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Date:   Fri, 4 Nov 2022 11:27:53 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Evan Green <evgreen@chromium.org>
Cc:     linux-kernel@vger.kernel.org, corbet@lwn.net,
        linux-pm@vger.kernel.org, rjw@rjwysocki.net, gwendal@chromium.org,
        apronin@chromium.org, Pavel Machek <pavel@ucw.cz>,
        Matthew Garrett <mgarrett@aurora.tech>,
        linux-integrity@vger.kernel.org, jejb@linux.ibm.com,
        zohar@linux.ibm.com, dlunev@google.com,
        Eric Biggers <ebiggers@kernel.org>,
        Ben Boeckel <me@benboeckel.net>, jarkko@kernel.org,
        Matthew Garrett <matthewgarrett@google.com>,
        Matthew Garrett <mjg59@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, Peter Huewe <peterhuewe@gmx.de>
Subject: Re: [PATCH v4 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use
Message-ID: <202211041127.55ED2921E5@keescook>
References: <20221103180120.752659-1-evgreen@chromium.org>
 <20221103105558.v4.3.I9ded8c8caad27403e9284dfc78ad6cbd845bc98d@changeid>
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On Thu, Nov 03, 2022 at 11:01:11AM -0700, Evan Green wrote:
> From: Matthew Garrett <matthewgarrett@google.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@google.com>
> Signed-off-by: Evan Green <evgreen@chromium.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