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Message-ID: <ae6dcadf-57d5-cb29-a361-d020f6333f59@schaufler-ca.com>
Date: Tue, 16 Feb 2021 07:26:30 -0800
From: Casey Schaufler <casey@...aufler-ca.com>
To: Mimi Zohar <zohar@...ux.ibm.com>, casey.schaufler@...el.com,
jmorris@...ei.org, linux-security-module@...r.kernel.org,
selinux@...r.kernel.org
Cc: linux-audit@...hat.com, keescook@...omium.org,
john.johansen@...onical.com, penguin-kernel@...ove.sakura.ne.jp,
paul@...l-moore.com, sds@...ho.nsa.gov,
linux-kernel@...r.kernel.org,
Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v24 04/25] IMA: avoid label collisions with stacked LSMs
On 2/14/2021 10:21 AM, Mimi Zohar wrote:
> 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@...aufler-ca.com>
>> To: Mimi Zohar <zohar@...ux.ibm.com>
>> To: linux-integrity@...r.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.
Thank you. I will add this.
>
>> 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];
You're right, that is better. I'll incorporate the change.
>
>
>> 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".
Which would be an improvement.
>
>> 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.
ima_rules_lsm seem to be more accurate. I'll fix it.
>
> thanks,
>
> Mimi
Thanks for the review and recommendations.
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