Jarkko,
> On 27.03.2024, at 16:40, Jarkko Sakkinen <jarkko@xxxxxxxxxx> 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@xxxxxx>
>> Signed-off-by: Richard Weinberger <richard@xxxxxx>
>> Co-developed-by: David Oberhollenzer <david.oberhollenzer@xxxxxxxxxxxxx>
>> Signed-off-by: David Oberhollenzer <david.oberhollenzer@xxxxxxxxxxxxx>
>> Signed-off-by: David Gstir <david@xxxxxxxxxxxxx>
>> ---
>> .../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
