2023-09-18 16:56:51

by David Gstir

[permalink] [raw]
Subject: [PATCH v3 3/3] doc: trusted-encrypted: add DCP as new trust source

Update the documentation for trusted and encrypted KEYS with DCP as new
trust source:

- Describe security properties of DCP trust source
- Describe key usage
- Document blob format

Co-developed-by: Richard Weinberger <[email protected]>
Signed-off-by: Richard Weinberger <[email protected]>
Co-developed-by: David Oberhollenzer <[email protected]>
Signed-off-by: David Oberhollenzer <[email protected]>
Signed-off-by: David Gstir <[email protected]>
.../security/keys/trusted-encrypted.rst | 85 +++++++++++++++++++
1 file changed, 85 insertions(+)

diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst
index 9bc9db8ec651..4452070afbe9 100644
--- a/Documentation/security/keys/trusted-encrypted.rst
+++ b/Documentation/security/keys/trusted-encrypted.rst
@@ -42,6 +42,14 @@ safe.
randomly generated and fused into each SoC at manufacturing time.
Otherwise, a common fixed test key is used instead.

+ (4) DCP (Data Co-Processor: crypto accelerator of various i.MX SoCs)
+ Rooted to a one-time programmable key (OTP) that is generally burnt
+ in the on-chip fuses and is accessible to the DCP encryption engine only.
+ DCP provides two keys that can be used as root of trust: the OTP key
+ and the UNIQUE key. Default is to use the UNIQUE key, but selecting
+ the OTP key can be done via a module parameter (dcp_use_otp_key).
* Execution isolation

(1) TPM
@@ -57,6 +65,12 @@ safe.

Fixed set of operations running in isolated execution environment.

+ (4) DCP
+ Fixed set of cryptographic operations running in isolated execution
+ environment. Only basic blob key encryption is executed there.
+ The actual key sealing/unsealing is done on main processor/kernel space.
* Optional binding to platform integrity state

(1) TPM
@@ -79,6 +93,11 @@ safe.
Relies on the High Assurance Boot (HAB) mechanism of NXP SoCs
for platform integrity.

+ (4) DCP
+ Relies on Secure/Trusted boot process (called HAB by vendor) for
+ platform integrity.
* Interfaces and APIs

(1) TPM
@@ -94,6 +113,11 @@ safe.

Interface is specific to silicon vendor.

+ (4) DCP
+ Vendor-specific API that is implemented as part of the DCP crypto driver in
+ ``drivers/crypto/mxs-dcp.c``.
* Threat model

The strength and appropriateness of a particular trust source for a given
@@ -129,6 +153,13 @@ selected trust source:
CAAM HWRNG, enable CRYPTO_DEV_FSL_CAAM_RNG_API and ensure the device
is probed.

+ * DCP (Data Co-Processor: crypto accelerator of various i.MX SoCs)
+ The DCP hardware device itself does not provide a dedicated RNG interface,
+ so the kernel default RNG is used. SoCs with DCP like the i.MX6ULL do have
+ a dedicated hardware RNG that is independent from DCP which can be enabled
+ to back the kernel RNG.
Users may override this by specifying ``trusted.rng=kernel`` on the kernel
command-line to override the used RNG with the kernel's random number pool.

@@ -231,6 +262,19 @@ Usage::
CAAM-specific format. The key length for new keys is always in bytes.
Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).

+Trusted Keys usage: DCP
+ keyctl add trusted name "new keylen" ring
+ keyctl add trusted name "load hex_blob" ring
+ keyctl print keyid
+"keyctl print" returns an ASCII hex copy of the sealed key, which is in format
+specific to this DCP key-blob implementation. The key length for new keys is
+always in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
Encrypted Keys usage

@@ -426,3 +470,44 @@ string length.
privkey is the binary representation of TPM2B_PUBLIC excluding the
initial TPM2B header which can be reconstructed from the ASN.1 octed
string length.
+DCP Blob Format
+The Data Co-Processor (DCP) provides hardware-bound AES keys using its
+AES encryption engine only. It does not provide direct key sealing/unsealing.
+To make DCP hardware encryption keys usable as trust source, we define
+our own custom format that uses a hardware-bound key to secure the sealing
+key stored in the key blob.
+Whenever a new trusted key using DCP is generated, we generate a random 128-bit
+blob encryption key (BEK) and 128-bit nonce. The BEK and nonce are used to
+encrypt the trusted key payload using AES-128-GCM.
+The BEK itself is encrypted using the hardware-bound key using the DCP's AES
+encryption engine with AES-128-ECB. The encrypted BEK, generated nonce,
+BEK-encrypted payload and authentication tag make up the blob format together
+with a version number, payload length and authentication tag::
+ /*
+ * struct dcp_blob_fmt - DCP BLOB format.
+ *
+ * @fmt_version: Format version, currently being %1
+ * @blob_key: Random AES 128 key which is used to encrypt @payload,
+ * @blob_key itself is encrypted with OTP or UNIQUE device key in
+ * AES-128-ECB mode by DCP.
+ * @nonce: Random nonce used for @payload encryption.
+ * @payload_len: Length of the plain text @payload.
+ * @payload: The payload itself, encrypted using AES-128-GCM and @blob_key,
+ * GCM auth tag of size AES_BLOCK_SIZE is attached at the end of it.
+ *
+ * The total size of a DCP BLOB is sizeof(struct dcp_blob_fmt) + @payload_len +
+ */
+ struct dcp_blob_fmt {
+ __u8 fmt_version;
+ __u8 blob_key[AES_KEYSIZE_128];
+ __u8 nonce[AES_KEYSIZE_128];
+ __le32 payload_len;
+ __u8 payload[];
+ } __packed;