Jarkko, > On 27.03.2024, at 16:40, Jarkko Sakkinen <jar...@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 <rich...@nod.at> >> Signed-off-by: Richard Weinberger <rich...@nod.at> >> Co-developed-by: David Oberhollenzer <david.oberhollen...@sigma-star.at> >> Signed-off-by: David Oberhollenzer <david.oberhollen...@sigma-star.at> >> Signed-off-by: David Gstir <da...@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