Re: [PATCH v6 7/8] IMA: add a built-in policy rule for critical data measurement

[Mimi Zohar <zohar@linux.ibm.com>]

Hi Lakshmi,

On Thu, 2020-11-19 at 15:26 -0800, Tushar Sugandhi wrote:
> From: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
> 
> The IMA hook to measure kernel critical data, namely
> ima_measure_critical_data(), could be called before a custom IMA policy
> is loaded.
> Define a new critical data builtin policy to allow measuring
> early kernel integrity critical data before a custom IMA policy is
> loaded.

Everything needing to be said seems to be included in the second
sentence.  Does the first sentence add anything?  "Define a new
critical data builtin policy" makes for a good Subject line.

> 
> Add critical data to built-in IMA rules if the kernel command line
> contains "ima_policy=critical_data".

The boot command line parameters are defined in Documentation/admin-
guide/kernel-parameters.txt.  Please update "ima_policy".

> 
> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
> ---
>  security/integrity/ima/ima_policy.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index c9e52dab0638..119604a3efa0 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -206,6 +206,10 @@ static struct ima_rule_entry secure_boot_rules[] __ro_after_init = {
>  	 .flags = IMA_FUNC | IMA_DIGSIG_REQUIRED},
>  };
> 
> +static struct ima_rule_entry critical_data_rules[] __ro_after_init = {
> +	{.action = MEASURE, .func = CRITICAL_DATA, .flags = IMA_FUNC},
> +};
> +
>  /* An array of architecture specific rules */
>  static struct ima_rule_entry *arch_policy_entry __ro_after_init;
>  
> @@ -228,6 +232,7 @@ __setup("ima_tcb", default_measure_policy_setup);
>  
>  static bool ima_use_appraise_tcb __initdata;
>  static bool ima_use_secure_boot __initdata;
> +static bool ima_use_critical_data __ro_after_init;

Unlike ima_fail_unverifiable_sigs, ima_use_critical_data is only used
during __init.  Please change "__ro_after_init" to "__initdata".  (The
critical data policy itself is defined properly as __ro_after_init.)

>  static bool ima_fail_unverifiable_sigs __ro_after_init;
>  static int __init policy_setup(char *str)
>  {
> @@ -242,6 +247,8 @@ static int __init policy_setup(char *str)
>  			ima_use_appraise_tcb = true;
>  		else if (strcmp(p, "secure_boot") == 0)
>  			ima_use_secure_boot = true;
> +		else if (strcmp(p, "critical_data") == 0)
> +			ima_use_critical_data = true;
>  		else if (strcmp(p, "fail_securely") == 0)
>  			ima_fail_unverifiable_sigs = true;
>  		else
> @@ -875,6 +882,11 @@ void __init ima_init_policy(void)
>  			  ARRAY_SIZE(default_appraise_rules),
>  			  IMA_DEFAULT_POLICY);
>  
> +	if (ima_use_critical_data)
> +		add_rules(critical_data_rules,
> +			  ARRAY_SIZE(critical_data_rules),
> +			  IMA_DEFAULT_POLICY);
> +
>  	ima_update_policy_flag();
>  }
>