48 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     LABBE Corentin <clabbe.montjoie@...il.com>
Cc:     srinivas.kandagatla@...aro.org, wens@...e.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH] nvmem: sunxi-sid: SID content is not a valid source of
 randomness

On Tue, Oct 25, 2016 at 07:38:55AM +0200, LABBE Corentin wrote:
> On Mon, Oct 24, 2016 at 10:10:20PM +0200, Maxime Ripard wrote:
> > On Sat, Oct 22, 2016 at 03:53:28PM +0200, Corentin Labbe wrote:
> > > Since SID's content is constant over reboot,
> > 
> > That's not true, at least not across all the Allwinner SoCs, and
> > especially not on the A10 and A20 that this driver supports.
> > 
> 
> On my cubieboard2 (A20)
> hexdump -C /sys/devices/platform/soc\@01c00000/1c23800.eeprom/sunxi-sid0/nvmem 
> 00000000  16 51 66 83 80 48 50 72  56 54 48 48 03 c2 75 72  |.Qf..HPrVTHH..ur|
> 00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> *
> 00000100  16 51 66 83 80 48 50 72  56 54 48 48 03 c2 75 72  |.Qf..HPrVTHH..ur|
> 00000110  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> *
> 00000200
> cubiedev ~ # reboot
> cubiedev ~ # hexdump -C /sys/devices/platform/soc\@01c00000/1c23800.eeprom/sunxi-sid0/nvmem 
> 00000000  16 51 66 83 80 48 50 72  56 54 48 48 03 c2 75 72  |.Qf..HPrVTHH..ur|
> 00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> *
> 00000100  16 51 66 83 80 48 50 72  56 54 48 48 03 c2 75 72  |.Qf..HPrVTHH..ur|
> 00000110  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> *
> 00000200
> 
> So clearly for me its constant.

It's constant across reboots, but not across devices. Each device have
a different SID content, therefore it's a relevant source of entropy
in the system.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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