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

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Date:   Wed, 20 Nov 2019 20:51:11 +0100
From:   Stephan Müller <smueller@...onox.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        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>,
        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

Am Mittwoch, 20. November 2019, 14:29:18 CET schrieb Greg Kroah-Hartman:

Hi Greg,

> On Wed, Nov 20, 2019 at 09:58:35AM +0100, Stephan Müller wrote:
> > Am Dienstag, 19. November 2019, 13:41:50 CET schrieb Greg Kroah-Hartman:
> > 
> > Hi Greg,
> > 
> > > On Tue, Nov 19, 2019 at 02:07:40AM -0800, Andy Lutomirski wrote:
> > > > > 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.
> > > 
> > > [10] is the aio char device node, so you better not try to overlap it or
> > > bad things will happen :(
> > 
> > Thanks for your insights.
> > 
> > Which device minor number could we use?
> 
> Get your own dynamic one by using a misc device if you _REALLY_ want to
> add yet-another-char-node-for-random-data.
> 
> But I would have thought that we all realize that this is not the way to
> do things.  Let's not have "random", "urandom", and "true_random" be
> something we want to totally confuse userspace with, that way is insane.
> 
> Please just make the existing userspace api "just work", don't add to
> the mess.

Thank you, I think we should follow that advise.

With that and considering Alexander's rightful remark we have a challenge. So, 
changing the syscall may not be the right way unless we find a way to restrict 
the permissions somehow (capability? LSM? None of that seems to be a good 
fit).

What about providing a /sys file? I.e. adding a file that:

a) has permissions 440 per default and maybe the ownership of root:root

b) allow user space to perform a chown/chgrp

c) only supports reading of data from user space

But then, how could we provide a common interface for the existing random.c 
and the LRNG?

Or should we use a proc file for that? If yes, I guess it should not be a 
sysctl, but a "regular" proc file that should allow a chown(2) operation. On 
the other hand, is proc the right place to provide a user space interface for 
exporting data to user?

Thanks a lot.

Ciao
Stephan