Re: [RFC PATCH] tpm: Allow the TPM2 pcr_extend HMAC capability to be disabled on boot

Jarkko Sakkinen
2024-11-06T23:42:32Z

On Thu Nov 7, 2024 at 1:40 AM EET, Jarkko Sakkinen wrote:
> On Thu Nov 7, 2024 at 1:22 AM EET, Jarkko Sakkinen wrote:
> > > I'm a bit confused here. It's TPM2_PCR_Extend we have the trouble with
> > > (as Mimi says in her email that you quoted) not TPM2_GetRandom.
> > >
> > > The random number generator reseed occurs in a kernel thread that fires
> > > about once a minute, so it doesn't show up in really any of the boot
> > > timings. Plus even with sessions added, what there now isn't a
> > > significant overhead even to the running kernel given it's asynchronous
> > > and called infrequently.
> >
> > Ah, right then we need the boot flag, and my earlier comments to the
> > parameter apply. I've never used IMA so I don't actually even know in
> > detail how it is using TPM.
> >
> > Now that I did some seek I mixed this up with the report:
> >
> > https://chaos.social/@gromit/113345582873908273
> >
> > Anyway concerning this issue and patch, my earlier comments still apply.
>
> Makes me wonder tho why do we then export tpm_get_random() in the first
> place? HWRNG does not needed that export, and the code does not have any
> of the mentioned features.

I mean specifically the code for tpm_get_random() and tpm2_get_random().

BR, Jarkko