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

Evan Green
2022-09-07T17:04:00Z

On Thu, Aug 25, 2022 at 8:02 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Tue, Aug 23, 2022 at 03:25:18PM -0700, 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>
>
> Same issues as the other patch.
>
> > ---
> > Matthew's original version of this patch is at:
> > https://patchwork.kernel.org/patch/12096491/
>
> Suggestion: find a lore URL instead for Matthew's patch,
> and then add "Link: <lore URL>" to your commit before
> sob's. It's a useful reference also in the commit log.
>
> >
> > 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;
> > + 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 69126a6770386e..dbf7f5552c6782 100644
> > --- a/drivers/char/tpm/tpm2-cmd.c
> > +++ b/drivers/char/tpm/tpm2-cmd.c
> > @@ -821,3 +821,25 @@ 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);
> > + __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
> > diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
> > index ffb35f0154c16c..6f51cd92c6400f 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)
>
> Split the export to a separate commit.

Will do all these, thanks for taking a look.