Re: [PATCH v1 3/3] KEYS: trusted: Introduce support for NXP CAAM-based trusted keys

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Mar 30, 2021 at 12:56:41PM +0530, Sumit Garg wrote:
> On Mon, 29 Mar 2021 at 01:07, Jarkko Sakkinen <jarkko@kernel.org> wrote:
> >
> > On Sat, Mar 27, 2021 at 01:41:24PM +0100, David Gstir wrote:
> > > Hi!
> > >
> > > > On 25.03.2021, at 06:26, Sumit Garg <sumit.garg@linaro.org> wrote:
> > > >
> > > > On Wed, 24 Mar 2021 at 19:37, Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
> > > >>
> > > >> Hello Sumit,
> > > >>
> > > >> On 24.03.21 11:47, Sumit Garg wrote:
> > > >>> On Wed, 24 Mar 2021 at 14:56, Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
> > > >>>>
> > > >>>> Hello Mimi,
> > > >>>>
> > > >>>> On 23.03.21 19:07, Mimi Zohar wrote:
> > > >>>>> On Tue, 2021-03-23 at 17:35 +0100, Ahmad Fatoum wrote:
> > > >>>>>> On 21.03.21 21:48, Horia Geantă wrote:
> > > >>>>>>> caam has random number generation capabilities, so it's worth using that
> > > >>>>>>> by implementing .get_random.
> > > >>>>>>
> > > >>>>>> If the CAAM HWRNG is already seeding the kernel RNG, why not use the kernel's?
> > > >>>>>>
> > > >>>>>> Makes for less code duplication IMO.
> > > >>>>>
> > > >>>>> Using kernel RNG, in general, for trusted keys has been discussed
> > > >>>>> before.   Please refer to Dave Safford's detailed explanation for not
> > > >>>>> using it [1].
> > > >>>>
> > > >>>> The argument seems to boil down to:
> > > >>>>
> > > >>>> - TPM RNG are known to be of good quality
> > > >>>> - Trusted keys always used it so far
> > > >>>>
> > > >>>> Both are fine by me for TPMs, but the CAAM backend is new code and neither point
> > > >>>> really applies.
> > > >>>>
> > > >>>> get_random_bytes_wait is already used for generating key material elsewhere.
> > > >>>> Why shouldn't new trusted key backends be able to do the same thing?
> > > >>>>
> > > >>>
> > > >>> Please refer to documented trusted keys behaviour here [1]. New
> > > >>> trusted key backends should align to this behaviour and in your case
> > > >>> CAAM offers HWRNG so we should be better using that.
> > > >>
> > > >> Why is it better?
> > > >>
> > > >> Can you explain what benefit a CAAM user would have if the trusted key
> > > >> randomness comes directly out of the CAAM instead of indirectly from
> > > >> the kernel entropy pool that is seeded by it?
> > > >
> > > > IMO, user trust in case of trusted keys comes from trusted keys
> > > > backend which is CAAM here. If a user doesn't trust that CAAM would
> > > > act as a reliable source for RNG then CAAM shouldn't be used as a
> > > > trust source in the first place.
> > > >
> > > > And I think building user's trust for kernel RNG implementation with
> > > > multiple entropy contributions is pretty difficult when compared with
> > > > CAAM HWRNG implementation.
> > >
> > > Generally speaking, I’d say trusting the CAAM RNG and trusting in it’s
> > > other features are two separate things. However, reading through the CAAM
> > > key blob spec I’ve got here, CAAM key blob keys (the keys that secure a blob’s
> > > content) are generated using its internal RNG. So I’d save if the CAAM RNG
> > > is insecure, so are generated key blobs. Maybe somebody with more insight
> > > into the CAAM internals can verify that, but I don’t see any point in using
> > > the kernel’s RNG as long as we let CAAM generate the key blob keys for us.
> >
> > Here's my long'ish analysis. Please read it to the end if by ever means
> > possible, and apologies, I usually try to keep usually my comms short, but
> > this requires some more meat than the usual.
> >
> > The Bad News
> > ============
> >
> > Now that we add multiple hardware trust sources for trusted keys, will
> > there ever be a scenario where a trusted key is originally sealed with a
> > backing hardware A, unsealed, and resealed with hardware B?
> >
> > The hardware and vendor neutral way to generate the key material would be
> > unconditionally always just the kernel RNG.
> >
> > CAAM is actually worse than TCG because it's not even a standards body, if
> > I got it right. Not a lot but at least a tiny fraction.
> >
> > This brings an open item in TEE patches: trusted_tee_get_random() is an
> > issue in generating kernel material. I would rather replace that with
> > kernel RNG *for now*, because the same open question applies also to ARM
> > TEE. It's also a single company controlled backing technology.
> >
> > By all practical means, I do trust ARM TEE in my personal life but this is
> > not important.
> >
> > CAAM *and* TEE backends break the golden rule of putting as little trust as
> > possible to anything, even not anything weird is clear at sight, as
> > security is essentially a game of known unknowns and unknown unknowns.
> >
> > Unfortunately, TPM trusted keys started this bad security practice, and
> > obviously it cannot be fixed without breaking uapi backwards compatibility.
> >
> > This leaves me exactly two rational options:
> >
> > A. Add a patch to remove trusted_tee_get_random() and use kernel RNG
> >    instead.
> > B. Drop the whole TEE patch set up until I have good reasons to believe
> >    that it's the best possible idea ever to use TEE RNG.
> >
> > Doing does (A) does not disclude of doing (B) later on, if someone some
> > day sends a patch with sound reasoning.
> >
> > It's also good to understand that when some day a vendor D, other than TCG,
> > CAAM or ARM, comes up, we need to go again this lenghty and messy
> > discussion. Now this already puts an already accepted patch set into a
> > risk, because by being a responsible maintainer I would have legit reasons
> > just simply to drop it.
> >
> > OK, but....
> >
> > The GOOD News
> > =============
> >
> > So there's actually option (C) that also fixes the TPM trustd keys issue:
> >
> > Add a new kernel patch, which:
> >
> > 1. Adds the use of kernel RNG as a boot option.
> > 2. If this boot option is not active, the subsystem will print a warning
> >    to klog denoting this.
> > 3. Default is of course vendor RNG given the bad design issue in the TPM
> >    trusted keys, but the warning in klog will help to address it at least
> >    a bit.
> > 4. Document all this to Documentation/security/keys/trusted-encrypted.rst.
> >
> > I'd prefer the choice between A, B and C be concluded rather sooner than
> > later.
> 
> Option (C) sounds reasonable to me but I would rather prefer an info
> message rather than warning as otherwise it would reflect that we are
> enforcing kernel RNG choice for a user to trust upon.

I gave some though on this.

I take TEE as it is but I'd expect the CAAM patch set sort out this option
with some patch.

/Jarkko