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Message-ID: <fd3cc87a-97ec-00ea-e480-f6597664c13a@amd.com>
Date: Mon, 31 Jul 2023 16:57:20 -0500
From: "Limonciello, Mario" <mario.limonciello@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
linux@...inikbrodowski.net
Cc: Jarkko Sakkinen <jarkko@...nel.org>,
Daniil Stas <daniil.stas@...teo.net>,
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 7/31/2023 2:30 PM, Linus Torvalds wrote:
> On Mon, 31 Jul 2023 at 12:18, Limonciello, Mario
> <mario.limonciello@....com> 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?
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