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

[Sumit Garg <sumit.garg@linaro.org>]

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.

-Sumit

>
> /Jarkko