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

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Date:   Tue, 19 Nov 2019 02:07:40 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Stephan Müller <smueller@...onox.de>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <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 Sun, Nov 17, 2019 at 3:11 AM Stephan Müller <smueller@...onox.de> wrote:
>
> Am Samstag, 16. November 2019, 17:09:09 CET schrieb Andy Lutomirski:
>
> Hi Andy,
>
> > > 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.
>
> I apologize for the misunderstanding. I assumed we would introduce such /dev/
> true_random at a later stage.
>
> If you agree, I can certainly add /dev/true_random right now that links with
> the TRNG and make /dev/random behave as discussed, i.e. behave exactly like
> getrandom(..., 0);

Given that your series is already a decently large ABI change, I think
it would be polite to users to make all the anticipated changes all at
once to reduce the amount of churn everyone needs to deal with.

>
> As this would introduce a new device file now, is there a special process that
> I need to follow or do I need to copy? Which major/minor number should I use?
>
> Looking into static const struct memdev devlist[] I see
>
>          [8] = { "random", 0666, &random_fops, 0 },
>          [9] = { "urandom", 0666, &urandom_fops, 0 },
>
> Shall a true_random be added here with [10]?

I am not at all an expert on chardevs, but this sounds generally
reasonable.  gregkh is probably the real authority here.