[PATCH 3/3] doc: trusted-encrypted: add DCP as new trust source

Richard Weinberger richard at nod.at
Mon Jun 14 20:16:20 UTC 2021

From: David Gstir <david at sigma-star.at>

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

Cc: Ahmad Fatoum <a.fatoum at pengutronix.de>
Cc: David Gstir <david at sigma-star.at>
Cc: David Howells <dhowells at redhat.com>
Cc: "David S. Miller" <davem at davemloft.net>
Cc: Fabio Estevam <festevam at gmail.com>
Cc: Herbert Xu <herbert at gondor.apana.org.au>
Cc: James Bottomley <jejb at linux.ibm.com>
Cc: James Morris <jmorris at namei.org>
Cc: Jarkko Sakkinen <jarkko at kernel.org>
Cc: Jonathan Corbet <corbet at lwn.net>
Cc: keyrings at vger.kernel.org
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-crypto at vger.kernel.org
Cc: linux-doc at vger.kernel.org
Cc: linux-integrity at vger.kernel.org
Cc: linux-kernel at vger.kernel.org
Cc: linux-security-module at vger.kernel.org
Cc: Mimi Zohar <zohar at linux.ibm.com>
Cc: NXP Linux Team <linux-imx at nxp.com>
Cc: Pengutronix Kernel Team <kernel at pengutronix.de>
Cc: Richard Weinberger <richard at nod.at>
Cc: Sascha Hauer <s.hauer at pengutronix.de>
Cc: "Serge E. Hallyn" <serge at hallyn.com>
Cc: Shawn Guo <shawnguo at kernel.org>
Co-developed-by: Richard Weinberger <richard at nod.at>
Signed-off-by: David Gstir <david at sigma-star.at>
 .../security/keys/trusted-encrypted.rst       | 84 ++++++++++++++++++-
 1 file changed, 83 insertions(+), 1 deletion(-)

diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst
index 80d5a5af62a1..e8413122e4bc 100644
--- a/Documentation/security/keys/trusted-encrypted.rst
+++ b/Documentation/security/keys/trusted-encrypted.rst
@@ -35,6 +35,11 @@ safe.
          Rooted to Hardware Unique Key (HUK) which is generally burnt in on-chip
          fuses and is accessible to TEE only.
+     (3) 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 accessbile to the DCP encryption engine only.
   *  Execution isolation
      (1) TPM
@@ -46,6 +51,12 @@ safe.
          Customizable set of operations running in isolated execution
          environment verified via Secure/Trusted boot process.
+     (3) 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
@@ -63,6 +74,11 @@ safe.
          Relies on Secure/Trusted boot process for platform integrity. It can
          be extended with TEE based measured boot process.
+     (3) DCP
+         Relies on Secure/Trusted boot process (called HAB by vendor) for
+         platform integrity.
   *  Interfaces and APIs
      (1) TPM
@@ -74,10 +90,14 @@ safe.
          TEEs have well-documented, standardized client interface and APIs. For
          more details refer to ``Documentation/staging/tee.rst``.
+     (3) 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 TPM or TEE for a given
+     The strength and appropriateness of a particular TPM, TEE or DCP for a given
      purpose must be assessed when using them to protect security-relevant data.
@@ -103,6 +123,14 @@ access control policy within the trust source.
      from platform specific hardware RNG or a software based Fortuna CSPRNG
      which can be seeded via multiple entropy sources.
+  * 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.
 Encrypted Keys
@@ -188,6 +216,19 @@ Usage::
 specific to TEE device implementation.  The key length for new keys is always
 in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
+Trusted Keys usage: DCP
+    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
@@ -370,3 +411,44 @@ 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 tusted 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 +
+     */
+    struct dcp_blob_fmt {
+            __u8 fmt_version;
+            __u8 blob_key[AES_KEYSIZE_128];
+            __u8 nonce[AES_KEYSIZE_128];
+            __le32 payload_len;
+            __u8 payload[0];
+    } __packed;

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