linux-kernel - Re: [RFC][PATCH 4/8] ima: Add digest_cache_measure and digest_cache

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Date: Thu, 07 Mar 2024 15:17:03 -0500
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Roberto Sassu <roberto.sassu@...weicloud.com>, corbet@....net,
        dmitry.kasatkin@...il.com, eric.snowberg@...cle.com,
        paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org,
        wufan@...ux.microsoft.com, pbrobinson@...il.com, zbyszek@...waw.pl,
        hch@....de, mjg59@...f.ucam.org, pmatilai@...hat.com, jannh@...gle.com,
        dhowells@...hat.com, jikos@...nel.org, mkoutny@...e.com,
        ppavlu@...e.com, petr.vorel@...il.com, petrtesarik@...weicloud.com,
        mzerqung@...inter.de, kgold@...ux.ibm.com,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [RFC][PATCH 4/8] ima: Add digest_cache_measure and
 digest_cache_appraise boot-time policies

On Wed, 2024-02-14 at 15:35 +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@...wei.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@...wei.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),