Thanks for the response Jarkko.
On 8/1/23 12:02, Jarkko Sakkinen wrote:
The short summary is cryptic to say the least.
Do you mean the patch subject line, or the description below?
"update counter" does not map it to have anything to do with PCRs.
Agreed. I noticed that when I was testing the patches.
The update counter is same for all PCRs. It was also the same for
the two hash algo's I tested it for (SHA1 and SHA256). But the spec
description and Kernel implementation requires to pass the
pcr_idx and hash algo to PCR_Read command to get the update counter.
Why not "tpm: Read pcrUpdateCounter field from TPM2_PCR_Read"?
As I said in the patch description below, update counter is only
needed for IMA measurements. None of the other code that calls
tpm2_pcr_read() use the update counter.
I was not sure if you were okay changing the function signature and
implementation of tpm2_pcr_read(). It felt disruptive.
But I can update tpm2_pcr_read() if you are ok with it.
Please let me know.
I also have a few more thoughts on this in the comment below.
On Tue Aug 1, 2023 at 9:19 PM EEST, Tushar Sugandhi wrote:
The TPM2_PCR_Read command returns TPM2_PCR_Read Response struct[1]. It
contains pcrUpdateCounter member which contains the current value of TPM
PCR update counter. The update counter provides the number of times the
PCRs are updated, which is essential for tracking changes and verifying
system integrity. Thus, subsystems (like IMA) should measure
pcrUpdateCounter value. Although tpm2_pcr_read_out struct is returned
by tpm2_pcr_read(), it is not used by it's caller function tpm_pcr_read().
Further, TPM2_PCR_Read Response struct and pcrUpdateCounter is not
available in tpm1_pcr_read().
PcrUpdateCounter is only needed in a specific case (IMA for measurements).
Changing tpm_pcr_read() and tpm2_pcr_read() function signature to return
tpm2_pcr_read_out struct would be a more disruptive change, since these
functions are used elsewhere too. Creating separate functions to get
pcrUpdateCounter when needed would be a cleaner approach.
Add a function, 'tpm2_pcr_get_update_counter()' to retrieve
the update counter for a given PCR index and algorithm ID on a TPM2 chip.
This function complements existing TPM functionalities such as reading
and extending PCRs, and enhances the ability to monitor PCR status
in the Linux Kernel.
[1] https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part3_Commands_pub.pdf
Section 22.4.2, Page 206.
Signed-off-by: Tushar Sugandhi <tusharsu@xxxxxxxxxxxxxxxxxxx>
---
drivers/char/tpm/tpm.h | 3 +++
drivers/char/tpm/tpm2-cmd.c | 48 +++++++++++++++++++++++++++++++++++++
2 files changed, 51 insertions(+)
diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
index 830014a26609..60489f21d3bd 100644
--- a/drivers/char/tpm/tpm.h
+++ b/drivers/char/tpm/tpm.h
@@ -288,6 +288,9 @@ static inline void tpm_add_ppi(struct tpm_chip *chip)
int tpm2_get_timeouts(struct tpm_chip *chip);
int tpm2_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
struct tpm_digest *digest, u16 *digest_size_ptr);
+int tpm2_pcr_get_update_counter(struct tpm_chip *chip,
+ u32 pcr_idx, u16 alg_id,
+ u32 *update_counter);
tpm_pcr_read_update_cnt()
I can rename 'get' -> 'read'
About 'tpm2' -> 'tpm':
I already have tpm_pcr_get_update_counter() defined in patch 2.
I was following the existing pattern here in patch 1 and 2 i. e.
- Implementing the tpm1/tpm2 specific functionality in
drivers/char/tpm/tpm.h, drivers/char/tpm/tpm2-cmd.c,
drivers/char/tpm/tpm1-cmd.c.
- And combining that functionality in drivers/char/tpm/tpm-interface.c
and exposing it to other subsystems (like IMA) through
include/linux/tpm.h
(patch 2 of this series)
BTW, if I understand correctly, the update counter is not available in
TPM 1.2.
Please let me know if you want me to expose the functionality directly
from drivers/char/tpm/tpm2-cmd.c and getting rid of patch #2 of this series.
~Tushar
int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
struct tpm_digest *digests);
int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max);
diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index 93545be190a5..55f4e102289a 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -216,6 +216,54 @@ int tpm2_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
return rc;
}
+/**
+ * tpm2_pcr_get_update_counter() - gets an update counter value for a PCR bank
+ * @chip: TPM chip to use
+ * @pcr_idx: PCR index used to retrieve the update counter
+ * @alg_id: alg id used to retrieve the update counter
+ * @update_counter: output update counter value
+ *
+ * Return: Same as with tpm_transmit_cmd.
+ */
+int tpm2_pcr_get_update_counter(struct tpm_chip *chip,
+ u32 pcr_idx, u16 alg_id, u32 *update_counter)
+{
+ int rc;
+ struct tpm_buf buf;
+ struct tpm2_pcr_read_out *read_out;
+ u8 pcr_select[TPM2_PCR_SELECT_MIN] = {0};
+
+ if (pcr_idx >= TPM2_PLATFORM_PCR)
+ return -EINVAL;
+
+ if (!update_counter)
+ return -EINVAL;
+
+ rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_PCR_READ);
+ if (rc)
+ return rc;
+
+ pcr_select[pcr_idx >> 3] = 1 << (pcr_idx & 0x7);
+
+ tpm_buf_append_u32(&buf, 1);
+ tpm_buf_append_u16(&buf, alg_id);
+ tpm_buf_append_u8(&buf, TPM2_PCR_SELECT_MIN);
+ tpm_buf_append(&buf, (const unsigned char *)pcr_select,
+ sizeof(pcr_select));
+
+ rc = tpm_transmit_cmd(chip, &buf, 0, "attempting to read a pcr value");
+ if (rc)
+ goto out;
+
+ read_out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
+
+ *update_counter = be32_to_cpu(read_out->update_cnt);
+
+out:
+ tpm_buf_destroy(&buf);
+ return rc;
+}
+
struct tpm2_null_auth_area {
__be32 handle;
__be16 nonce_size;
--
2.25.1
BR, Jarkko
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