[PATCH v7 6/6] docs: trusted-encrypted: add DCP as new trust source
Jarkko Sakkinen
jarkko at kernel.org
Thu Mar 28 18:47:07 UTC 2024
On Thu Mar 28, 2024 at 10:05 AM EET, David Gstir wrote:
> Jarkko,
>
> > On 27.03.2024, at 16:40, Jarkko Sakkinen <jarkko at 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 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 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.
Yeah, I don't enforce you to convert all the existing work to this
model, but we could use this as a reference for that work.
The patch set is overally in a pretty good shape I think :-)
BR, Jarkko
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