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 <richard@xxxxxx>
Signed-off-by: Richard Weinberger <richard@xxxxxx>
Co-developed-by: David Oberhollenzer <david.oberhollenzer@xxxxxxxxxxxxx>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@xxxxxxxxxxxxx>
Signed-off-by: David Gstir <david@xxxxxxxxxxxxx>
---
.../security/keys/trusted-encrypted.rst | 53 +++++++++++++++++++
security/keys/trusted-keys/trusted_dcp.c | 19 +++++++
2 files changed, 72 insertions(+)
diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst
index e989b9802f92..f4d7e162d5e4 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
+-----------------------
+
+Usage::
+
+ 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 +470,12 @@ 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
+---------------
+
+.. kernel-doc:: security/keys/trusted-keys/trusted_dcp.c
+ :doc: dcp blob format
+
+.. kernel-doc:: security/keys/trusted-keys/trusted_dcp.c
+ :identifiers: struct dcp_blob_fmt
diff --git a/security/keys/trusted-keys/trusted_dcp.c b/security/keys/trusted-keys/trusted_dcp.c
index 16c44aafeab3..b5f81a05be36 100644
--- a/security/keys/trusted-keys/trusted_dcp.c
+++ b/security/keys/trusted-keys/trusted_dcp.c
@@ -19,6 +19,25 @@
#define DCP_BLOB_VERSION 1
#define DCP_BLOB_AUTHLEN 16
+/**
+ * DOC: 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.
*
--
2.35.3
