docs: 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 <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>
Co-developed-by: David Oberhollenzer <[e-mail]>
Signed-off-by: David Oberhollenzer <[e-mail]>
Signed-off-by: David Gstir <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>

Documentation/security/keys/trusted-encrypted.rst

···
          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
···
      (3) CAAM
 
          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
 
···
          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
···
      (3) CAAM
 
          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 normal kernel random number generator is used. To seed it from the
      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.
···
 "keyctl print" returns an ASCII hex copy of the sealed key, which is in a
 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
 --------------------
···
 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

security/keys/trusted-keys/trusted_dcp.c

···
 #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.
  *
  * @fmt_version: Format version, currently being %1.