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Date:   Mon, 31 Jul 2023 14:18:21 -0500
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Subject: Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs
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To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Jarkko Sakkinen <jarkko@kernel.org>
Cc:     Daniil Stas <daniil.stas@posteo.net>,
        James.Bottomley@hansenpartnership.com, Jason@zx2c4.com,
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On 7/31/2023 2:05 PM, Linus Torvalds wrote:
> On Mon, 31 Jul 2023 at 03:53, Jarkko Sakkinen <jarkko@kernel.org> wrote:
>>
>> I quickly carved up a patch (attached), which is only compile tested
>> because I do not have any AMD hardware at hand.
> 
> 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().

> Because honestly, even if AMD is the one that has had stuttering
> issues, the bigger argument is that there is simply no _point_ in
> supporting randomness from a firmware source.
> 

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.

They both source the exact same hardware IP, but RDRAND is a *lot* more 
direct.

> There is no way anybody should believe that a firmware TPM generates
> better randomness than we do natively.
> 
> And there are many reasons to _not_ believe it. The AMD problem is
> just the most user-visible one.
> 
> Now, I'm not saying that a fTPM needs to be disabled in general - but
> I really feel like we should just do
> 
>   static int tpm_add_hwrng(struct tpm_chip *chip)
>   {
>          if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM))
>                  return 0;
>          // If it's not hardware, don't treat it as such
>          if (tpm_is_fTPM(chip))
>                  return 0;
>          [...]
> 
> and be done with it.
> 
> But hey, if we have no way to see that whole "this is firmware
> emulation", then just blocking AMD might be the only way.
> 
>                 Linus