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

[Sumit Garg]

On Wed, 24 Mar 2021 at 19:37, Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> wrote:
>
> Hello Sumit,
>
> On 24.03.21 11:47, Sumit Garg wrote:
> > On Wed, 24 Mar 2021 at 14:56, Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> 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.

-Sumit

>
> > Also, do update documentation corresponding to CAAM as a trusted keys backend.
>
> Yes. The documentation should be updated for CAAM and it should describe
> how the key material is derived. Will do so for v2.
>
> Cheers,
> Ahmad
>
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd.git/tree/Documentation/security/keys/trusted-encrypted.rst#n87
> >
> > -Sumit
> >
> >> Cheers,
> >> Ahmad
> >>
> >>>
> >>> thanks,
> >>>
> >>> Mimi
> >>>
> >>> [1]
> >>> https://lore.kernel.org/linux-integrity/BCA04D5D9A3B764C9B7405BBA4D4A3C035F2A38B@xxxxxxxxxxxxxxxxxxxxxxxx/
> >>>
> >>>
> >>>
> >>
> >> --
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> >
>
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