[RFC][PATCH 4/8] ima: Add digest_cache_measure and digest_cache_appraise boot-time policies

[Mimi Zohar]

On Wed, 2024-02-14 at 15:35 +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu at huawei.com>
> 
> Specify the 'digest_cache_measure' boot-time policy with 'ima_policy=' in
> the kernel command line

The 'built-in' policies may be specified on the boot command line.  Please
update Subject line, to user the term "built-in" as well as here.

>  to add the following rule at the beginning of the
> IMA policy, before other rules:

Comments below...

> 
> measure func=DIGEST_LIST_CHECK pcr=12
> 
> which will measure digest lists into PCR 12 (or the value in
> CONFIG_IMA_DIGEST_CACHE_MEASURE_PCR_IDX).
> 
> 'digest_cache_measure' also adds 'digest_cache=content pcr=12' to the other
> measure rules, if they have a compatible IMA hook. The PCR value still
> comes from CONFIG_IMA_DIGEST_CACHE_MEASURE_PCR_IDX.
> 
> Specify 'digest_cache_appraise' to add the following rule at the beginning,
> before other rules:
> 
> appraise func=DIGEST_LIST_CHECK appraise_type=imasig|modsig
> 
> which will appraise digest lists with IMA signatures or module-style
> appended signatures.
> 
> 'digest_cache_appraise' also adds 'digest_cache=content' to the other
> appraise rules, if they have a compatible IMA hook.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> ---
>  .../admin-guide/kernel-parameters.txt         | 15 ++++++-
>  security/integrity/ima/Kconfig                | 10 +++++
>  security/integrity/ima/ima_policy.c           | 45 +++++++++++++++++++
>  3 files changed, 69 insertions(+), 1 deletion(-)

[...]
 
> @@ -971,6 +1006,16 @@ void __init ima_init_policy(void)
>  {
>  	int build_appraise_entries, arch_entries;
>  
> +	/*
> +	 * We need to load digest cache rules at the beginning, to avoid dont_
> +	 * rules causing ours to not be reached.
> +	 */

"lockdown" trusts IMA to measure and appraise kernel modules, if the rule
exists.  Placing the digest_cache first breaks this trust.

>From a trusted and secure boot perspective, the architecture specific policy
rules should not be ignored. Putting the digest_cache before any other rules
will limit others from being able to use digest_cache.

Instead of putting the digest_cache_{measure,appraise} built-in policies first,
skip loading the dont_measure_rules.


Mimi

> +	if (ima_digest_cache_measure)
> +		add_rules(&measure_digest_cache_rule, 1, IMA_DEFAULT_POLICY);
> +
> +	if (ima_digest_cache_appraise)
> +		add_rules(&appraise_digest_cache_rule, 1, IMA_DEFAULT_POLICY);
> +
>  	/* if !ima_policy, we load NO default rules */
>  	if (ima_policy)
>  		add_rules(dont_measure_rules, ARRAY_SIZE(dont_measure_rules),