[PATCH 0/1] add sysfs exports for TPM 2 PCR registers

[PATCH 0/1] add sysfs exports for TPM 2 PCR registers

[James Bottomley]

At last year's plumbers conference it was agreed in principle to
export TPM 2 PCRs via sysfs.  We also agreed we should conform to
sysfs rules of one value per file, which rules out the "pcrs" file
format of TPM 1.2 which has every PCR value in the same file.

I added these files using device groups, so one group per bank hash of
the TPM.  Using an emulator which supports a variety of hashes, you
can see the structure of the group files:

root@testdeb:~# ls -F /sys/class/tpm/tpm0/
dev      pcr-sha1/    pcr-sha384/  power/      tpm_version_major
device@  pcr-sha256/  pcr-sha512/  subsystem@  uevent

As a future enhancement, we could use the group is_visible function to
remove files corresponding to PCRs which don't exist.  The reason this
isn't present is that so far I've never seen a TPM with a missing PCR.

James

---

James Bottomley (1):
  tpm: add sysfs exports for all banks of PCR registers

 drivers/char/tpm/tpm-sysfs.c | 178 +++++++++++++++++++++++++++++++++++
 include/linux/tpm.h          |   8 +-
 2 files changed, 185 insertions(+), 1 deletion(-)

-- 
2.26.2

[PATCH 1/1] tpm: add sysfs exports for all banks of PCR registers

[James Bottomley]

use macro magic to create sysfs per hash groups with 24 PCR files in
them one for each possible agile hash of the TPM.  The files are
plugged in to a read function which is TPM version agnostic, so this
works also for TPM 1.2 although the hash is only sha1 in that case.
For every hash the TPM supports, a group named pcr-<hash> is created
and each of the PCR read files placed under it.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
---
 drivers/char/tpm/tpm-sysfs.c | 178 +++++++++++++++++++++++++++++++++++
 include/linux/tpm.h          |   8 +-
 2 files changed, 185 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm-sysfs.c b/drivers/char/tpm/tpm-sysfs.c
index d52bf4df0bca..d81ec5e437c1 100644
--- a/drivers/char/tpm/tpm-sysfs.c
+++ b/drivers/char/tpm/tpm-sysfs.c
@@ -348,9 +348,187 @@ static const struct attribute_group tpm2_dev_group = {
 	.attrs = tpm2_dev_attrs,
 };
 
