Re: [Linux-ima-user] [RFC] i.MX6 CAAM blob generator for IMA/EVM initialization

On Wed, 2016-01-27 at 11:04 +0100, Steffen Trumtrar wrote:

Hi!

Mimi Zohar writes:

quoted

On Mon, 2015-11-09 at 16:18 +0100, Steffen Trumtrar wrote:

quoted

Hi!

The RFC Patch attached after this cover letter is mostly for illustration
purposes, so please don't waste too much time reviewing the code ;-)

For context I'll try to describe the problem that this patch tries to solve.

I need to be able to boot an EVM signed (and dongled) rootfs. The CAAM on
the i.MX6 has support for an OTP key and can en/decrypt data.
It also has a feature for generating red blobs: basically a chunk of data,
that is encrypted with the OTP key, which can be saved on some medium as a
secret to decrypt the EVM HMAC secret for one specific device.

To open the rootfs, the secret is handed from the bootloader to the kernel
as a base64 encoded string via the cmdline to an initramfs.
In the initramfs the sysfs file "modifier" is set to something starting with
"kernel:evm" and the base64 string is written to the sysfs file "blob".
The CAAM than decodes the red blob and, in case of "kernel:evm", initializes
the EVM or otherwise writes the result to "payload" if the modifier starts
with "user:". Therefore a blob that was generated for EVM never leaves the
kernel on decryption.
Generation of blobs goes like: echoing "modifier" to something and echoing
the payload to "payload". The red blob can than be read from "blob".


So, the sysfs interface is not the best option, I guess. The question is:
What is the right approach for a setup like this?
I need to:
  - be able to encrypt the secret and store it somewhere
  - to load the stored secret and decrypt it later
  - initialize IMA/EVM with the secret

Would something like
  - security/keys/encrypted-keys/encrypted.c
be the correct approach?

Instead of using the CAAM for OTP encrypting/decrypting, can it be used
to load the EVM key directly?  Dmitry's patches, which will be
upstreamed in 4.5

https://git.kernel.org/cgit/linux/kernel/git/zohar/linux-integrity.git/log/?h=for-next-4.5?   adds support for a crypto device to directly load the EVM key.

quoted

The patches look good and I use them for loading the EVM key from the
CAAM driver. But I still need the OTP decryption functionality.

The key data that I hand to evm_set_key must be device specific but I
don't want to use the fused OTP in the CAAM directly.
The OTP is used to protect multiple random keys. Therefore I need to
generate encrypted blobs that I can store on some unsecure memory
(EEPROM, NAND,...) and be able to hand that later back to the CAAM
module, to then get back an IMA/EVM, ecryptfs, $something key.

quoted

FYI, the EVM key is an encrypted key, which encrypts/decrypts either a
trusted or user type key.

So the normal approach would be to have a key in the kernel keyring
and decrypt it with the key loaded with evm_set_key?

Sorry, I should have said the encrypted key is encrypted/decrypted using
the trusted or user type key.

Can I somehow use the keyring framework as an abstraction around my
blobbing/deblobbing functionality?
So that the "keyring" calls into the crypto driver to decrypt the data
and uses the crypto driver to encrypt the keys when I want to "dump"
them?

Definitely.   It sounds like you want the equivalent functionality as
the TPM based trusted keys using OTP on the CAAM.

From Documentation/security/keys-trusted-encrypted.txt:

"Trusted Keys use a TPM both to generate and to seal the keys.  Keys are
sealed
under a 2048 bit RSA key in the TPM, and optionally sealed to specified
PCR
(integrity measurement) values, and only unsealed by the TPM, if PCRs
and blob
integrity verifications match."

Mimi