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

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Message-Id: <D5FHR6UVEH4G.1OE6D5PDU26X5@kernel.org>
Date: Thu, 07 Nov 2024 01:40:31 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Jarkko Sakkinen" <jarkko@...nel.org>, "James Bottomley"
 <James.Bottomley@...senPartnership.com>, "Mimi Zohar"
 <zohar@...ux.ibm.com>, <linux-integrity@...r.kernel.org>
Cc: <roberto.sassu@...wei.com>, <mapengyu@...il.com>, "Paul Moore"
 <paul@...l-moore.com>, <linux-kernel@...r.kernel.org>,
 <christian@...sel.eu>
Subject: Re: [RFC PATCH] tpm: Allow the TPM2 pcr_extend HMAC capability to
 be disabled on boot

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.

BR, Jarkko