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From:   Stephan Mueller <smueller@chronox.de>
To:     Andy Lutomirski <luto@kernel.org>
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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Subject: Re: [PATCH v25 10/12] LRNG - add TRNG support
Date:   Tue, 19 Nov 2019 11:46:51 +0100
Message-ID: <25235737.Vo4cOZQZay@tauon.chronox.de>
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Am Dienstag, 19. November 2019, 11:07:40 CET schrieb Andy Lutomirski:

Hi Andy,

> On Sun, Nov 17, 2019 at 3:11 AM Stephan M=C3=BCller <smueller@chronox.de>=
 wrote:
> > Am Samstag, 16. November 2019, 17:09:09 CET schrieb Andy Lutomirski:
> >=20
> > Hi Andy,
> >=20
> > > > On Nov 16, 2019, at 1:40 AM, Stephan M=C3=BCller <smueller@chronox.=
de>
> > > > wrote:
> > > >=20
> > > > =EF=BB=BFThe True Random Number Generator (TRNG) provides a random =
number
> > > > generator with prediction resistance (SP800-90A terminology) or an
> > > > NTG.1
> > > > (AIS 31 terminology).
> > >=20
> > > ...
> > >=20
> > > > The secondary DRNGs seed from the TRNG if it is present. In additio=
n,
> > > > the /dev/random device accesses the TRNG.
> > > >=20
> > > > If the TRNG is disabled, the secondary DRNGs seed from the entropy
> > > > pool
> > > > and /dev/random behaves like getrandom(2).
> > >=20
> > > As mentioned before, I don=E2=80=99t like this API.  An application t=
hat, for
> > > some
> > > reason, needs a TRNG, should have an API by which it either gets a TR=
NG
> > > or
> > > an error. Similarly, an application that wants cryptographically secu=
re
> > > random numbers efficiently should have an API that does that.  With y=
our
> > > design, /dev/random tries to cater to both use cases, but one of the =
use
> > > cases fails depending on kernel config.
> > >=20
> > > 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 A=
PI
> > > that is better than /dev/random.
> >=20
> > I apologize for the misunderstanding. I assumed we would introduce such
> > /dev/ true_random at a later stage.
> >=20
> > 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);
>=20
> 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.

I fully agree with that. This is the reason for my question to ask for how =
to=20
approach adding such additional interface for the TRNG. Considering that th=
ere=20
are use cases for the blocking interfaces, I would like to have it present=
=20
from the start.

That said, are you planning to keep the blocking_pool considering our=20
discussion?

But may I ask why you say that there are ABI changes? The LRNG set tries to=
 be=20
100% API and ABI compliant to the existing random.c interfaces.

The only change is the runtime-behavior (e.g. time when something is seeded=
),=20
performance of the interrupt handling and the kernel/user interfaces that m=
ay=20
be noticeable by users. I.e. there are timing changes, but no technical=20
changes.

The LRNG though would add one optional interface for kernel internal use an=
d=20
then the discussed /dev/true_random interface.
>=20
> > As this would introduce a new device file now, is there a special proce=
ss
> > that I need to follow or do I need to copy? Which major/minor number
> > should I use?
> >=20
> > Looking into static const struct memdev devlist[] I see
> >=20
> >          [8] =3D { "random", 0666, &random_fops, 0 },
> >          [9] =3D { "urandom", 0666, &urandom_fops, 0 },
> >=20
> > Shall a true_random be added here with [10]?
>=20
> I am not at all an expert on chardevs, but this sounds generally
> reasonable.  gregkh is probably the real authority here.

Thank you for the hint. Then I would like to wait for Greg on this.

Ciao
Stephan