Message-ID: <9872655.prSdhymlXK@positron.chronox.de>
Date:   Thu, 26 Dec 2019 10:29:00 +0100
From:   Stephan Müller <smueller@...onox.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Ted Ts'o <tytso@....edu>, LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Willy Tarreau <w@....eu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH v3 0/8] Rework random blocking

Am Montag, 23. Dezember 2019, 09:20:43 CET schrieb Andy Lutomirski:

Hi Andy,
> 
> There are some open questions and future work here:
> 
> Should the kernel provide an interface to get software-generated
> "true random" numbers?  I can think of only one legitimate reason to
> use such an interface: compliance with government standards.  If the
> kernel provides such an interface going forward, I think it should
> be a brand new character device, and it should have a default mode
> 0440 or similar.  Software-generated "true random numbers" are a
> very limited resource, and resource exhaustion is a big deal.  Ask
> anyone who has twiddled their thumbs while waiting for gnupg to
> generate a key.  If we think the kernel might do such a thing, then
> patches 5-8 could be tabled for now.

What about offering a compile-time option to enable or disable such code? 
Note, with the existing random.c code base, there is no need to have a 
separate blocking_pool. The ChaCha20 DRNG could be used for that very same 
purpose, provided that in case these true random numbers are generated when 
the Chacha20 DRNG received an equal amount of "unused" entropy.
> 
> Alternatively, perhaps the kernel should instead provide a
> privileged interface to read out raw samples from the various
> entropy sources, and users who care could have a user daemon that
> does something intelligent with them.  This would push the mess of
> trying to comply with whatever standards are involved to userspace.
> Userspace could then export "true randomness" via CUSE if it is so
> inclined, or could have a socket with a well-known name, or whatever
> else seems appropriate.

With the patch set v26 of my LRNG I offer another possible alternative 
avoiding any additional character device file and preventing the starvation of 
legitimate use cases: the LRNG has an entropy pool that leaves different 
levels of entropy in the pool depending on the use cases of this data.

If an unprivileged caller requests true random data, at least 1024 bits of 
entropy is left in the pool. I.e. all entropy above that point is available 
for this request type. Note, even namespaces fall into this category 
considering that unprivileged users can create a user name space in which they 
can become root.

If a non-blocking DRNG serving /dev/urandom or getrandom(2) needs reseeding, 
at least 512 bits of entropy is left in the pool. Each DRNG seeding operation 
requires at least 128 bits and at most 256 bits of entropy. This means that at 
least 2 reseed operations worth of entropy is found in the entropy pool even 
though massive amount of true random numbers are requested by unprivileged 
users.

If a privileged caller requests true random numbers, the entropy pool is 
allowed to be exhausted.

Access to the true random number generator is provided with getrandom(2) and 
the GRND_TRUERANDOM flag. If the true random number generator (TRNG) is not 
compiled or not present, -EOPNOTSUPP is returned.

Entire patch set:

Reviewed-by: Stephan Müller <smueller@...onox.de>

Ciao
Stephan

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