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

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Sep 13, 2022 at 08:26:09AM -0400, Stefan Berger wrote:
> 
> 
> 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...

BTW, Why do we want to support TPM 1.2 at all.

I would not support it for new features. This could be just TPM2 only
feeature.

BR, Jarkko