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

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Date:   Tue, 1 Aug 2023 11:42:11 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Daniil Stas <daniil.stas@...teo.net>,
        Mario Limonciello <mario.limonciello@....com>,
        James.Bottomley@...senpartnership.com, Jason@...c4.com,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        regressions@...mhuis.info, stable@...r.kernel.org
Subject: Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs

On Tue, 1 Aug 2023 at 11:28, Jarkko Sakkinen <jarkko@...nel.org> 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.

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?

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 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