On Sat, 2019-12-07 at 21:11 -0800, 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@xxxxxxxxxxxxxxxxxxxxx>
> ---
> include/keys/trusted-type.h | 1 +
> include/linux/tpm.h | 2 ++
> security/keys/trusted-keys/trusted_tpm2.c | 57 ++++++++++++++++++++++---------
> 3 files changed, 44 insertions(+), 16 deletions(-)
>
> diff --git a/include/keys/trusted-type.h b/include/keys/trusted-type.h
> index a94c03a61d8f..4728e13aada8 100644
> --- a/include/keys/trusted-type.h
> +++ b/include/keys/trusted-type.h
> @@ -22,6 +22,7 @@ struct trusted_key_payload {
> unsigned int key_len;
> unsigned int blob_len;
> unsigned char migratable;
> + unsigned char old_format;
> unsigned char key[MAX_KEY_SIZE + 1];
> unsigned char blob[MAX_BLOB_SIZE];
> };
> 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 4efc7b64d1cd..a34ab6f90f76 100644
> --- a/security/keys/trusted-keys/trusted_tpm2.c
> +++ b/security/keys/trusted-keys/trusted_tpm2.c
> @@ -207,6 +207,7 @@ int tpm2_seal_trusted(struct tpm_chip *chip,
> unsigned int blob_len;
> struct tpm_buf buf;
> u32 hash;
> + u32 flags;
> int i;
> int rc;
>
> @@ -235,29 +236,30 @@ int tpm2_seal_trusted(struct tpm_chip *chip,
> TPM_DIGEST_SIZE);
>
> /* sensitive */
> - tpm_buf_append_u16(&buf, 4 + TPM_DIGEST_SIZE + payload->key_len + 1);
> + tpm_buf_append_u16(&buf, 4 + TPM_DIGEST_SIZE + payload->key_len);
>
> tpm_buf_append_u16(&buf, TPM_DIGEST_SIZE);
> tpm_buf_append(&buf, options->blobauth, TPM_DIGEST_SIZE);
> - 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);
> @@ -330,13 +332,16 @@ 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)
> + if (rc) {
> /* old form */
> blob = payload->blob;
> + payload->old_format = 1;
> + }
>
> /* new format carries keyhandle but old format doesn't */
> if (!options->keyhandle)
> @@ -347,6 +352,16 @@ static int tpm2_load_cmd(struct tpm_chip *chip,
> return -E2BIG;
>
> public_len = be16_to_cpup((__be16 *) &blob[2 + private_len]);
> +
> + pub = blob + 2 + private_len + 2;
> + /* key attributes are always at offset 4 */
> + attrs = get_unaligned_be32(pub + 4);
At this point I don't believe you've checked yet that payload->blob_len
is sufficient to know that these bytes exist.
I think you're reading 'private_len' from non-existent bytes too, if
payload->blob_len is zero or one? Which I think was there before you
started, but you touched it last...
> + 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;
> @@ -427,7 +442,7 @@ 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;
> }
> @@ -438,9 +453,19 @@ 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:
Attachment:
smime.p7s
Description: S/MIME cryptographic signature
