[PATCH 3/3] doc: trusted-encrypted: add DCP as new trust source
Ahmad Fatoum
a.fatoum at pengutronix.de
Wed Jul 14 09:32:12 UTC 2021
Hello Richard,
Hello David,
On 14.06.21 22:16, Richard Weinberger wrote:
> 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.
s/accessbile/accessible/ . In the code you differentiate between UNIQUE and OTP.
Here you use OTP to mean both. Perhaps explicitly mention this?
> +
> * 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
> +-----------------------
> +
> +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
> --------------------
>
> @@ -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
s/tusted/trusted/
> +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[0];
[] ?
> + } __packed;
>
Cheers,
Ahmad
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