[2/9] tpm: Allow PCR 23 to be restricted to kernel-only use

Message ID 20210220013255.1083202-3-matthewgarrett@google.com
State New
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  • [1/9] tpm: Add support for in-kernel resetting of PCRs
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Commit Message

Matthew Garrett Feb. 20, 2021, 1:32 a.m.
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.

Signed-off-by: Matthew Garrett <mjg59@google.com>
---
 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(-)

Comments

Jarkko Sakkinen Feb. 24, 2021, 5:12 p.m. | #1
On Sat, Feb 20, 2021 at 01:32:48AM +0000, Matthew Garrett wrote:
> +#define TPM_RESTRICTED_PCR 23


As stupid it may sound, I'd just change this to:

#define TPM_PCR_23 23

It documents to the code that we are dealing with PCR 23, which just a
plain number doesn't. By naming it as TPM_RESTRICED_TPM you have to
unnecessarily xref to its definition. It obfuscates rather than clarifies
anything important.

/Jarkko
James Bottomley Feb. 24, 2021, 6 p.m. | #2
On Sat, 2021-02-20 at 01:32 +0000, Matthew Garrett wrote:
> 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.


I don't understand this ... the localities seem to work fine on all the
systems I have ... is this some embedded thing?

>  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 seems a bit arbitrary: You're removing this PCR from user space
accessibility, but PCR 23 is defined as "Application Support" how can
we be sure no application will actually want to use it (and then fail)?

Since PCRs are very scarce, why not use a NV index instead.  They're
still a bounded resource, but most TPMs have far more of them than they
do PCRs, and the address space is much bigger so picking a nice
arbitrary 24 bit value reduces the chance of collisions.

James
Matthew Garrett Feb. 28, 2021, 7:59 a.m. | #3
On Wed, Feb 24, 2021 at 10:00:53AM -0800, James Bottomley wrote:
> On Sat, 2021-02-20 at 01:32 +0000, Matthew Garrett wrote:

> > 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.

> 

> I don't understand this ... the localities seem to work fine on all the

> systems I have ... is this some embedded thing?


I haven't made it work on an HP Z440 or a Lenovo P520. So now I'm
wondering whether having chipsets with TXT support (even if it's turned
off) confuse this point. Sigh. I'd really prefer to use localities than
a PCR, so if it works on client platforms I'd be inclined to say we'll
do a self-test and go for that, and workstation vendors can just
recommend their customers use UPSes or something.

> >  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 seems a bit arbitrary: You're removing this PCR from user space

> accessibility, but PCR 23 is defined as "Application Support" how can

> we be sure no application will actually want to use it (and then fail)?


Absolutely no way of guaranteeing that, and enabling this option is
certainly an ABI break.

> Since PCRs are very scarce, why not use a NV index instead.  They're

> still a bounded resource, but most TPMs have far more of them than they

> do PCRs, and the address space is much bigger so picking a nice

> arbitrary 24 bit value reduces the chance of collisions.


How many write cycles do we expect the NV to survive? But I'll find a
client system with a TPM and play with locality support there - maybe we
can just avoid this problem anyway.

Patch

diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
index a18c314da211..bba30fb16a2e 100644
--- a/drivers/char/tpm/Kconfig
+++ b/drivers/char/tpm/Kconfig
@@ -190,4 +190,14 @@  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 1784530b8387..d3db4fd76257 100644
--- a/drivers/char/tpm/tpm-dev-common.c
+++ b/drivers/char/tpm/tpm-dev-common.c
@@ -193,6 +193,14 @@  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 746f7696bdc0..8eed5016d733 100644
--- a/drivers/char/tpm/tpm.h
+++ b/drivers/char/tpm/tpm.h
@@ -232,6 +232,8 @@  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);
@@ -245,4 +247,23 @@  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 36990e9d2dc1..2dab1647d89c 100644
--- a/drivers/char/tpm/tpm1-cmd.c
+++ b/drivers/char/tpm/tpm1-cmd.c
@@ -840,3 +840,38 @@  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;
+	u32 *pcr;
+	int pos;
+
+	switch (be32_to_cpu(header->ordinal)) {
+	case TPM_ORD_PCR_EXTEND:
+		if (size < (TPM_HEADER_SIZE + sizeof(u32)))
+			return -EINVAL;
+		pcr = (u32 *)&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 9609ae8086c6..7dbd4590dee8 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -795,3 +795,25 @@  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);
+	u32 *handle;
+
+	switch (cc) {
+	case TPM2_CC_PCR_EXTEND:
+	case TPM2_CC_PCR_RESET:
+		if (size < (TPM_HEADER_SIZE + sizeof(u32)))
+			return -EINVAL;
+
+		handle = (u32 *)&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 784b8b3cb903..76a993492962 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)
 {