linux-kernel - Re: [PATCH v0 2/2] IMA: Call queue functions to measure keys

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Date:   Mon, 02 Dec 2019 19:02:33 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Lakshmi Ramasubramanian <nramas@...ux.microsoft.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
Subject: Re: [PATCH v0 2/2] IMA: Call queue functions to measure keys

Hi Lakshmi,

On Tue, 2019-11-26 at 18:52 -0800, Lakshmi Ramasubramanian wrote:
> Keys should be queued for measurement if custom IMA policies have
> not yet been applied. Keys queued for measurement, if any, need to be
> processed when custom IMA policies have been applied.

Please start with the problem description.  For example, measuring
keys requires loading a custom IMA policy.

> 
> This patch adds the call to ima_queue_key_for_measurement() in
> the IMA hook function if ima_process_keys_for_measurement flag is set
> to false. And, the call to ima_process_queued_keys_for_measurement()
> when custom IMA policies have been applied in ima_update_policy().

This reads like pseudo code.  Please summarize the purpose of this
patch.

> 
> NOTE:
> If the kernel is built with CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE
> enabled then the IMA policy should be applied as custom IMA policies.
> 
> Keys will be queued up until custom policies are applied and processed
> when custom policies have been applied.
> 
> Signed-off-by: Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
> ---
>  security/integrity/ima/ima_asymmetric_keys.c | 16 ++++++++++++++++
>  security/integrity/ima/ima_policy.c          | 12 ++++++++++++
>  2 files changed, 28 insertions(+)
> 
> diff --git a/security/integrity/ima/ima_asymmetric_keys.c b/security/integrity/ima/ima_asymmetric_keys.c
> index 10deb77b22a0..adb7a307190f 100644
> --- a/security/integrity/ima/ima_asymmetric_keys.c
> +++ b/security/integrity/ima/ima_asymmetric_keys.c
> @@ -157,6 +157,8 @@ void ima_post_key_create_or_update(struct key *keyring, struct key *key,
>  				   const void *payload, size_t payload_len,
>  				   unsigned long flags, bool create)
>  {
> +	bool key_queued = false;
> +
>  	/* Only asymmetric keys are handled by this hook. */
>  	if (key->type != &key_type_asymmetric)
>  		return;
> @@ -164,6 +166,20 @@ void ima_post_key_create_or_update(struct key *keyring, struct key *key,
>  	if (!payload || (payload_len == 0))
>  		return;
>  
> +	if (!ima_process_keys_for_measurement)
> +		key_queued = ima_queue_key_for_measurement(keyring,
> +							   payload,
> +							   payload_len);
> +
> +	/*
> +	 * Need to check again if the key was queued or not because
> +	 * ima_process_keys_for_measurement could have flipped from
> +	 * false to true after it was checked above, but before the key
> +	 * could be queued by ima_queue_key_for_measurement().
> +	 */

You're describing a race condition.

> +	if (key_queued)
> +		return;
> +
>  	/*
>  	 * keyring->description points to the name of the keyring
>  	 * (such as ".builtin_trusted_keys", ".ima", etc.) to
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 78b25f083fe1..a2e30a90f97d 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -812,6 +812,18 @@ void ima_update_policy(void)
>  		kfree(arch_policy_entry);
>  	}
>  	ima_update_policy_flag();
> +
> +	/*
> +	 * Custom IMA policies have been setup.
> +	 * Process key(s) queued up for measurement now.
> +	 *
> +	 * NOTE:
> +	 *   Custom IMA policies always overwrite builtin policies
> +	 *   (policies compiled in code). If one wants measurement
> +	 *   of asymmetric keys then it has to be configured in
> +	 *   custom policies and updated here.
> +	 */

The "NOTE" is over commenting the code and belongs in the patch
description.

> +	ima_process_queued_keys_for_measurement();

Overwriting the initial policy is highly recommended, but not everyone
defines a custom policy.  Should there be a time limit or some other
criteria before deleting the key measurement queue?

Mimi

>  }
>  
>  /* Keep the enumeration in sync with the policy_tokens! */