linux-kernel - Re: [PATCH v5 1/4] IMA: Add func to measure LSM state and policy

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Message-ID: <c2f0c4cc-67a9-d467-1b2c-7edaea47c9d6@linux.microsoft.com>
Date:   Thu, 30 Jul 2020 08:15:34 -0700
From:   Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
To:     Tyler Hicks <tyhicks@...ux.microsoft.com>
Cc:     zohar@...ux.ibm.com, stephen.smalley.work@...il.com,
        casey@...aufler-ca.com, sashal@...nel.org, jmorris@...ei.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/4] IMA: Add func to measure LSM state and policy

On 7/30/20 8:02 AM, Tyler Hicks wrote:

>> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
>> index 07f033634b27..a0f5c39d9084 100644
>> --- a/security/integrity/ima/ima_policy.c
>> +++ b/security/integrity/ima/ima_policy.c
>> @@ -442,13 +442,20 @@ static bool ima_match_rules(struct ima_rule_entry *rule, struct inode *inode,
>>   {
>>   	int i;
>>   
>> -	if (func == KEY_CHECK) {
>> -		return (rule->flags & IMA_FUNC) && (rule->func == func) &&
>> -		       ima_match_keyring(rule, keyring, cred);
>> -	}
>>   	if ((rule->flags & IMA_FUNC) &&
>>   	    (rule->func != func && func != POST_SETATTR))
>>   		return false;
>> +
>> +	switch (func) {
>> +	case KEY_CHECK:
>> +		return ima_match_keyring(rule, keyring, cred);
>> +	case LSM_STATE:
>> +	case LSM_POLICY:
>> +		return true;
>> +	default:
>> +		break;
>> +	}
>> +
>>   	if ((rule->flags & IMA_MASK) &&
>>   	    (rule->mask != mask && func != POST_SETATTR))
>>   		return false;
>> @@ -1044,6 +1051,18 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
>>   		if (ima_rule_contains_lsm_cond(entry))
>>   			return false;
>>   
>> +		break;
>> +	case LSM_STATE:
>> +	case LSM_POLICY:
>> +		if (entry->action & ~(MEASURE | DONT_MEASURE))
>> +			return false;
>> +
>> +		if (entry->flags & ~(IMA_FUNC | IMA_PCR))
>> +			return false;
>> +
>> +		if (ima_rule_contains_lsm_cond(entry))
>> +			return false;
>> +
>>   		break;
>>   	default:
>>   		return false;
>> @@ -1176,6 +1195,10 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
>>   				entry->func = KEXEC_CMDLINE;
>>   			else if (strcmp(args[0].from, "KEY_CHECK") == 0)
>>   				entry->func = KEY_CHECK;
>> +			else if (strcmp(args[0].from, "LSM_STATE") == 0)
>> +				entry->func = LSM_STATE;
>> +			else if (strcmp(args[0].from, "LSM_POLICY") == 0)
>> +				entry->func = LSM_POLICY;
> 
> This patch generally looks really good to me with the exception of one
> thing...
> 
> We should only accept rules with these specified hook functions when an
> LSM that has measurement support is enabled. This messes up the ordering
> of your patch series but it could be as simple as doing this:
> 
> 			else if (IS_ENABLED(CONFIG_SECURITY_SELINUX) &&
> 				 strcmp(args[0].from, "LSM_STATE") == 0)
> 				 entry->func = LSM_STATE;
> 
> Or you could do something a little more complex, like what's done with
> CONFIG_IMA_LSM_RULES. You could create a CONFIG_IMA_MEASURE_LSM option
> that's default enabled but depends on CONFIG_SECURITY_SELINUX and then
> check for IS_ENABLED(CONFIG_IMA_MEASURE_LSM) in ima_parse_rule().
> 
> I'd personally opt for just placing the
> IS_ENABLED(CONFIG_SECURITY_SELINUX) check directly into
> ima_parse_rule().
> 

The LSM hook can be used by any security module (not just SELinux) to 
measure their data.

I have implemented measurement in SELinux to illustrate the usage. 
Maybe, I can add the check you have suggested for now and when more 
security modules start using this IMA policy additional checks can be 
added as appropriate.

thanks,
  -lakshmi