Re: [PATCH v4 01/10] IMA: Defined an IMA hook to measure keys on key create or update

On Thu, 2019-11-07 at 10:42 -0800, Lakshmi Ramasubramanian wrote:

On 11/6/2019 7:40 PM, Mimi Zohar wrote:

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I would move the patch that defines the "keyring=" policy option prior
to this one.  Include the call to process_buffer_measurement() in this
patch.  A subsequent patch would add support to defer measuring the
key, by calling a function named something like
ima_queue_key_measurement().

As I'd stated in the other response, I wanted to isolate all key related
code in a separate C file and build it if and only if all CONFIG
dependencies are met.

The basic measuring of keys shouldn't be any different than any other
policy rule, other than it is a key and not a file.  This is the
reason that I keep saying start out with the basics and then add
support to defer measuring keys on the trusted keyrings.

I'll make the changes, rearrange the patches and send an updated set.

I do have a few questions since I am still not fully understanding the 
requirements you are targeting. Appreciate if you could please clarify.

As you already know, I am using the "public key" of the given asymmetric 
key as the "buffer" to measure in process_buffer_measurement().

The measurement decision is not based on whether the keyring is a 
trusted one or an untrusted one. As long as the IMA policy allows 
(through the "keyrings=" option) the key will be measured.

We should be able to measure all keys being loaded onto any keyring or
onto a specific "keyring=".   This shouldn't be any different than any
other policy rule.  Once you have this basic feature working, you
would address loading keys during early boot.

Do you want only trusted keyrings to be allowed in the measurement?
In my opinion, that decision should be deferred to whoever is setting up 
the IMA policy.

Right, but it shouldn't be limited to just "trusted" keyrings.  This
way you can first test loading keys onto any keyring.

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Only the queueing code needed for measuring keys on the trusted
keyrings would be in a separate file.

The decision to process key immediately or defer (queue) is based on 
whether IMA has been initialized or not. Keyring is not used for this 
decision.

Could you please clarify how queuing is related to keyring's 
trustworthiness?

The check for whether the key is an asymmetric one or not, and 
extracting the "public key" if it is an asymmetric key needs to be in a 
separate file to handle the CONFIG dependencies in IMA.

Queuing the keys should be independent of measuring the keys.
 Initially you would start with just measuring the key.  From a high
level it would look like:

    ima_post_key_create_or_update(...)
    {
       "measure key based on
    policy(key, keyring, ...)"
    }

This requires the IMA "keyring=" policy option support be defined
first.

Subsequently you would add key queuing support, and then update
ima_post_key_create_or_update().  It would look like:

        ima_post_key_create_or_update(...)
        {
            if (custom policy is loaded)
               "measure key based on policy(key, keyring, ...)"
            else
                "queue key(key, keyring)"
        }

Mimi