[PATCH v5 15/17] KEYS: trusted: Add session encryption protection to the seal/unseal path

James Bottomley
2023-11-27T19:20:04Z

If some entity is snooping the TPM bus, the can see the data going in
to be sealed and the data coming out as it is unsealed. Add parameter
and response encryption to these cases to ensure that no secrets are
leaked even if the bus is snooped.

As part of doing this conversion it was discovered that policy
sessions can't work with HMAC protected authority because of missing
pieces (the tpm Nonce). I've added code to work the same way as
before, which will result in potential authority exposure (while still
adding security for the command and the returned blob), and a fixme to
redo the API to get rid of this security hole.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

---

v2: fix unseal with policy and password
v3: fix session memory leak
---
security/keys/trusted-keys/trusted_tpm2.c | 88 ++++++++++++++++-------
1 file changed, 61 insertions(+), 27 deletions(-)

diff --git a/security/keys/trusted-keys/trusted_tpm2.c b/security/keys/trusted-keys/trusted_tpm2.c
index 97b1dfca2dba..dfeec06301ce 100644
--- a/security/keys/trusted-keys/trusted_tpm2.c
+++ b/security/keys/trusted-keys/trusted_tpm2.c
@@ -253,26 +253,26 @@ int tpm2_seal_trusted(struct tpm_chip *chip,
if (rc)
return rc;

+ rc = tpm2_start_auth_session(chip);
+ if (rc)
+ goto out_put;
+
rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_CREATE);
if (rc) {
- tpm_put_ops(chip);
- return rc;
+ tpm2_end_auth_session(chip);
+ goto out_put;
}

rc = tpm_buf_init_sized(&sized);
if (rc) {
tpm_buf_destroy(&buf);
- tpm_put_ops(chip);
- return rc;
+ tpm2_end_auth_session(chip);
+ goto out_put;
}

- tpm_buf_reset(&buf, TPM2_ST_SESSIONS, TPM2_CC_CREATE);
- tpm_buf_append_u32(&buf, options->keyhandle);
- tpm2_buf_append_auth(&buf, TPM2_RS_PW,
- NULL /* nonce */, 0,
- 0 /* session_attributes */,
- options->keyauth /* hmac */,
- TPM_DIGEST_SIZE);
+ tpm_buf_append_name(chip, &buf, options->keyhandle, NULL);
+ tpm_buf_append_hmac_session(chip, &buf, TPM2_SA_DECRYPT,
+ options->keyauth, TPM_DIGEST_SIZE);

/* sensitive */
tpm_buf_append_u16(&sized, options->blobauth_len);
@@ -314,10 +314,13 @@ int tpm2_seal_trusted(struct tpm_chip *chip,

if (buf.flags & TPM_BUF_OVERFLOW) {
rc = -E2BIG;
+ tpm2_end_auth_session(chip);
goto out;
}

+ tpm_buf_fill_hmac_session(chip, &buf);
rc = tpm_transmit_cmd(chip, &buf, 4, "sealing data");
+ rc = tpm_buf_check_hmac_response(chip, &buf, rc);
if (rc)
goto out;

@@ -348,6 +351,7 @@ int tpm2_seal_trusted(struct tpm_chip *chip,
else
payload->blob_len = blob_len;

+out_put:
tpm_put_ops(chip);
return rc;
}
@@ -417,25 +421,31 @@ static int tpm2_load_cmd(struct tpm_chip *chip,
if (blob_len > payload->blob_len)
return -E2BIG;

- rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_LOAD);
+ rc = tpm2_start_auth_session(chip);
if (rc)
return rc;

- tpm_buf_append_u32(&buf, options->keyhandle);
- tpm2_buf_append_auth(&buf, TPM2_RS_PW,
- NULL /* nonce */, 0,
- 0 /* session_attributes */,
- options->keyauth /* hmac */,
- TPM_DIGEST_SIZE);
+ rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_LOAD);
+ if (rc) {
+ tpm2_end_auth_session(chip);
+ return rc;
+ }
+
+ tpm_buf_append_name(chip, &buf, options->keyhandle, NULL);
+ tpm_buf_append_hmac_session(chip, &buf, 0, options->keyauth,
+ TPM_DIGEST_SIZE);

tpm_buf_append(&buf, blob, blob_len);

if (buf.flags & TPM_BUF_OVERFLOW) {
rc = -E2BIG;
+ tpm2_end_auth_session(chip);
goto out;
}

+ tpm_buf_fill_hmac_session(chip, &buf);
rc = tpm_transmit_cmd(chip, &buf, 4, "loading blob");
+ rc = tpm_buf_check_hmac_response(chip, &buf, rc);
if (!rc)
*blob_handle = be32_to_cpup(
(__be32 *) &buf.data[TPM_HEADER_SIZE]);
@@ -473,20 +483,44 @@ static int tpm2_unseal_cmd(struct tpm_chip *chip,
u8 *data;
int rc;

- rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_UNSEAL);
+ rc = tpm2_start_auth_session(chip);
if (rc)
return rc;

- tpm_buf_append_u32(&buf, blob_handle);
- tpm2_buf_append_auth(&buf,
- options->policyhandle ?
- options->policyhandle : TPM2_RS_PW,
- NULL /* nonce */, 0,
- TPM2_SA_CONTINUE_SESSION,
- options->blobauth /* hmac */,
- options->blobauth_len);
+ rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_UNSEAL);
+ if (rc) {
+ tpm2_end_auth_session(chip);
+ return rc;
+ }
+
+ tpm_buf_append_name(chip, &buf, blob_handle, NULL);
+
+ if (!options->policyhandle) {
+ tpm_buf_append_hmac_session(chip, &buf, TPM2_SA_ENCRYPT,
+ options->blobauth,
+ options->blobauth_len);
+ } else {
+ /*
+ * FIXME: The policy session was generated outside the
+ * kernel so we don't known the nonce and thus can't
+ * calculate a HMAC on it. Therefore, the user can
+ * only really use TPM2_PolicyPassword and we must
+ * send down the plain text password, which could be
+ * intercepted. We can still encrypt the returned
+ * key, but that's small comfort since the interposer
+ * could repeat our actions with the exfiltrated
+ * password.
+ */
+ tpm2_buf_append_auth(&buf, options->policyhandle,
+ NULL /* nonce */, 0, 0,
+ options->blobauth, options->blobauth_len);
+ tpm_buf_append_hmac_session_opt(chip, &buf, TPM2_SA_ENCRYPT,
+ NULL, 0);
+ }

+ tpm_buf_fill_hmac_session(chip, &buf);
rc = tpm_transmit_cmd(chip, &buf, 6, "unsealing");
+ rc = tpm_buf_check_hmac_response(chip, &buf, rc);
if (rc > 0)
rc = -EPERM;

--
2.35.3