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

[Limonciello, Mario]

On 7/31/2023 2:30 PM, Linus Torvalds wrote:
> On Mon, 31 Jul 2023 at 12:18, Limonciello, Mario
> <mario.limonciello@xxxxxxx> wrote:
> > > Is there some way to just see "this is a fTPM"?
> >
> > How many fTPM implementations are there?  We're talking like less than 5
> > right?  Maybe just check against a static list when
> > calling tpm_add_hwrng().
>
> Sounds sane. But I was hoping for some direct way to just query "are
> you a firmware SMI hook, or real hardware".
>
> It would be lovely to avoid the list, because maybe AMD does - or in
> the past have done - discrete TPM hardware?  So it might not be as
> easy as just checking against the manufacturer..
>
> That said, maybe it really doesn't matter. I'm perfectly fine with
> just the "check for AMD as a manufacturer" too.

Jarko's patch seems conceptually fine for now for the fire of the day if 
that's the consensus on the direction for this.

> In fact, I'd be perfectly happy with not using the TPM for run-time
> randomness at all, and purely doing it for the bootup entropy, which
> is where I feel it matters a lot m ore.
>
> > I've had some discussions today with a variety of people on this problem
> > and there is no advantage to get RNG through the fTPM over RDRAND.
>
> Ack.
>
> And that's true even if you _trust_ the fTPM.
>
> That said, I see no real downside to using the TPM (whether firmware
> or discrete) to just add to the boot-time "we'll gather entropy for
> our random number generator from any source".
>
> So it's purely the runtime randomness where I feel that the upside
> just isn't there, and the downsides are real.
>
>                    Linus

Are you thinking then to unregister the tpm hwrng "sometime" after boot?

What would be the right timing/event for this? Maybe rootfs_initcall?