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

Kees Cook
2022-11-04T18:28:04Z

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