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

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue Aug 1, 2023 at 9:52 PM EEST, Jarkko Sakkinen wrote:
> On Tue Aug 1, 2023 at 6:04 AM EEST, Mario Limonciello wrote:
> > On 7/31/23 18:40, Jason A. Donenfeld wrote:
> > > Hi all,
> > > 
> > > I've been tracking this issue with Mario on various threads and
> > > bugzilla for a while now. My suggestion over at bugzilla was to just
> > > disable all current AMD fTPMs by bumping the check for a major version
> > > number, so that the hardware people can reenable it i it's ever fixed,
> > > but only if this is something that the hardware people would actually
> > > respect. As I understand it, Mario was going to check into it and see.
> > > Failing that, yea, just disabling hwrng on fTPM seems like a fine
> > > enough thing to do.
> > > 
> > > The reason I'm not too concerned about that is twofold:
> > > - Systems with fTPM all have RDRAND anyway, so there's no entropy problem.
> > > - fTPM *probably* uses the same random source as RDRAND -- the
> > > TRNG_OUT MMIO register -- so it's not really doing much more than what
> > > we already have available.
> >
> > Yeah I have conversations ongoing about this topic, but also I concluded
> > your suspicion is correct.  They both get their values from the 
> > integrated CCP HW IP.
> >
> > > 
> > > So this all seems fine. And Jarkko's patch seems more or less the
> > > straight forward way of disabling it. But with that said, in order of
> > > priority, maybe we should first try these:
> > > 
> > > 1) Adjust the version check to a major-place fTPM version that AMD's
> > > hardware team pinky swears will have this bug fixed. (Though, I can
> > > already imagine somebody on the list shouting, "we don't trust
> > > hardware teams to do anything with unreleased stuff!", which could be
> > > valid.)
> >
> > I find it very likely the actual root cause is similar to what Linus 
> > suggested.  If that's the case I don't think the bug can be fixed
> > by just an fTPM fix but would rather require a BIOS fix.
> >
> > This to me strengthens the argument to either not register fTPM as RNG 
> > in the first place or just use TPM for boot time entropy.
> >
> > > 2) Remove the version check, but add some other query to detect AMD
> > > fTPM vs realTPM, and ban fTPM.
> >
> > AMD doesn't make dTPMs, only fTPMs.  It's tempting to try to use 
> > TPM2_PT_VENDOR_TPM_TYPE, but this actually is a vendor specific value.
> >
> > I don't see a reliable way in the spec to do this.
> >
> > > - Remove the version check, and just check for AMD; this is Jarrko's patch.
> >
> > I have a counter-proposal to Jarkko's patch attached.  This has two 
> > notable changes:
> >
> > 1) It only disables RNG generation in the case of having RDRAND or RDSEED.
> > 2) It also matches Intel PTT.
> >
> > I still do also think Linus' idea of TPMs only providing boot time 
> > entropy is worth weighing out.
>
> You should add something like TPM_CHIP_HWRNG_DISABLED instead and
> set this in tpm_crb before calling tpm_chip_register().
>
> Nothing else concerning AMD hardware should be done in tpm-chip.c.
> It should only check TPM_CHIP_HWRNG_DISABLED in the beginning of
> tpm_add_hwrng().

In English: I think adding the function to tpm-chip.c was a really
bad idea in the first place, so let's revert that decisions and do
this correctly in tpm_crb.c.

BR, Jarkko