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

Jarkko Sakkinen
2022-09-21T18:06:04Z

On Wed, Sep 21, 2022 at 09:02:29PM +0300, Jarkko Sakkinen wrote:
> On Wed, Sep 21, 2022 at 08:35:35AM -0700, Evan Green wrote:
> > On Mon, Sep 19, 2022 at 9:51 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
> > >
> > > 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.
> >
> > I didn't know about the TPM1.2 tunnelling thing, thanks Stefan. Yes,
> > maybe in light of that and Jarkko's comment we shouldn't bend over
> > backwards to make this work on TPM1 and just make it a TPM2-only
> > feature.
> >
> > Downstream of this decision, in the other patch, "Add support for
> > in-kernel resetting of PCRs", my instinct is to keep the addition of
> > tpm1_pcr_reset() just so the newly introduced generic tpm_pcr_reset()
> > is fully implemented. Let me know if instead I should also drop the
> > tpm1 side of that as well, in the name of "don't add stuff you're not
> > using".
> > -Evan
>
> You should drop TPM 1.2 support.
>
> General policy with TPM 1.2:
>
> 1. Support legacy.
> 2. Do no extend the functionality.

Why? Because we do not want to add legacy to something that is dying.

BR, Jarkko