linux-kernel - Re: [PATCH v25 10/12] LRNG

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Message-Id: <DDB907EA-3FCC-40C7-B55B-A84BC77FD7A1@amacapital.net>
Date:   Sat, 16 Nov 2019 08:09:09 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Stephan Müller <smueller@...onox.de>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        "Theodore Y. Ts'o" <tytso@....edu>, Willy Tarreau <w@....eu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Vito Caputo <vcaputo@...garu.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        zhangjs <zachary@...shancloud.com>,
        Andy Lutomirski <luto@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Nicolai Stange <nstange@...e.de>,
        "Peter, Matthias" <matthias.peter@....bund.de>,
        Marcelo Henrique Cerri <marcelo.cerri@...onical.com>,
        Roman Drahtmueller <draht@...altsekun.de>,
        Neil Horman <nhorman@...hat.com>
Subject: Re: [PATCH v25 10/12] LRNG - add TRNG support



> On Nov 16, 2019, at 1:40 AM, Stephan Müller <smueller@...onox.de> wrote:
> 
> The True Random Number Generator (TRNG) provides a random number
> generator with prediction resistance (SP800-90A terminology) or an NTG.1
> (AIS 31 terminology).
> 

...

> The secondary DRNGs seed from the TRNG if it is present. In addition,
> the /dev/random device accesses the TRNG.
> 
> If the TRNG is disabled, the secondary DRNGs seed from the entropy pool
> and /dev/random behaves like getrandom(2).

As mentioned before, I don’t like this API.  An application that, for some reason, needs a TRNG, should have an API by which it either gets a TRNG or an error. Similarly, an application that wants cryptographically secure random numbers efficiently should have an API that does that.  With your design, /dev/random tries to cater to both use cases, but one of the use cases fails depending on kernel config.

I think /dev/random should wait for enough entropy to initialize the system but should not block after that. A TRNG should have an entirely new API that is better than /dev/random.