kernel: prevent root process to exec su, which makes app can escape root profile by exec it twice

kernel/core_hook.c

@@ -60,6 +60,10 @@ void escape_to_root(void)
 
 	cred = (struct cred *)__task_cred(current);
 
+	if (cred->euid.val == 0) {
+		pr_warn("Already root, don't escape!\n");
+		return;
+	}
 	struct root_profile *profile = ksu_get_root_profile(cred->uid.val);
 
 	cred->uid.val = profile->uid;