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Date:   Fri, 28 Jul 2023 16:01:45 -0500
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Subject: Re: [PATCH 1/1] tpm: disable hwrng for fTPM on some AMD designs
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Daniil Stas <daniil.stas@posteo.net>
Cc:     James.Bottomley@hansenpartnership.com, Jason@zx2c4.com,
        jarkko@kernel.org, linux-integrity@vger.kernel.org,
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On 7/28/2023 3:41 PM, Linus Torvalds wrote:
> On Thu, 27 Jul 2023 at 10:05, Daniil Stas <daniil.stas@posteo.net> wrote:
>>
>> Here is the bug report I created:
>> https://bugzilla.kernel.org/show_bug.cgi?id=217719
> 
> Let's just disable the stupid fTPM hwrnd thing.
> 
> Maybe use it for the boot-time "gather entropy from different
> sources", but clearly it should *not* be used at runtime.
> 
> Why would anybody use that crud when any machine that has it
> supposedly fixed (which apparently didn't turn out to be true after
> all) would also have the CPU rdrand instruction that doesn't have the
> problem?

It /seems/ to be a separate problem, but yes I agree with your point.

> 
> If you don't trust the CPU rdrand implementation (and that has had
> bugs too - see clear_rdrand_cpuid_bit() and x86_init_rdrand()), why
> would you trust the fTPM version that has caused even *more* problems?

That's exactly why I was asking in the kernel bugzilla if something 
similar gets tripped up by RDRAND.

I've got a patch that tears it out entirely for AMD fTPMs in the 
bugzilla, but I would prefer to discuss this with BIOS people before 
going that direction.

> 
> So I don't see any downside to just saying "that fTPM thing is not
> working". Even if it ends up working in the future, there are
> alternatives that aren't any worse.
> 
>                   Linus