diff mbox series

[v10,5/8] security: keys: trusted: Make sealed key properly interoperable

Message ID 20200616160229.8018-6-James.Bottomley@HansenPartnership.com (mailing list archive)
State Rejected
Headers show
Series TPM 2.0 trusted keys with attached policy | expand

Commit Message

James Bottomley June 16, 2020, 4:02 p.m. UTC
The current implementation appends a migratable flag to the end of a
key, meaning the format isn't exactly interoperable because the using
party needs to know to strip this extra byte.  However, all other
consumers of TPM sealed blobs expect the unseal to return exactly the
key.  Since TPM2 keys have a key property flag that corresponds to
migratable, use that flag instead and make the actual key the only
sealed quantity.  This is secure because the key properties are bound
to a hash in the private part, so if they're altered the key won't
load.

Backwards compatibility is implemented by detecting whether we're
loading a new format key or not and correctly setting migratable from
the last byte of old format keys.

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

---

v2: added length checks to untrusted payload
v8: recover patch
---
 include/linux/tpm.h                       |  2 +
 security/keys/trusted-keys/trusted_tpm2.c | 54 ++++++++++++++++-------
 2 files changed, 40 insertions(+), 16 deletions(-)

Comments

Jarkko Sakkinen June 17, 2020, 11:46 p.m. UTC | #1
On Tue, Jun 16, 2020 at 09:02:26AM -0700, James Bottomley wrote:
> The current implementation appends a migratable flag to the end of a
> key, meaning the format isn't exactly interoperable because the using
> party needs to know to strip this extra byte.  However, all other
> consumers of TPM sealed blobs expect the unseal to return exactly the
> key.  Since TPM2 keys have a key property flag that corresponds to
> migratable, use that flag instead and make the actual key the only
> sealed quantity.  This is secure because the key properties are bound
> to a hash in the private part, so if they're altered the key won't
> load.
> 
> Backwards compatibility is implemented by detecting whether we're
> loading a new format key or not and correctly setting migratable from
> the last byte of old format keys.
> 
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

/Jarkko
diff mbox series

Patch

diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 03e9b184411b..cd46ab27baa5 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -297,6 +297,8 @@  struct tpm_buf {
 };
 
 enum tpm2_object_attributes {
+	TPM2_OA_FIXED_TPM		= BIT(1),
+	TPM2_OA_FIXED_PARENT		= BIT(4),
 	TPM2_OA_USER_WITH_AUTH		= BIT(6),
 };
 
diff --git a/security/keys/trusted-keys/trusted_tpm2.c b/security/keys/trusted-keys/trusted_tpm2.c
index 51d845da8728..905c5ca4d51c 100644
--- a/security/keys/trusted-keys/trusted_tpm2.c
+++ b/security/keys/trusted-keys/trusted_tpm2.c
@@ -231,6 +231,7 @@  int tpm2_seal_trusted(struct tpm_chip *chip,
 	unsigned int blob_len;
 	struct tpm_buf buf;
 	u32 hash;
+	u32 flags;
 	int i;
 	int rc;
 
@@ -259,32 +260,32 @@  int tpm2_seal_trusted(struct tpm_chip *chip,
 			     TPM_DIGEST_SIZE);
 
 	/* sensitive */
-	tpm_buf_append_u16(&buf, 4 + options->blobauth_len +
-			   payload->key_len + 1);
+	tpm_buf_append_u16(&buf, 4 + options->blobauth_len + payload->key_len);
 
 	tpm_buf_append_u16(&buf, options->blobauth_len);
 	if (options->blobauth_len)
 		tpm_buf_append(&buf, options->blobauth, options->blobauth_len);
 
-	tpm_buf_append_u16(&buf, payload->key_len + 1);
+	tpm_buf_append_u16(&buf, payload->key_len);
 	tpm_buf_append(&buf, payload->key, payload->key_len);
-	tpm_buf_append_u8(&buf, payload->migratable);
 
 	/* public */
 	tpm_buf_append_u16(&buf, 14 + options->policydigest_len);
 	tpm_buf_append_u16(&buf, TPM_ALG_KEYEDHASH);
 	tpm_buf_append_u16(&buf, hash);
 
+	/* key properties */
+	flags = 0;
+	flags |= options->policydigest_len ? 0 : TPM2_OA_USER_WITH_AUTH;
+	flags |= payload->migratable ? (TPM2_OA_FIXED_TPM |
+					TPM2_OA_FIXED_PARENT) : 0;
+	tpm_buf_append_u32(&buf, flags);
+
 	/* policy */
-	if (options->policydigest_len) {
-		tpm_buf_append_u32(&buf, 0);
-		tpm_buf_append_u16(&buf, options->policydigest_len);
+	tpm_buf_append_u16(&buf, options->policydigest_len);
+	if (options->policydigest_len)
 		tpm_buf_append(&buf, options->policydigest,
 			       options->policydigest_len);
-	} else {
-		tpm_buf_append_u32(&buf, TPM2_OA_USER_WITH_AUTH);
-		tpm_buf_append_u16(&buf, 0);
-	}
 
 	/* public parameters */
 	tpm_buf_append_u16(&buf, TPM_ALG_NULL);
@@ -357,8 +358,9 @@  static int tpm2_load_cmd(struct tpm_chip *chip,
 	unsigned int private_len;
 	unsigned int public_len;
 	unsigned int blob_len;
-	u8 *blob;
+	u8 *blob, *pub;
 	int rc;
+	u32 attrs;
 
 	rc = tpm2_key_decode(payload, options, &blob);
 	if (rc) {
@@ -385,6 +387,16 @@  static int tpm2_load_cmd(struct tpm_chip *chip,
 	if (private_len + 2 + public_len + 2 > payload->blob_len)
 		return -E2BIG;
 
+	pub = blob + 2 + private_len + 2;
+	/* key attributes are always at offset 4 */
+	attrs = get_unaligned_be32(pub + 4);
+
+	if ((attrs & (TPM2_OA_FIXED_TPM | TPM2_OA_FIXED_PARENT)) ==
+	    (TPM2_OA_FIXED_TPM | TPM2_OA_FIXED_PARENT))
+		payload->migratable = 0;
+	else
+		payload->migratable = 1;
+
 	blob_len = private_len + public_len + 4;
 	if (blob_len > payload->blob_len)
 		return -E2BIG;
@@ -465,7 +477,7 @@  static int tpm2_unseal_cmd(struct tpm_chip *chip,
 	if (!rc) {
 		data_len = be16_to_cpup(
 			(__be16 *) &buf.data[TPM_HEADER_SIZE + 4]);
-		if (data_len < MIN_KEY_SIZE ||  data_len > MAX_KEY_SIZE + 1) {
+		if (data_len < MIN_KEY_SIZE ||  data_len > MAX_KEY_SIZE) {
 			rc = -EFAULT;
 			goto out;
 		}
@@ -476,9 +488,19 @@  static int tpm2_unseal_cmd(struct tpm_chip *chip,
 		}
 		data = &buf.data[TPM_HEADER_SIZE + 6];
 
-		memcpy(payload->key, data, data_len - 1);
-		payload->key_len = data_len - 1;
-		payload->migratable = data[data_len - 1];
+		if (payload->old_format) {
+			/* migratable flag is at the end of the key */
+			memcpy(payload->key, data, data_len - 1);
+			payload->key_len = data_len - 1;
+			payload->migratable = data[data_len - 1];
+		} else {
+			/*
+			 * migratable flag already collected from key
+			 * attributes
+			 */
+			memcpy(payload->key, data, data_len);
+			payload->key_len = data_len;
+		}
 	}
 
 out: