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

Jarkko Sakkinen
2022-11-07T11:40:14Z

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
>
> drivers/char/tpm/Kconfig | 12 ++++++++++++
> drivers/char/tpm/tpm-dev-common.c | 8 ++++++++
> drivers/char/tpm/tpm.h | 19 +++++++++++++++++++
> drivers/char/tpm/tpm1-cmd.c | 13 +++++++++++++
> drivers/char/tpm/tpm2-cmd.c | 22 ++++++++++++++++++++++
> 5 files changed, 74 insertions(+)
>
> diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
> index 927088b2c3d3f2..c8ed54c66e399a 100644
> --- a/drivers/char/tpm/Kconfig
> +++ b/drivers/char/tpm/Kconfig
> @@ -211,4 +211,16 @@ 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. This is a TPM2-only feature, and if enabled
> + on a TPM1 machine will cause all usermode TPM commands to return EPERM due
> + to the complications introduced by tunnelled sessions in TPM1.2.
> 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 f1e0f490176f01..c0845e3f9eda17 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -245,4 +245,23 @@ 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 cf64c738510529..1869e89215fcb9 100644
> --- a/drivers/char/tpm/tpm1-cmd.c
> +++ b/drivers/char/tpm/tpm1-cmd.c
> @@ -811,3 +811,16 @@ 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)
> +{
> + /*
> + * Restrict all usermode commands on TPM1.2. Ideally we'd just restrict
> + * TPM_ORD_PCR_EXTEND and TPM_ORD_PCR_RESET, but TPM1.2 also supports
> + * tunnelled transport sessions where the kernel would be unable to filter
> + * commands.
> + */
> + return -EPERM;
> +}
> +#endif
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 303ce2ea02a4b0..e0503cfd7bcfee 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -778,3 +778,25 @@ int tpm2_find_cc(struct tpm_chip *chip, u32 cc)
>
> return -1;
> }
> +
> +#ifdef CONFIG_TCG_TPM_RESTRICT_PCR
> +int tpm2_cmd_restricted(struct tpm_chip *chip, u8 *buffer, size_t size)
> +{
> + int cc = tpm2_find_and_validate_cc(chip, NULL, buffer, size);
> + __be32 *handle;
> +
> + switch (cc) {
> + case TPM2_CC_PCR_EXTEND:
> + case TPM2_CC_PCR_RESET:
> + if (size < (TPM_HEADER_SIZE + sizeof(u32)))
> + return -EINVAL;
> +
> + handle = (__be32 *)&buffer[TPM_HEADER_SIZE];
> + if (be32_to_cpu(*handle) == TPM_RESTRICTED_PCR)
> + return -EPERM;
> + break;
> + }
> +
> + return 0;
> +}
> +#endif
> --
> 2.38.1.431.g37b22c650d-goog
>

This looks otherwise good but I have still one remark: what is the reason
for restricting PCR23 for TPM 1.x?

BR, Jarkko