[PATCH v3 0/3] DCP as trusted keys backend

David Gstir david at sigma-star.at
Tue Sep 19 00:18:22 AEST 2023

This is a revival of the previous patch set submitted by Richard Weinberger:

v2 is here:

v2 -> v3:
- Addressed review comments from Jarkko Sakkinen
v1 -> v2:
- Revive and rebase to latest version
- Include review comments from Ahmad Fatoum

The Data CoProcessor (DCP) is an IP core built into many NXP SoCs such
as i.mx6ull.

Similar to the CAAM engine used in more powerful SoCs, DCP can AES-
encrypt/decrypt user data using a unique, never-disclosed,
device-specific key. Unlike CAAM though, it cannot directly wrap and
unwrap blobs in hardware. As DCP offers only the bare minimum feature
set and a blob mechanism needs aid from software. A blob in this case
is a piece of sensitive data (e.g. a key) that is encrypted and
authenticated using the device-specific key so that unwrapping can only
be done on the hardware where the blob was wrapped.

This patch series adds a DCP based, trusted-key backend and is similar
in spirit to the one by Ahmad Fatoum [0] that does the same for CAAM.
It is of interest for similar use cases as the CAAM patch set, but for
lower end devices, where CAAM is not available.

Because constructing and parsing the blob has to happen in software,
we needed to decide on a blob format and chose the following:

struct dcp_blob_fmt {
	__u8 fmt_version;
	__u8 blob_key[AES_KEYSIZE_128];
	__u8 nonce[AES_KEYSIZE_128];
	__le32 payload_len;
	__u8 payload[];
} __packed;

The `fmt_version` is currently 1.

The encrypted key is stored in the payload area. It is AES-128-GCM
encrypted using `blob_key` and `nonce`, GCM auth tag is attached at
the end of the payload (`payload_len` does not include the size of
the auth tag).

The `blob_key` itself is encrypted in AES-128-ECB mode by DCP using
the OTP or UNIQUE device key. A new `blob_key` and `nonce` are generated
randomly, when sealing/exporting the DCP blob.

This patchset was tested with dm-crypt on an i.MX6ULL board.

[0] https://lore.kernel.org/keyrings/20220513145705.2080323-1-a.fatoum@pengutronix.de/

David Gstir (3):
  crypto: mxs-dcp: Add support for hardware provided keys
  KEYS: trusted: Introduce support for NXP DCP-based trusted keys
  doc: trusted-encrypted: add DCP as new trust source

 .../admin-guide/kernel-parameters.txt         |  13 +
 .../security/keys/trusted-encrypted.rst       |  85 +++++
 MAINTAINERS                                   |   9 +
 drivers/crypto/mxs-dcp.c                      | 104 +++++-
 include/keys/trusted_dcp.h                    |  11 +
 include/soc/fsl/dcp.h                         |  17 +
 security/keys/trusted-keys/Kconfig            |   9 +-
 security/keys/trusted-keys/Makefile           |   2 +
 security/keys/trusted-keys/trusted_core.c     |   6 +-
 security/keys/trusted-keys/trusted_dcp.c      | 311 ++++++++++++++++++
 10 files changed, 554 insertions(+), 13 deletions(-)
 create mode 100644 include/keys/trusted_dcp.h
 create mode 100644 include/soc/fsl/dcp.h
 create mode 100644 security/keys/trusted-keys/trusted_dcp.c


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