IMA: policy can be updated zero times

Commit "IMA: policy can now be updated multiple times" assumed that the
policy would be updated at least once.

If there are zero updates, the temporary list head object will get added
to the policy list, and later dereferenced as an IMA policy object, which
means that invalid memory will be accessed.

Changelog:
- Move list_empty() test to ima_release_policy(), before audit msg - Mimi

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/integrity/ima/ima.h

@@ -166,6 +166,7 @@ void ima_update_policy(void);
 void ima_update_policy_flag(void);
 ssize_t ima_parse_add_rule(char *);
 void ima_delete_rules(void);
+int ima_check_policy(void);
 void *ima_policy_start(struct seq_file *m, loff_t *pos);
 void *ima_policy_next(struct seq_file *m, void *v, loff_t *pos);
 void ima_policy_stop(struct seq_file *m, void *v);

security/integrity/ima/ima_fs.c

@@ -355,6 +355,11 @@ static int ima_release_policy(struct inode *inode, struct file *file)
 	if ((file->f_flags & O_ACCMODE) == O_RDONLY)
 		return 0;
 
+	if (valid_policy && ima_check_policy() < 0) {
+		cause = "failed";
+		valid_policy = 0;
+	}
+
 	pr_info("IMA: policy update %s\n", cause);
 	integrity_audit_msg(AUDIT_INTEGRITY_STATUS, NULL, NULL,
 			    "policy_update", cause, !valid_policy, 0);

security/integrity/ima/ima_policy.c

@@ -416,6 +416,14 @@ void __init ima_init_policy(void)
 	ima_rules = &ima_default_rules;
 }
 
+/* Make sure we have a valid policy, at least containing some rules. */
+int ima_check_policy()
+{
+	if (list_empty(&ima_temp_rules))
+		return -EINVAL;
+	return 0;
+}
+
 /**
  * ima_update_policy - update default_rules with new measure rules
  *