Re: [PATCH v2 02/10] tpm: Allow PCR 23 to be restricted to kernel-only use

Stefan Berger
2022-09-13T12:26:33Z

On 8/23/22 18:25, Evan Green wrote:
> From: Matthew Garrett <matthewgarrett@google.com>
>
> 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.
>
> From: Matthew Garrett <mjg59@google.com>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
>
> Signed-off-by: Evan Green <evgreen@chromium.org>
> ---
> Matthew's original version of this patch is at:
> https://patchwork.kernel.org/patch/12096491/
>
> Changes in v2:
> - Fixed sparse warnings
>
> drivers/char/tpm/Kconfig | 10 +++++++++
> drivers/char/tpm/tpm-dev-common.c | 8 +++++++
> drivers/char/tpm/tpm.h | 21 +++++++++++++++++++
> drivers/char/tpm/tpm1-cmd.c | 35 +++++++++++++++++++++++++++++++
> drivers/char/tpm/tpm2-cmd.c | 22 +++++++++++++++++++
> drivers/char/tpm/tpm2-space.c | 2 +-
> 6 files changed, 97 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
> index 927088b2c3d3f2..4483b61a428b11 100644
> --- a/drivers/char/tpm/Kconfig
> +++ b/drivers/char/tpm/Kconfig
> @@ -211,4 +211,14 @@ config TCG_FTPM_TEE
> This driver proxies for firmware TPM running in TEE.
>
> source "drivers/char/tpm/st33zp24/Kconfig"
> +
> +config TCG_TPM_RESTRICT_PCR
> + bool "Restrict userland access to PCR 23"
> + depends on TCG_TPM
> + help
> + If set, block userland from extending or resetting PCR 23. This
> + allows it to be restricted to in-kernel use, preventing userland
> + from being able to make use of data sealed to the TPM by the kernel.
> + This is required for secure hibernation support, but should be left
> + disabled if any userland may require access to PCR23.
> endif # TCG_TPM
> diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
> index dc4c0a0a512903..7a4e618c7d1942 100644
> --- a/drivers/char/tpm/tpm-dev-common.c
> +++ b/drivers/char/tpm/tpm-dev-common.c
> @@ -198,6 +198,14 @@ ssize_t tpm_common_write(struct file *file, const char __user *buf,
> priv->response_read = false;
> *off = 0;
>
> + if (priv->chip->flags & TPM_CHIP_FLAG_TPM2)
> + ret = tpm2_cmd_restricted(priv->chip, priv->data_buffer, size);
> + else
> + ret = tpm1_cmd_restricted(priv->chip, priv->data_buffer, size);
> +
> + if (ret)
> + goto out;
> +
> /*
> * If in nonblocking mode schedule an async job to send
> * the command return the size.
> diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> index a80b341d38eb8c..077c3ca0a127ba 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -229,6 +229,8 @@ void tpm2_shutdown(struct tpm_chip *chip, u16 shutdown_type);
> unsigned long tpm2_calc_ordinal_duration(struct tpm_chip *chip, u32 ordinal);
> int tpm2_probe(struct tpm_chip *chip);
> int tpm2_get_cc_attrs_tbl(struct tpm_chip *chip);
> +int tpm_find_and_validate_cc(struct tpm_chip *chip, struct tpm_space *space,
> + const void *buf, size_t bufsiz);
> int tpm2_find_cc(struct tpm_chip *chip, u32 cc);
> int tpm2_init_space(struct tpm_space *space, unsigned int buf_size);
> void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space);
> @@ -244,4 +246,23 @@ void tpm_bios_log_setup(struct tpm_chip *chip);
> void tpm_bios_log_teardown(struct tpm_chip *chip);
> int tpm_dev_common_init(void);
> void tpm_dev_common_exit(void);
> +
> +#ifdef CONFIG_TCG_TPM_RESTRICT_PCR
> +#define TPM_RESTRICTED_PCR 23
> +
> +int tpm1_cmd_restricted(struct tpm_chip *chip, u8 *buffer, size_t size);
> +int tpm2_cmd_restricted(struct tpm_chip *chip, u8 *buffer, size_t size);
> +#else
> +static inline int tpm1_cmd_restricted(struct tpm_chip *chip, u8 *buffer,
> + size_t size)
> +{
> + return 0;
> +}
> +
> +static inline int tpm2_cmd_restricted(struct tpm_chip *chip, u8 *buffer,
> + size_t size)
> +{
> + return 0;
> +}
> +#endif
> #endif
> diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
> index 8ec743dec26544..318e75ae42fb85 100644
> --- a/drivers/char/tpm/tpm1-cmd.c
> +++ b/drivers/char/tpm/tpm1-cmd.c
> @@ -845,3 +845,38 @@ int tpm1_get_pcr_allocation(struct tpm_chip *chip)
>
> return 0;
> }
> +
> +#ifdef CONFIG_TCG_TPM_RESTRICT_PCR
> +int tpm1_cmd_restricted(struct tpm_chip *chip, u8 *buffer, size_t size)
> +{
> + struct tpm_header *header = (struct tpm_header *)buffer;
> + char len, offset;
> + __be32 *pcr;
> + int pos;
> +
> + switch (be32_to_cpu(header->ordinal)) {
> + case TPM_ORD_PCR_EXTEND:
> + if (size < (TPM_HEADER_SIZE + sizeof(u32)))
> + return -EINVAL;
> + pcr = (__be32 *)&buffer[TPM_HEADER_SIZE];
> + if (be32_to_cpu(*pcr) == TPM_RESTRICTED_PCR)
> + return -EPERM;

FYI: TPM 1.2 has transport sessions where the command is tunneled in an
encrypted channel and this check could be circumvented...