linux-kernel - Re: [PATCH] tpm: Opt-in in disable PCR encryption on TPM2 chips

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Message-Id: <D5FQC7G8OUAO.1A449Z8ADEA0E@kernel.org>
Date: Thu, 07 Nov 2024 08:24:06 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Mimi Zohar" <zohar@...ux.ibm.com>, <linux-integrity@...r.kernel.org>,
 "Jonathan Corbet" <corbet@....net>, "Peter Huewe" <peterhuewe@....de>,
 "Jason Gunthorpe" <jgg@...pe.ca>, "James Bottomley"
 <James.Bottomley@...senPartnership.com>
Cc: "Roberto Sassu" <roberto.sassu@...wei.com>, <linux-doc@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: Opt-in in disable PCR encryption on TPM2 chips

On Thu Nov 7, 2024 at 4:48 AM EET, Mimi Zohar wrote:
> On Thu, 2024-11-07 at 02:51 +0200, Jarkko Sakkinen wrote:
> > On Thu Nov 7, 2024 at 2:47 AM EET, Jarkko Sakkinen wrote:
> > > From: Mimi Zohar <zohar@...ux.ibm.com>
> > > 
> > > The initial encrypted HMAC session feature added TPM bus encryption to
> > > various in-kernel TPM operations. This can cause performance bottlenecks
> > > with IMA, as it heavily utilizes PCR extend operations.
>
> The patch Subject line and problem description aren't quite right.  In the case
> of TPM pcr_extend, the session isn't being encrypted, only HMAC'ed.  According
> to James, it's the HMAC itself that is causing the performance degradation. I
> would remove the word "encrypted" throughout.

I have to say I disagree with that. Encryption is the feature we get
with HMAC and is more understandable for most. HMAC is implemnetation
detail.


BR, Jarkko