Re: [PATCH v8 6/6] docs: trusted-encrypted: add DCP as new trust source

From: Bagas Sanjaya
Date: Wed Apr 03 2024 - 09:27:04 EST


On Wed, Apr 03, 2024 at 09:21:22AM +0200, David Gstir wrote:
> 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.
> *

The doc LGTM, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx>

--
An old man doll... just what I always wanted! - Clara

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