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

[Jarkko Sakkinen]

On Mon Sep 18, 2023 at 5:18 PM EEST, David Gstir wrote:
> 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 at nod.at>
> Signed-off-by: Richard Weinberger <richard at nod.at>
> Co-developed-by: David Oberhollenzer <david.oberhollenzer at sigma-star.at>
> Signed-off-by: David Oberhollenzer <david.oberhollenzer at sigma-star.at>
> Signed-off-by: David Gstir <david at sigma-star.at>
> ---
>  .../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
> +-----------------------
> +
> +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 +470,44 @@ 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.

"When a new trusted key using DCP is created, a random 128-bit
blob encryption key (BEK) and 128-bit nonce are generated."

... or along the lines.

BR, Jarkko