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

[Stefan Berger <stefanb@linux.ibm.com>]

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...