linux-kernel - Re: [PATCH v0 1/2] IMA: Defined queue functions

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Date:   Mon, 2 Dec 2019 10:39:25 -0800
From:   Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>, linux-integrity@...r.kernel.org
Cc:     eric.snowberg@...cle.com, dhowells@...hat.com,
        matthewgarrett@...gle.com, sashal@...nel.org,
        jamorris@...ux.microsoft.com, linux-kernel@...r.kernel.org,
        keyrings@...r.kernel.org, Janne Karhunen <janne.karhunen@...il.com>
Subject: Re: [PATCH v0 1/2] IMA: Defined queue functions

On 12/2/19 10:00 AM, Mimi Zohar wrote:

> 
> ima_update_policy() is called from multiple places.  Initially, it is
> called before a custom policy has been loaded.  The call to
> ima_process_queued_keys_for_measurement() needs to be moved to within
> the test, otherwise it runs the risk of dropping "key" measurements.

static const struct file_operations ima_measure_policy_ops = {
	.release = ima_release_policy,
};

ima_update_policy() is called from ima_release_policy() function.

On my test machine I have the IMA policy in /etc/ima/ima-policy file. 
When IMA policy is setup from this file, I see ima_release_policy() 
called (which in turn calls ima_update_policy()).

How can I have ima_update_policy() called before a custom policy is loaded?

If CONFIG_IMA_WRITE_POLICY is enabled, IMA policy can be updated at 
runtime - which would invoke ima_update_policy().

Is there any other way ima_update_policy() can get called?

> All the queued keys need to be processed at the same time.  Afterwards
> the queue should be deleted.  Unfortunately, the current queue locking
> assumes ima_process_queued_keys_for_measurement() is called multiple
> times.

When ima_process_queued_keys_for_measurement() is called the first time, 
the flag ima_process_keys_for_measurement is set to true and all queued 
keys are processed at that time.

The queue is not used after this point - In the IMA hook the key is 
processed immediately when ima_process_keys_for_measurement is set to true.

ima_process_queued_keys_for_measurement() can be called multiple times, 
but on 2nd and subsequent calls it will be a NOP.

> 
> Perhaps using the RCU method of walking lists would help.  I need to
> think about it some more.
> 
> Mimi

Please let me know how using RCU method would help.

thanks,
  -lakshmi