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

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

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

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

Mimi