On Fri, Nov 11, 2022 at 03:16:28PM -0800, Evan Green wrote:
> Introduce a new Kconfig, TCG_TPM_RESTRICT_PCR, which if enabled
TCG_TPM_RESTRICT_PCR => TCG_TPM2_RESTRICT_PCR
> For systems with TPM1 devices, having this Kconfig enabled completely
> restricts usermode's access to the TPM.
This doesn't appear to actually be the case.
> +config TCG_TPM2_RESTRICT_PCR
> + bool "Restrict userland access to PCR 23 on TPM2 devices"
> + depends on TCG_TPM
I assume you also considered making this a once-settable sysctl, or similar?
I guess this kconfig is fine for now, but IMO it does violate the concept of
"kernel provides mechanism, not policy".
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 303ce2ea02a4b0..3bc5546fddc792 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_TPM2_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;
get_unaligned_be32((__be32 *)&buffer[TPM_HEADER_SIZE]),
to avoid an unaligned memory access.
> + break;
> + }
> +
> + return 0;
So, if tpm2_find_and_validate_cc() returns an error code, the command is *not*
restricted, even if it uses one of the forbidden command codes. Are you sure
there are no loopholes here?
- Eric
