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SubjectRe: [PATCH 2/9] tpm: Allow PCR 23 to be restricted to kernel-only use
On Sat, Feb 20, 2021 at 01:32:48AM +0000, Matthew Garrett wrote:
> 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.

Does this leave room to use them *if* they are available? Not saying
that this patch set must support them, but neither would like to
disclude them from unforseeable future.

>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> ---
> 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 a18c314da211..bba30fb16a2e 100644
> --- a/drivers/char/tpm/Kconfig
> +++ b/drivers/char/tpm/Kconfig
> @@ -190,4 +190,14 @@ 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 1784530b8387..d3db4fd76257 100644
> --- a/drivers/char/tpm/tpm-dev-common.c
> +++ b/drivers/char/tpm/tpm-dev-common.c
> @@ -193,6 +193,14 @@ 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;
> +

I have to admit my knowledge is limited here. I'm not sure how widely is 23
used by the pre-existing user space in the wild.

> /*
> * 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 746f7696bdc0..8eed5016d733 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -232,6 +232,8 @@ 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);
> @@ -245,4 +247,23 @@ 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 36990e9d2dc1..2dab1647d89c 100644
> --- a/drivers/char/tpm/tpm1-cmd.c
> +++ b/drivers/char/tpm/tpm1-cmd.c
> @@ -840,3 +840,38 @@ 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;
> + u32 *pcr;
> + int pos;
> +
> + switch (be32_to_cpu(header->ordinal)) {
> + case TPM_ORD_PCR_EXTEND:
> + if (size < (TPM_HEADER_SIZE + sizeof(u32)))
> + return -EINVAL;
> + pcr = (u32 *)&buffer[TPM_HEADER_SIZE];
> + if (be32_to_cpu(*pcr) == TPM_RESTRICTED_PCR)
> + return -EPERM;
> + break;
> + case TPM_ORD_PCR_RESET:
> + if (size < (TPM_HEADER_SIZE + 1))
> + return -EINVAL;
> + len = buffer[TPM_HEADER_SIZE];
> + if (size < (TPM_HEADER_SIZE + 1 + len))
> + return -EINVAL;
> + offset = TPM_RESTRICTED_PCR/3;
> + if (len < offset)
> + break;
> + pos = TPM_HEADER_SIZE + 1 + offset;
> + if (buffer[pos] & (1 << (TPM_RESTRICTED_PCR - 2 * offset)))
> + return -EPERM;
> + break;
> + }
> +
> + return 0;
> +}
> +#endif
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 9609ae8086c6..7dbd4590dee8 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -795,3 +795,25 @@ 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 = tpm_find_and_validate_cc(chip, NULL, buffer, size);
> + u32 *handle;
> +
> + switch (cc) {
> + case TPM2_CC_PCR_EXTEND:
> + case TPM2_CC_PCR_RESET:
> + if (size < (TPM_HEADER_SIZE + sizeof(u32)))
> + return -EINVAL;
> +
> + handle = (u32 *)&buffer[TPM_HEADER_SIZE];
> + if (be32_to_cpu(*handle) == TPM_RESTRICTED_PCR)
> + return -EPERM;
> + break;
> + }
> +
> + return 0;
> +}
> +#endif
> diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
> index 784b8b3cb903..76a993492962 100644
> --- a/drivers/char/tpm/tpm2-space.c
> +++ b/drivers/char/tpm/tpm2-space.c
> @@ -262,7 +262,7 @@ static int tpm2_map_command(struct tpm_chip *chip, u32 cc, u8 *cmd)
> return 0;
> }
>
> -static int tpm_find_and_validate_cc(struct tpm_chip *chip,
> +int tpm_find_and_validate_cc(struct tpm_chip *chip,
> struct tpm_space *space,
> const void *cmd, size_t len)
> {
> --
> 2.30.0.617.g56c4b15f3c-goog
>
>

What are the consumer use cases anyway? Why wouldn't locality based
solution make sense to those use cases where this makes sense.

Finally, where does hibernate make sense? :-)

This comes more down to on how lay out the requirements.

/Jarkko

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