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security: keys: trusted: Make sealed key properly interoperable
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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@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
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James Bottomley authored and jarkkojs committed Apr 14, 2021
1 parent f221974 commit e5fb5d2
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Showing 2 changed files with 40 additions and 15 deletions.
2 changes: 2 additions & 0 deletions include/linux/tpm.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -305,6 +305,8 @@ 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),
};

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53 changes: 38 additions & 15 deletions security/keys/trusted-keys/trusted_tpm2.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -231,6 +231,7 @@ int tpm2_seal_trusted(struct tpm_chip *chip,
int blob_len = 0;
struct tpm_buf buf;
u32 hash;
u32 flags;
int i;
int rc;

Expand Down Expand Up @@ -265,31 +266,32 @@ 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);
Expand Down Expand Up @@ -364,8 +366,9 @@ 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) {
Expand All @@ -392,6 +395,16 @@ 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;
Expand Down Expand Up @@ -472,7 +485,7 @@ 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;
}
Expand All @@ -483,9 +496,19 @@ 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:
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