+struct tpm_pcr_attr {
+	int alg_id;
+	int pcr;
+	struct device_attribute attr;
+};
+
+#define to_tpm_pcr_attr(a) container_of(a, struct tpm_pcr_attr, attr)
+
+static ssize_t pcr_value_show(struct device *dev,
+			      struct device_attribute *attr,
+			      char *buf)
+{
+	struct tpm_pcr_attr *ha = to_tpm_pcr_attr(attr);
+	struct tpm_chip *chip = to_tpm_chip(dev);
+	struct tpm_digest digest;
+	int i;
+	int digest_size = 0;
+	int rc;
+	char *str = buf;
+
+	for (i = 0; i < chip->nr_allocated_banks; i++)
+		if (ha->alg_id == chip->allocated_banks[i].alg_id)
+			digest_size = chip->allocated_banks[i].digest_size;
+	/* should never happen */
+	if (!digest_size)
+		return -EINVAL;
+
+	digest.alg_id = ha->alg_id;
+	rc = tpm_pcr_read(chip, ha->pcr, &digest);
+	if (rc)
+		return rc;
+	for (i = 0; i < digest_size; i++)
+		str += sprintf(str, "%02X", digest.digest[i]);
+	str += sprintf(str, "\n");
+
+	return str - buf;
+}
+
+/*
+ * The following set of defines represents all the magic to build
+ * the per hash attribute groups for displaying each bank of PCRs.
+ * The only slight problem with this approach is that every PCR is
+ * hard coded to be present, so you don't know if an PCR is missing
+ * until a cat of the file returns -EINVAL
+ *
+ * Also note you must ignore checkpatch warnings in this macro
+ * code. This is deep macro magic that checkpatch.pl doesn't
+ * understand.
+ */
+
+/* Note, this must match TPM2_PLATFORM_PCR which is fixed at 24. */
+#define _TPM_HELPER(_alg, _hash, F) \
+	F(_alg, _hash, 0)	    \
+	F(_alg, _hash, 1)	    \
+	F(_alg, _hash, 2)	    \
+	F(_alg, _hash, 3)	    \
+	F(_alg, _hash, 4)	    \
+	F(_alg, _hash, 5)	    \
+	F(_alg, _hash, 6)	    \
+	F(_alg, _hash, 7)	    \
+	F(_alg, _hash, 8)	    \
+	F(_alg, _hash, 9)	    \
+	F(_alg, _hash, 10)	    \
+	F(_alg, _hash, 11)	    \
+	F(_alg, _hash, 12)	    \
+	F(_alg, _hash, 13)	    \
+	F(_alg, _hash, 14)	    \
+	F(_alg, _hash, 15)	    \
+	F(_alg, _hash, 16)	    \
+	F(_alg, _hash, 17)	    \
+	F(_alg, _hash, 18)	    \
+	F(_alg, _hash, 19)	    \
+	F(_alg, _hash, 20)	    \
+	F(_alg, _hash, 21)	    \
+	F(_alg, _hash, 22)	    \
+	F(_alg, _hash, 23)
+
+/* ignore checkpatch warning about trailing ; in macro. */
+#define PCR_ATTR(_alg, _hash, _pcr)				   \
+	static struct tpm_pcr_attr dev_attr_pcr_##_hash##_##_pcr = {	\
+		.alg_id = _alg,					   \
+		.pcr = _pcr,					   \
+		.attr = {					   \
+			.attr = {				   \
+				.name = __stringify(_pcr),	   \
+				.mode = 0444			   \
+			},					   \
+			.show = pcr_value_show			   \
+		}						   \
+	};
+
+#define PCR_ATTRS(_alg, _hash)			\
+	_TPM_HELPER(_alg, _hash, PCR_ATTR)
+
+/* ignore checkpatch warning about trailing , in macro. */
+#define PCR_ATTR_VAL(_alg, _hash, _pcr)		\
+	&dev_attr_pcr_##_hash##_##_pcr.attr.attr,
+
+#define PCR_ATTR_GROUP_ARRAY(_alg, _hash)		       \
+	static struct attribute *pcr_group_attrs_##_hash[] = { \
+		_TPM_HELPER(_alg, _hash, PCR_ATTR_VAL)	       \
+		NULL					       \
+	}
+
+#define PCR_ATTR_GROUP(_alg, _hash)			    \
+	static struct attribute_group pcr_group_##_hash = { \
+		.name = "pcr-" __stringify(_hash),	    \
+		.attrs = pcr_group_attrs_##_hash	    \
+	}
+
+#define PCR_ATTR_BUILD(_alg, _hash)	   \
+	PCR_ATTRS(_alg, _hash)		   \
+	PCR_ATTR_GROUP_ARRAY(_alg, _hash); \
+	PCR_ATTR_GROUP(_alg, _hash)
+/*
+ * End of macro structure to build an attribute group containing 24
+ * PCR value files for each supported hash algorithm
+ */
+
+/*
+ * The next set of macros implements the cleverness for each hash to
+ * build a static attribute group called pcr_group_<hash> which can be
+ * added to chip->groups[].
+ *
+ * The first argument is the TPM algorithm id and the second is the
+ * hash used as both the suffix and the group name.  Note: the group
+ * name is a directory in the top level tpm class with the name
+ * pcr-<hash>, so it must not clash with any other names already
+ * in the sysfs directory.
+ */
+PCR_ATTR_BUILD(TPM_ALG_SHA1, sha1);
+PCR_ATTR_BUILD(TPM_ALG_SHA256, sha256);
+PCR_ATTR_BUILD(TPM_ALG_SHA384, sha384);
+PCR_ATTR_BUILD(TPM_ALG_SHA512, sha512);
+PCR_ATTR_BUILD(TPM_ALG_SM3_256, sm3);
+
+
 void tpm_sysfs_add_device(struct tpm_chip *chip)
 {
+	int i;
+
 	WARN_ON(chip->groups_cnt != 0);
+
+	/* add one group for each bank hash */
+	for (i = 0; i < chip->nr_allocated_banks; i++) {
+		switch (chip->allocated_banks[i].alg_id) {
+		case TPM_ALG_SHA1:
+			chip->groups[chip->groups_cnt++] = &pcr_group_sha1;
+			break;
+		case TPM_ALG_SHA256:
+			chip->groups[chip->groups_cnt++] = &pcr_group_sha256;
+			break;
+		case TPM_ALG_SHA384:
+			chip->groups[chip->groups_cnt++] = &pcr_group_sha384;
+			break;
+		case TPM_ALG_SHA512:
+			chip->groups[chip->groups_cnt++] = &pcr_group_sha512;
+			break;
+		case TPM_ALG_SM3_256:
+			chip->groups[chip->groups_cnt++] = &pcr_group_sm3;
+			break;
+		default:
+			/*
+			 * If this warning triggers, send a patch to
+			 * add both a PCR_ATTR_BUILD() macro above for
+			 * the missing algorithm as well as an
+			 * additional case in this switch statement.
+			 */
+			WARN(1, "TPM with unsupported bank algorthm 0x%04x",
+			     chip->allocated_banks[i].alg_id);
+			break;
+		}
+	}
+
+	/*
+	 * This will only trigger if someone has added an additional
+	 * hash to the tpm_algorithms enum without incrementing
+	 * TPM_MAX_HASHES
+	 */
+	WARN_ON(chip->groups_cnt >= TPM_MAX_HASHES);
+
 	if (chip->flags & TPM_CHIP_FLAG_TPM2)
 		chip->groups[chip->groups_cnt++] = &tpm2_dev_group;
 	else
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 5026a06977e1..f73dc4f9fbf4 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -42,6 +42,12 @@ enum tpm_algorithms {
 	TPM_ALG_SM3_256		= 0x0012,
 };
 
+/*
+ * maximum number of hashing algorithms a TPM can have.  This is
+ * basically a count of every hash in tpm_algorithms above
+ */
+#define TPM_MAX_HASHES	5
+
 struct tpm_digest {
 	u16 alg_id;
 	u8 digest[TPM_MAX_DIGEST_SIZE];
@@ -145,7 +151,7 @@ struct tpm_chip {
 
 	struct dentry *bios_dir[TPM_NUM_EVENT_LOG_FILES];
 
-	const struct attribute_group *groups[3];
+	const struct attribute_group *groups[3 + TPM_MAX_HASHES];
 	unsigned int groups_cnt;
 
 	u32 nr_allocated_banks;
-- 
2.26.2

Re: [PATCH 1/1] tpm: add sysfs exports for all banks of PCR registers

[Mimi Zohar]

Hi James,

On Mon, 2020-07-20 at 08:00 -0700, James Bottomley wrote:
> use macro magic to create sysfs per hash groups with 24 PCR files in
> them one for each possible agile hash of the TPM.  The files are
> plugged in to a read function which is TPM version agnostic, so this
> works also for TPM 1.2 although the hash is only sha1 in that case.
> For every hash the TPM supports, a group named pcr-<hash> is created
> and each of the PCR read files placed under it.
> 
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

Nice!  Being able to read the TPM 2.0 PCRs, without requiring a TSS,
will really simplify regression testing - re-calculating the IMA
"boot_aggregate" and verifying the IMA measurement list.

With the following code snippet all of the PCRs for all the exported
TPM banks are displayed, but unfortunately the digests are not ordered
or prefixed with the PCR.

banks=$(echo -n $(ls -d /sys/class/tpm/tpm0/pcr-*))

for bank in ${banks[@]}; do
        echo "$bank:"
        find "${bank}/" -type f -exec cat {} \;
        echo " "
done

Either this code snippet needs to be fixed or, perhaps, instead of
returning just the digest, the digest could be prefixed with the PCR
number (eg. PCR-00:<digest>).

FYI, with this patch on a system with TPM 1.2, IMA goes into TPM-
bypass mode.

