Re: [PATCH v24 04/25] IMA: avoid label collisions with stacked LSMs

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

Hi Casey,

On Tue, 2021-01-26 at 08:40 -0800, Casey Schaufler wrote:
> Integrity measurement may filter on security module information
> and needs to be clear in the case of multiple active security
> modules which applies. Provide a boot option ima_rules_lsm= to
> allow the user to specify an active securty module to apply
> filters to. If not specified, use the first registered module
> that supports the audit_rule_match() LSM hook. Allow the user
> to specify in the IMA policy an lsm= option to specify the
> security module to use for a particular rule.

Thanks, Casey.

(This patch description line length seems short.)

> 
> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> To: Mimi Zohar <zohar@linux.ibm.com>
> To: linux-integrity@vger.kernel.org
> ---
>  Documentation/ABI/testing/ima_policy |  8 +++-
>  security/integrity/ima/ima_policy.c  | 64 ++++++++++++++++++++++------
>  2 files changed, 57 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
> index e35263f97fc1..a7943d40466f 100644
> --- a/Documentation/ABI/testing/ima_policy
> +++ b/Documentation/ABI/testing/ima_policy
> @@ -25,7 +25,7 @@ Description:
>  			base:	[[func=] [mask=] [fsmagic=] [fsuuid=] [uid=]
>  				[euid=] [fowner=] [fsname=]]
>  			lsm:	[[subj_user=] [subj_role=] [subj_type=]
> -				 [obj_user=] [obj_role=] [obj_type=]]
> +				 [obj_user=] [obj_role=] [obj_type=] [lsm=]]

"[lsm=]" either requires all LSM rules types (e.g. {subj/obj}_user,
role, type) to be exactly the same for multiple LSMs or all of the LSM
rule types are applicable to only a single LSM.  Supporting multiple
LSMs with exactly the same LSM labels doesn't seem worth the effort.  
Keep it simple - a single rule, containing any LSM rule types, is
applicable to a single LSM.

>  			option:	[[appraise_type=]] [template=] [permit_directio]
>  				[appraise_flag=] [keyrings=]
>  		  base:
> @@ -114,6 +114,12 @@ Description:
> 
>  			measure subj_user=_ func=FILE_CHECK mask=MAY_READ
> 
> +		It is possible to explicitly specify which security
> +		module a rule applies to using lsm=.  If the security
> +		modules specified is not active on the system the rule
> +		will be rejected.  If lsm= is not specified the first
> +		security module registered on the system will be assumed.
> +
>  		Example of measure rules using alternate PCRs::
> 
>  			measure func=KEXEC_KERNEL_CHECK pcr=4
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 8002683003e6..de72b719c90c 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -82,6 +82,7 @@ struct ima_rule_entry {
>  		void *rules[LSMBLOB_ENTRIES]; /* LSM file metadata specific */
>  		char *args_p;	/* audit value */
>  		int type;	/* audit type */
> +		int which_lsm; /* which of the rules to use */
>  	} lsm[MAX_LSM_RULES];

Even if we wanted to support multiple LSMs within the same rule having
both "rules[LSMBLOB_ENTRIES]" and "which_lsm" shouldn't be necessary.  
The LSMBLOB_ENTRIES should already identify the LSM.

To support a single LSM per policy rule, "which_lsm" should be defined
outside of lsm[MAX_LSM_RULES].  This will simplify the rest of the code
(e.g. matching/freeing rules).

	int which_lsm;          /* which of the rules to use */
	struct {
                void *rule;        /* LSM file metadata specific */
                char *args_p;   /* audit value */
                int type;       /* audit type */
        } lsm[MAX_LSM_RULES];


>  	char *fsname;
>  	struct ima_rule_opt_list *keyrings; /* Measure keys added to these keyrings */
> @@ -90,17 +91,15 @@ struct ima_rule_entry {
> 
>  /**
>   * ima_lsm_isset - Is a rule set for any of the active security modules
> - * @rules: The set of IMA rules to check
> + * @entry: the rule entry to examine
> + * @lsm_rule: the specific rule type in question
>   *
> - * If a rule is set for any LSM return true, otherwise return false.
> + * If a rule is set return true, otherwise return false.
>   */
> -static inline bool ima_lsm_isset(void *rules[])
> +static inline bool ima_lsm_isset(struct ima_rule_entry *entry, int lsm_rule)
>  {
> -	int i;
> -
> -	for (i = 0; i < LSMBLOB_ENTRIES; i++)
> -		if (rules[i])
> -			return true;
> +	if (entry->lsm[lsm_rule].rules[entry->lsm[lsm_rule].which_lsm])
> +		return true;

If each IMA policy rule is limited to a specific LSM, then the test
would be "entry->which_lsm".

>  	return false;
>  }
> 
> @@ -273,6 +272,20 @@ static int __init default_appraise_policy_setup(char *str)
>  }
>  __setup("ima_appraise_tcb", default_appraise_policy_setup);
> 
> +static int ima_rule_lsm __ro_after_init;
> +
> +static int __init ima_rule_lsm_init(char *str)
> +{
> +	ima_rule_lsm = lsm_name_to_slot(str);
> +	if (ima_rule_lsm < 0) {
> +		ima_rule_lsm = 0;
> +		pr_err("rule lsm \"%s\" not registered", str);
> +	}
> +
> +	return 1;
> +}
> +__setup("ima_rule_lsm=", ima_rule_lsm_init);

The patch description refers to "ima_rules_lsm=".  Please update one or
the other.

thanks,

Mimi