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

Mimi Zohar
2021-02-14T18:23:05Z

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