Re: [PATCH v7 6/6] docs: trusted-encrypted: add DCP as new trust source

[David Gstir <david@sigma-star.at>]

Jarkko,

> On 27.03.2024, at 16:40, Jarkko Sakkinen <jarkko@kernel.org> wrote:
> 
> On Wed Mar 27, 2024 at 10:24 AM EET, 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@nod.at>
>> Signed-off-by: Richard Weinberger <richard@nod.at>
>> Co-developed-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
>> Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
>> Signed-off-by: David Gstir <david@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 e989b9802f92..81fb3540bb20 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.
>> +
>> +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
>> +     * @blob_key: Random AES 128 key which is used to encrypt @payload,
>> +     *            @blob_key itself is encrypted with OTP or UNIQUE device key in
>> +     *            AES-128-ECB mode by DCP.
>> +     * @nonce: Random nonce used for @payload encryption.
>> +     * @payload_len: Length of the plain text @payload.
>> +     * @payload: The payload itself, encrypted using AES-128-GCM and @blob_key,
>> +     *           GCM auth tag of size AES_BLOCK_SIZE is attached at the end of it.
>> +     *
>> +     * The total size of a DCP BLOB is sizeof(struct dcp_blob_fmt) + @payload_len +
>> +     * AES_BLOCK_SIZE.
>> +     */
>> +    struct dcp_blob_fmt {
>> +            __u8 fmt_version;
>> +            __u8 blob_key[AES_KEYSIZE_128];
>> +            __u8 nonce[AES_KEYSIZE_128];
>> +            __le32 payload_len;
>> +            __u8 payload[];
>> +    } __packed;
> 
> I'm thinking here given that you need to replicate the same thing that
> is in the source files. E.g. Documentation/gpu/i915.rst.
> 
> The rationale would so many sources so maybe it would make sense to
> maintain this in the source code.
> 
> Also this documents how to generally insert documentation inline:
> https://docs.kernel.org/doc-guide/kernel-doc.html
> 
> I.e. I'm feeling that this is good time to improve scalability so that
> documentation will keep up to date. Also then backend specific patches
> mostly go to their subdirectories and not to Documentation/ subtree
> (or that would be more rare case).
> 
> So a good chance to do more than just a new backend for the benefit
> of the trusted keys subsystem :-)
> 
> Also, later on if something is changed e.g. in the above struct you
> don't have to do matching update to the documentation so it will save
> time too (over time).

sound good! I’ll maintain the blob format documentation to the source and insert 
a reference in the documentation. Thanks for pointing that out!

Is there anything else I can improve for this patchset? I’d like to include that in v8
too and make it the last iteration of this patchset.

Thanks,
David