linux-kernel - Re: One question about trusted key of keyring in Linux kernel.

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Message-Id: <1573659978.17949.83.camel@linux.ibm.com>
Date:   Wed, 13 Nov 2019 10:46:18 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     "Zhao, Shirley" <shirley.zhao@...el.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>
Cc:     "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "'Mauro Carvalho Chehab'" <mchehab+samsung@...nel.org>
Subject: Re: One question about trusted key of keyring in Linux kernel.

On Wed, 2019-11-13 at 01:22 +0000, Zhao, Shirley wrote:
> Hi, all,
> 
> This is Shirley from Intel. I have one question about trusted key of
> keyring in kernel. Please help.
> 
> According the to description in https://github.com/torvalds/linux/bl
> ob/master/Documentation/security/keys/trusted-encrypted.rst.
> Trusted key will be saved in TPM with PCR policy protected.

"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, ..."

Trusted keys are not TPM keys.  They are not stored in the TPM.

> 
> Then, I running the following command to create a trusted key.
> keyctl add trusted test_trusted "new 32 keyhandle=0x81000001" @u
> 
> I also tried the following command, it can add one trusted key, too.
> keyctl add trusted test_trusted "new 32 keyhandle=0x81000001
> pcrinfo=`cat pcr7.blob`" @u
> 
> But after reboot, this key will be removed.
> I need to re-added during boot.

Right, they need to be re-loaded on boot.  Refer to the dracut
module /modules.d/97masterkey for loading a trusted key during boot.

> 
> Then the question is since this key is saved in TPM, how to get it
> back from TPM?

Trusted keys are not stored in the TPM.  Refer to the ima-evm-utils
README for examples of creating a trusted key (kmk) and an encrypted
key (evm-key).

> 
> From the document, I need to use "keyctl pipe" to save the key into
> a blob, then load it.
> But the blob contend key text, and this is a file on hard disk, it
> is not safe to protect the key.
> 
> So what can TPM do here?

The hex ascii encoded trusted key is sealed under the TPM SRK.

Mimi