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

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Date:   Mon, 31 Jul 2023 11:07:39 +0000
From:   "Jarkko Sakkinen" <jarkko@...nel.org>
To:     "Daniil Stas" <daniil.stas@...teo.net>
Cc:     <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>,
        <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs

On Mon Jul 31, 2023 at 10:28 AM UTC, Daniil Stas wrote:
> On Mon, 31 Jul 2023 10:14:05 +0000
> "Jarkko Sakkinen" <jarkko@...nel.org> wrote:
>
> > On Fri Jul 28, 2023 at 8:18 PM UTC, Daniil Stas wrote:
> > > On Fri, 28 Jul 2023 19:30:18 +0000
> > > "Jarkko Sakkinen" <jarkko@...nel.org> wrote:
> > >  
> > > > On Thu Jul 27, 2023 at 3:38 PM UTC, Daniil Stas wrote:  
> >  [...]  
> > > > 
> > > > This is needs a bit more elaboration in order to be comprehended.
> > > > 
> > > > Do you mean by "stutter" unexpected delays and when do they
> > > > happen?
> > > > 
> > > > BR, Jarkko  
> > >
> > > Yes, unexpected delays. They just happen randomly.
> > > You can google "AMD fTPM stuttering", there are a lot of examples.
> > > Here is one: https://www.youtube.com/watch?v=TYnRL-x6DVI  
> > 
> > What if you make tpm_amd_is_rng_defective() to unconditonally return
> > true? Does this make the problem dissappear, or not?
> > 
> > BR, Jarkko
>
> I already tried compiling kernel without CONFIG_HW_RANDOM_TPM enabled,
> which does the same.
> Yes, it removes the issue.

Thank you, just wanted sanity check that I exactly know what is going on.

BR, Jarkko