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

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Message-ID: <84fcb3e29f3aa1ea7a5b638307e500608bc8b11a.camel@linux.ibm.com>
Date: Wed, 06 Nov 2024 18:52:50 -0500
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
        James Bottomley
	 <James.Bottomley@...senPartnership.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, 2024-11-07 at 01:22 +0200, Jarkko Sakkinen wrote:
> On Thu Nov 7, 2024 at 12:52 AM EET, James Bottomley 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.

Huh?  A simple explanation is that IMA-measurement maintains a measurement list,
similar to the pre-boot event log.  Each IMA-measurement record extends the TPM
PCR (default PCR 10).

Assuming IMA is enabled in the kernel, then just add "ima_policy=tcb" or
"ima_policy=critical_data" on the boot command line.  To view the measurement
records, cat <securityfs>/integrity/ima/ascii_runtime_measurements.  Normally
the IMA policy specified on the boot command line is replaced with a finer
grained custom policy.

Mimi