linux-kernel - Re: [PATCH v4 03/11] tpm: Allow PCR 23 to be restricted to kernel-only use

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Date:   Fri, 4 Nov 2022 11:27:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Evan Green <evgreen@...omium.org>
Cc:     linux-kernel@...r.kernel.org, corbet@....net,
        linux-pm@...r.kernel.org, rjw@...ysocki.net, gwendal@...omium.org,
        apronin@...omium.org, Pavel Machek <pavel@....cz>,
        Matthew Garrett <mgarrett@...ora.tech>,
        linux-integrity@...r.kernel.org, jejb@...ux.ibm.com,
        zohar@...ux.ibm.com, dlunev@...gle.com,
        Eric Biggers <ebiggers@...nel.org>,
        Ben Boeckel <me@...boeckel.net>, jarkko@...nel.org,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH v4 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use

On Thu, Nov 03, 2022 at 11:01:11AM -0700, Evan Green wrote:
> From: Matthew Garrett <matthewgarrett@...gle.com>
> 
> Introduce a new Kconfig, TCG_TPM_RESTRICT_PCR, which if enabled
> restricts usermode's ability to extend or reset PCR 23.
> 
> Under certain circumstances it might be desirable to enable the creation
> of TPM-backed secrets that are only accessible to the kernel. In an
> ideal world this could be achieved by using TPM localities, but these
> don't appear to be available on consumer systems. An alternative is to
> simply block userland from modifying one of the resettable PCRs, leaving
> it available to the kernel. If the kernel ensures that no userland can
> access the TPM while it is carrying out work, it can reset PCR 23,
> extend it to an arbitrary value, create or load a secret, and then reset
> the PCR again. Even if userland somehow obtains the sealed material, it
> will be unable to unseal it since PCR 23 will never be in the
> appropriate state.
> 
> This Kconfig is only properly supported for systems with TPM2 devices.
> For systems with TPM1 devices, having this Kconfig enabled completely
> restricts usermode's access to the TPM. TPM1 contains support for
> tunnelled transports, which usermode could use to smuggle commands
> through that this Kconfig is attempting to restrict.
> 
> Link: https://lore.kernel.org/lkml/20210220013255.1083202-3-matthewgarrett@google.com/
> Signed-off-by: Matthew Garrett <mjg59@...gle.com>
> Signed-off-by: Evan Green <evgreen@...omium.org>
> ---
> 
> Changes in v4:
>  - Augment the commit message (Jarkko)
> 
> Changes in v3:
>  - Fix up commit message (Jarkko)
>  - tpm2_find_and_validate_cc() was split (Jarkko)
>  - Simply fully restrict TPM1 since v2 failed to account for tunnelled
>    transport sessions (Stefan and Jarkko).
> 
> Changes in v2:
>  - Fixed sparse warnings

Since you've changed this patch from the original, I would follow the
same advice I gave here:
https://lore.kernel.org/lkml/202209201620.A886373@keescook/

>
-- 
Kees Cook