[PATCH v5 13/17] tpm: add hmac checks to tpm2_pcr_extend()

James Bottomley
2023-11-27T19:18:24Z

tpm2_pcr_extend() is used by trusted keys to extend a PCR to prevent a
key from being re-loaded until the next reboot. To use this
functionality securely, that extend must be protected by a session
hmac. This patch adds HMAC protection so tampering with the
tpm2_pcr_extend() command in flight is detected.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
---
drivers/char/tpm/tpm2-cmd.c | 27 ++++++++++-----------------
1 file changed, 10 insertions(+), 17 deletions(-)

diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index b0e72fb563d9..a53a843294ed 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -216,13 +216,6 @@ int tpm2_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
return rc;
}

-struct tpm2_null_auth_area {
- __be32 handle;
- __be16 nonce_size;
- u8 attributes;
- __be16 auth_size;
-} __packed;
-
/**
* tpm2_pcr_extend() - extend a PCR value
*
@@ -236,24 +229,22 @@ int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
struct tpm_digest *digests)
{
struct tpm_buf buf;
- struct tpm2_null_auth_area auth_area;
int rc;
int i;

- rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_PCR_EXTEND);
+ rc = tpm2_start_auth_session(chip);
if (rc)
return rc;

- tpm_buf_append_u32(&buf, pcr_idx);
+ rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_PCR_EXTEND);
+ if (rc) {
+ tpm2_end_auth_session(chip);
+ return rc;
+ }

- auth_area.handle = cpu_to_be32(TPM2_RS_PW);
- auth_area.nonce_size = 0;
- auth_area.attributes = 0;
- auth_area.auth_size = 0;
+ tpm_buf_append_name(chip, &buf, pcr_idx, NULL);
+ tpm_buf_append_hmac_session(chip, &buf, 0, NULL, 0);

- tpm_buf_append_u32(&buf, sizeof(struct tpm2_null_auth_area));
- tpm_buf_append(&buf, (const unsigned char *)&auth_area,
- sizeof(auth_area));
tpm_buf_append_u32(&buf, chip->nr_allocated_banks);

for (i = 0; i < chip->nr_allocated_banks; i++) {
@@ -262,7 +253,9 @@ int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
chip->allocated_banks[i].digest_size);
}

+ tpm_buf_fill_hmac_session(chip, &buf);
rc = tpm_transmit_cmd(chip, &buf, 0, "attempting extend a PCR value");
+ rc = tpm_buf_check_hmac_response(chip, &buf, rc);

tpm_buf_destroy(&buf);

--
2.35.3