linux-kernel - Re: [PATCH v2] ima: move APPRAISE_BOOTPARAM dependency on ARCH

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Date:   Mon, 22 Jun 2020 15:28:13 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Bruno Meneguele <bmeneg@...hat.com>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     erichte@...ux.ibm.com, nayna@...ux.ibm.com, stable@...r.kernel.org
Subject: Re: [PATCH v2] ima: move APPRAISE_BOOTPARAM dependency on
 ARCH_POLICY to runtime

On Mon, 2020-06-22 at 14:27 -0300, Bruno Meneguele wrote:
> IMA_APPRAISE_BOOTPARAM has been marked as dependent on !IMA_ARCH_POLICY in
> compile time, enforcing the appraisal whenever the kernel had the arch
> policy option enabled.
> 
> However it breaks systems where the option is actually set but the system
> wasn't booted in a "secure boot" platform. In this scenario, anytime the
> an appraisal policy (i.e. ima_policy=appraisal_tcb) is used it will be
> forced, giving no chance to the user set the 'fix' state (ima_appraise=fix)
> to actually measure system's files.
> 
> This patch remove this compile time dependency and move it to a runtime
> decision, based on the arch policy loading failure/success.
> 
> Cc: stable@...r.kernel.org
> Fixes: d958083a8f64 ("x86/ima: define arch_get_ima_policy() for x86")
> Signed-off-by: Bruno Meneguele <bmeneg@...hat.com>
> ---
> changes from v1:
> 	- removed "ima:" prefix from pr_info() message
> 
>  security/integrity/ima/Kconfig      | 2 +-
>  security/integrity/ima/ima_policy.c | 8 ++++++--
>  2 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
> index edde88dbe576..62dc11a5af01 100644
> --- a/security/integrity/ima/Kconfig
> +++ b/security/integrity/ima/Kconfig
> @@ -232,7 +232,7 @@ config IMA_APPRAISE_REQUIRE_POLICY_SIGS
>  
>  config IMA_APPRAISE_BOOTPARAM
>  	bool "ima_appraise boot parameter"
> -	depends on IMA_APPRAISE && !IMA_ARCH_POLICY
> +	depends on IMA_APPRAISE
>  	default y
>  	help
>  	  This option enables the different "ima_appraise=" modes
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index e493063a3c34..c876617d4210 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -733,11 +733,15 @@ void __init ima_init_policy(void)
>  	 * (Highest priority)
>  	 */
>  	arch_entries = ima_init_arch_policy();
> -	if (!arch_entries)
> +	if (!arch_entries) {
>  		pr_info("No architecture policies found\n");
> -	else
> +	} else {
> +		/* Force appraisal, preventing runtime xattr changes */
> +		pr_info("setting IMA appraisal to enforced\n");
> +		ima_appraise = IMA_APPRAISE_ENFORCE;
>  		add_rules(arch_policy_entry, arch_entries,
>  			  IMA_DEFAULT_POLICY | IMA_CUSTOM_POLICY);
> +	}
>  
>  	/*
>  	 * Insert the builtin "secure_boot" policy rules requiring file

CONFIG_IMA_APPRAISE_BOOTPARAM controls the "ima_appraise" mode bits.  
The mode bits are or'ed with the MODULES, FIRMWARE, POLICY, and KEXEC
bits, which have already been set in ima_init_arch_policy().

>From ima.h:
/* Appraise integrity measurements */
#define IMA_APPRAISE_ENFORCE    0x01
#define IMA_APPRAISE_FIX        0x02
#define IMA_APPRAISE_LOG        0x04
#define IMA_APPRAISE_MODULES    0x08
#define IMA_APPRAISE_FIRMWARE   0x10
#define IMA_APPRAISE_POLICY     0x20
#define IMA_APPRAISE_KEXEC      0x40

As Nayna pointed out, only when an architecture specific "secure boot"
policy is loaded, is this applicable. 

Mimi