[RFC] IMA LSM based rule race condition issue on 4.19 LTS

Guozihua (Scott)
2022-12-09T07:04:19Z

Hi community.

Previously our team reported a race condition in IMA relates to LSM
based rules which would case IMA to match files that should be filtered
out under normal condition. The issue was originally analyzed and fixed
on mainstream. The patch and the discussion could be found here:
https://lore.kernel.org/all/20220921125804.59490-1-guozihua@huawei.com/

After that, we did a regression test on 4.19 LTS and the same issue
arises. Further analysis reveled that the issue is from a completely
different cause.

The cause is that selinux_audit_rule_init() would set the rule (which is
a second level pointer) to NULL immediately after called. The relevant
codes are as shown:

security/selinux/ss/services.c:
> int selinux_audit_rule_init(u32 field, u32 op, char *rulestr, void **vrule)
> {
> struct selinux_state *state = &selinux_state;
> struct policydb *policydb = &state->ss->policydb;
> struct selinux_audit_rule *tmprule;
> struct role_datum *roledatum;
> struct type_datum *typedatum;
> struct user_datum *userdatum;
> struct selinux_audit_rule **rule = (struct selinux_audit_rule **)vrule;
> int rc = 0;
>
> *rule = NULL;
*rule is set to NULL here, which means the rule on IMA side is also NULL.
>
> if (!state->initialized)
> return -EOPNOTSUPP;
...
> out:
> read_unlock(&state->ss->policy_rwlock);
>
> if (rc) {
> selinux_audit_rule_free(tmprule);
> tmprule = NULL;
> }
>
> *rule = tmprule;
rule is updated at the end of the function.
>
> return rc;
> }

security/integrity/ima/ima_policy.c:
> static bool ima_match_rules(struct ima_rule_entry *rule, struct inode *inode,
> const struct cred *cred, u32 secid,
> enum ima_hooks func, int mask)
> {...
> for (i = 0; i < MAX_LSM_RULES; i++) {
> int rc = 0;
> u32 osid;
> int retried = 0;
>
> if (!rule->lsm[i].rule)
> continue;
Setting rule to NULL would lead to LSM based rule matching being skipped.
> retry:
> switch (i) {

To solve this issue, there are multiple approaches we might take and I
would like some input from the community.

The first proposed solution would be to change
selinux_audit_rule_init(). Remove the set to NULL bit and update the
rule pointer with cmpxchg.

> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index a9f2bc8443bd..aa74b04ccaf7 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -3297,10 +3297,9 @@ int selinux_audit_rule_init(u32 field, u32 op, char *rulestr, void **vrule)
> struct type_datum *typedatum;
> struct user_datum *userdatum;
> struct selinux_audit_rule **rule = (struct selinux_audit_rule **)vrule;
> + struct selinux_audit_rule *orig = rule;
> int rc = 0;
>
> - *rule = NULL;
> -
> if (!state->initialized)
> return -EOPNOTSUPP;
>
> @@ -3382,7 +3381,8 @@ int selinux_audit_rule_init(u32 field, u32 op, char *rulestr, void **vrule)
> tmprule = NULL;
> }
>
> - *rule = tmprule;
> + if (cmpxchg(rule, orig, tmprule) != orig)
> + selinux_audit_rule_free(tmprule);
>
> return rc;
> }

This solution would be an easy fix, but might influence other modules
calling selinux_audit_rule_init() directly or indirectly (on 4.19 LTS,
only auditfilter and IMA it seems). And it might be worth returning an
error code such as -EAGAIN.

Or, we can access rules via RCU, similar to what we do on 5.10. This
could means more code change and testing.

Reported-by: Huaxin Lu <luhuaxin1@huawei.com>
--
Best
GUO Zihua