doc: trusted-encrypted: describe new CAAM trust source

Update documentation for trusted key use with the Cryptographic
Acceleration and Assurance Module (CAAM), an IP on NXP SoCs.
Reviewed-by: Pankaj Gupta <pankaj.gupta@nxp.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Documentation/security/keys/trusted-encrypted.rst

@@ -35,6 +35,13 @@ safe.
          Rooted to Hardware Unique Key (HUK) which is generally burnt in on-chip
          fuses and is accessible to TEE only.
 
+     (3) CAAM (Cryptographic Acceleration and Assurance Module: IP on NXP SoCs)
+
+         When High Assurance Boot (HAB) is enabled and the CAAM is in secure
+         mode, trust is rooted to the OTPMK, a never-disclosed 256-bit key
+         randomly generated and fused into each SoC at manufacturing time.
+         Otherwise, a common fixed test key is used instead.
+
   *  Execution isolation
 
      (1) TPM
@@ -46,6 +53,10 @@ safe.
          Customizable set of operations running in isolated execution
          environment verified via Secure/Trusted boot process.
 
+     (3) CAAM
+
+         Fixed set of operations running in isolated execution environment.
+
   * Optional binding to platform integrity state
 
      (1) TPM
@@ -63,6 +74,11 @@ safe.
          Relies on Secure/Trusted boot process for platform integrity. It can
          be extended with TEE based measured boot process.
 
+     (3) CAAM
+
+         Relies on the High Assurance Boot (HAB) mechanism of NXP SoCs
+         for platform integrity.
+
   *  Interfaces and APIs
 
      (1) TPM
@@ -74,10 +90,13 @@ safe.
          TEEs have well-documented, standardized client interface and APIs. For
          more details refer to ``Documentation/staging/tee.rst``.
 
+     (3) CAAM
+
+         Interface is specific to silicon vendor.
 
   *  Threat model
 
-     The strength and appropriateness of a particular TPM or TEE for a given
+     The strength and appropriateness of a particular trust source for a given
      purpose must be assessed when using them to protect security-relevant data.
 
 
@@ -104,6 +123,12 @@ selected trust source:
      from platform specific hardware RNG or a software based Fortuna CSPRNG
      which can be seeded via multiple entropy sources.
 
+  *  CAAM: Kernel RNG
+
+     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.
+
 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.
 
@@ -193,6 +218,19 @@ Usage::
 specific to TEE device implementation.  The key length for new keys is always
 in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
 
+Trusted Keys usage: CAAM
+------------------------
+
+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 a
+CAAM-specific format.  The key length for new keys is always in bytes.
+Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
+
 Encrypted Keys usage
 --------------------