From dmesg (with some extra debugging):
tpm_chip_register: tpm_add_legacy_sysfs failed
tpm_tis: probe of 00:05 failed with error -2

Mimi

Re: [PATCH 1/1] tpm: add sysfs exports for all banks of PCR registers

[James Bottomley]

On Tue, 2020-07-21 at 08:30 -0400, Mimi Zohar wrote:
> Hi James,
> 
> On Mon, 2020-07-20 at 08:00 -0700, James Bottomley wrote:
> > use macro magic to create sysfs per hash groups with 24 PCR files
> > in them one for each possible agile hash of the TPM.  The files are
> > plugged in to a read function which is TPM version agnostic, so
> > this works also for TPM 1.2 although the hash is only sha1 in that
> > case. For every hash the TPM supports, a group named pcr-<hash> is
> > created and each of the PCR read files placed under it.
> > 
> > Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.c
> > om>
> 
> Nice!  Being able to read the TPM 2.0 PCRs, without requiring a TSS,
> will really simplify regression testing - re-calculating the IMA
> "boot_aggregate" and verifying the IMA measurement list.
> 
> With the following code snippet all of the PCRs for all the exported
> TPM banks are displayed, but unfortunately the digests are not
> ordered or prefixed with the PCR.
> 
> banks=$(echo -n $(ls -d /sys/class/tpm/tpm0/pcr-*))
> 
> for bank in ${banks[@]}; do
>         echo "$bank:"
>         find "${bank}/" -type f -exec cat {} \;
>         echo " "
> done
> 
> Either this code snippet needs to be fixed or, perhaps, instead of
> returning just the digest, the digest could be prefixed with the PCR
> number (eg. PCR-00:<digest>).

I think the sysfs output should be a pure hash to save having to
massage it.  However, if you want the 1.2 format, this is the script I
use:

a=0; while [ $a -lt 24 ]; do printf "PCR-%02d: " $a; cat
/sys/class/tpm/tpm0/pcr-sha1/$a; a=$[$a+1]; done

> FYI, with this patch on a system with TPM 1.2, IMA goes into TPM-
> bypass mode.

Hm, got to say I ran this in a virtual environment with the emulator
and I don't have a TPM 1.2.  I'm sure I have a physical system with TPM
1.2 somewhere I can dig out.

> From dmesg (with some extra debugging):
> tpm_chip_register: tpm_add_legacy_sysfs failed
> tpm_tis: probe of 00:05 failed with error -2

Oh, it's assuming chip->groups[0] is the unnamed group.  I can put that
back.  I only added the pcr groups first so they could be counted to
make sure a warning triggered if we had too many.

James

Re: [PATCH 1/1] tpm: add sysfs exports for all banks of PCR registers

[Mimi Zohar]

On Tue, 2020-07-21 at 08:06 -0700, James Bottomley wrote:
> On Tue, 2020-07-21 at 08:30 -0400, Mimi Zohar wrote:

> I think the sysfs output should be a pure hash to save having to
> massage it.  However, if you want the 1.2 format, this is the script I
> use:
> 
> a=0; while [ $a -lt 24 ]; do printf "PCR-%02d: " $a; cat
> /sys/class/tpm/tpm0/pcr-sha1/$a; a=$[$a+1]; done

The TPM 1.2 format is a bit different, but this provides the PCRs and
digests sequentially.

thanks,

Mimi

Re: [PATCH 1/1] tpm: add sysfs exports for all banks of PCR registers

[James Bottomley]

On Tue, 2020-07-21 at 11:47 -0400, Mimi Zohar wrote:
> On Tue, 2020-07-21 at 08:06 -0700, James Bottomley wrote:
> > On Tue, 2020-07-21 at 08:30 -0400, Mimi Zohar wrote:
> > I think the sysfs output should be a pure hash to save having to
> > massage it.  However, if you want the 1.2 format, this is the
> > script I
> > use:
> > 
> > a=0; while [ $a -lt 24 ]; do printf "PCR-%02d: " $a; cat
> > /sys/class/tpm/tpm0/pcr-sha1/$a; a=$[$a+1]; done
> 
> The TPM 1.2 format is a bit different,

You mean the spaces between each byte?  I can persuade the script to do
that, but there didn't seem to be much point given that everything I've
seen that uses the hashes has to remove it.

>  but this provides the PCRs and digests sequentially.

Right.

I've got the v2 that should work on TPM 1.2 if you can test it for me? 
My TPM 1.2 machine is somewhat infrastructure critical, so I have
difficulty rebooting it.

James