Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs

[Jarkko Sakkinen]

On Tue Aug 1, 2023 at 9:42 PM EEST, Linus Torvalds wrote:
> On Tue, 1 Aug 2023 at 11:28, Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
> >
> > I would disable it inside tpm_crb driver, which is the driver used
> > for fTPM's: they are identified by MSFT0101 ACPI identifier.
> >
> > I think the right scope is still AMD because we don't have such
> > regressions with Intel fTPM.
>
> I'm ok with that.
>
> > I.e. I would move the helper I created inside tpm_crb driver, and
> > a new flag, let's say "TPM_CHIP_FLAG_HWRNG_DISABLED", which tpm_crb
> > sets before calling tpm_chip_register().
> >
> > Finally, tpm_add_hwrng() needs the following invariant:
> >
> >         if (chip->flags & TPM_CHIP_FLAG_HWRNG_DISABLED)
> >                 return 0;
> >
> > How does this sound? I can refine this quickly from my first trial.
>
> Sounds fine.

Mario, it would be good if you could send a fix candidate but take my
suggestion for a new TPM chip flag into account, while doing it. Please
send it as a separate patch, not attachment to this thread.

I can test and ack it, if it looks reasonable.

> My only worry comes from my ignorance: do these fTPM devices *always*
> end up being enumerated through CRB, or do they potentially look
> "normal enough" that you can actually end up using them even without
> having that CRB driver loaded?

I know that QEMU has TPM passthrough but I don't know how it behaves
exactly.

> Put another way: is the CRB driver the _only_ way they are visible, or
> could some people hit on this through the TPM TIS interface if they
> have CRB disabled?

I'm not aware of such implementations.

> I see, for example, that qemu ends up emulating the TIS layer, and it
> might end up forwarding the TPM requests to something that is natively
> CRB?
>
> But again: I don't know enough about CRB vs TIS, so the above may be a
> stupid question.
>
>            Linus

I would focus exactly what is known not to work and disable exactly
that.

If someone still wants to enable TPM on such hardware, we can later
on add a kernel command-line flag to enforce hwrng. This ofc based
on user feedback, not something I would add right now.

BR, Jarkko