Re: [PATCH v4 03/11] tpm: Allow PCR 23 to be restricted to kernel-only use

Jarkko Sakkinen
2022-11-23T23:05:08Z

On Mon, Nov 07, 2022 at 10:15:27AM -0800, Evan Green wrote:
> On Mon, Nov 7, 2022 at 3:40 AM Jarkko Sakkinen <jarkko@kernel.org> wrote:
> >
> > On Thu, Nov 03, 2022 at 11:01:11AM -0700, Evan Green wrote:
> > > From: Matthew Garrett <matthewgarrett@google.com>
> > >
> > > Introduce a new Kconfig, TCG_TPM_RESTRICT_PCR, which if enabled
> > > restricts usermode's ability to extend or reset PCR 23.
> > >
> > > 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.
> > >
> > > This Kconfig is only properly supported for systems with TPM2 devices.
> > > For systems with TPM1 devices, having this Kconfig enabled completely
> > > restricts usermode's access to the TPM. TPM1 contains support for
> > > tunnelled transports, which usermode could use to smuggle commands
> > > through that this Kconfig is attempting to restrict.
> > >
> > > Link: https://lore.kernel.org/lkml/20210220013255.1083202-3-matthewgarrett@google.com/
> > > Signed-off-by: Matthew Garrett <mjg59@google.com>
> > > Signed-off-by: Evan Green <evgreen@chromium.org>
> > > ---
> > >
> > > Changes in v4:
> > > - Augment the commit message (Jarkko)
> > >
> > > Changes in v3:
> > > - Fix up commit message (Jarkko)
> > > - tpm2_find_and_validate_cc() was split (Jarkko)
> > > - Simply fully restrict TPM1 since v2 failed to account for tunnelled
> > > transport sessions (Stefan and Jarkko).
> > >
> > > Changes in v2:
> > > - Fixed sparse warnings
> > >
> > > drivers/char/tpm/Kconfig | 12 ++++++++++++
> > > drivers/char/tpm/tpm-dev-common.c | 8 ++++++++
> > > drivers/char/tpm/tpm.h | 19 +++++++++++++++++++
> > > drivers/char/tpm/tpm1-cmd.c | 13 +++++++++++++
> > > drivers/char/tpm/tpm2-cmd.c | 22 ++++++++++++++++++++++
> > > 5 files changed, 74 insertions(+)
> > >
> > > diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
> > > index 927088b2c3d3f2..c8ed54c66e399a 100644
> > > --- a/drivers/char/tpm/Kconfig
> > > +++ b/drivers/char/tpm/Kconfig
> > > @@ -211,4 +211,16 @@ 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. This is a TPM2-only feature, and if enabled
> > > + on a TPM1 machine will cause all usermode TPM commands to return EPERM due
> > > + to the complications introduced by tunnelled sessions in TPM1.2.
> > > 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 f1e0f490176f01..c0845e3f9eda17 100644
> > > --- a/drivers/char/tpm/tpm.h
> > > +++ b/drivers/char/tpm/tpm.h
> > > @@ -245,4 +245,23 @@ 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 cf64c738510529..1869e89215fcb9 100644
> > > --- a/drivers/char/tpm/tpm1-cmd.c
> > > +++ b/drivers/char/tpm/tpm1-cmd.c
> > > @@ -811,3 +811,16 @@ 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)
> > > +{
> > > + /*
> > > + * Restrict all usermode commands on TPM1.2. Ideally we'd just restrict
> > > + * TPM_ORD_PCR_EXTEND and TPM_ORD_PCR_RESET, but TPM1.2 also supports
> > > + * tunnelled transport sessions where the kernel would be unable to filter
> > > + * commands.
> > > + */
> > > + return -EPERM;
> > > +}
> > > +#endif
> > > diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> > > index 303ce2ea02a4b0..e0503cfd7bcfee 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_TPM_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;
> > > + break;
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +#endif
> > > --
> > > 2.38.1.431.g37b22c650d-goog
> > >
> >
> > This looks otherwise good but I have still one remark: what is the reason
> > for restricting PCR23 for TPM 1.x?
>
> Mostly I was trying to do the least surprising thing for someone who
> had compiled with this RESTRICT_PCR Kconfig enabled but booted a TPM1
> system. If we do nothing for TPM1, then the encrypted hibernation
> mechanism appears to work fine, but leaves a gaping hole where
> usermode can manipulate PCR23 themselves to create forged encrypted
> hibernate images. Denying all usermode access makes the Kconfig
> correct on TPM1 systems, at the expense of all usermode access (rather
> than just access to PCR23).

OK, I buy this. Can you add inline comment perhaps denoting this?

BR, Jarkko