landlock: Change landlock_restrict_self(2) check ordering

According to the Landlock goal to be a security feature available to
unprivileges processes, it makes more sense to first check for
no_new_privs before checking anything else (i.e. syscall arguments).

Merge inval_fd_enforce and unpriv_enforce_without_no_new_privs tests
into the new restrict_self_checks_ordering. This is similar to the
previous commit checking other syscalls.

Link: https://lore.kernel.org/r/[e-mail]
Cc: [e-mail]
Signed-off-by: Mickaël Salaün <[e-mail]>

security/landlock/syscalls.c

···
 	if (!landlock_initialized)
 		return -EOPNOTSUPP;
 
-	/* No flag for now. */
-	if (flags)
-		return -EINVAL;
-
 	/*
 	 * Similar checks as for seccomp(2), except that an -EPERM may be
 	 * returned.
···
 	if (!task_no_new_privs(current) &&
 	    !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN))
 		return -EPERM;
+
+	/* No flag for now. */
+	if (flags)
+		return -EINVAL;
 
 	/* Gets and checks the ruleset. */
 	ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ);

tools/testing/selftests/landlock/base_test.c

···
 	ASSERT_EQ(0, close(ruleset_fd));
 }
 
-TEST(inval_fd_enforce)
+/* Tests ordering of syscall argument and permission checks. */
+TEST(restrict_self_checks_ordering)
 {
+	const struct landlock_ruleset_attr ruleset_attr = {
+		.handled_access_fs = LANDLOCK_ACCESS_FS_EXECUTE,
+	};
+	struct landlock_path_beneath_attr path_beneath_attr = {
+		.allowed_access = LANDLOCK_ACCESS_FS_EXECUTE,
+		.parent_fd = -1,
+	};
+	const int ruleset_fd =
+		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+
+	ASSERT_LE(0, ruleset_fd);
+	path_beneath_attr.parent_fd =
+		open("/tmp", O_PATH | O_NOFOLLOW | O_DIRECTORY | O_CLOEXEC);
+	ASSERT_LE(0, path_beneath_attr.parent_fd);
+	ASSERT_EQ(0, landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
+				       &path_beneath_attr, 0));
+	ASSERT_EQ(0, close(path_beneath_attr.parent_fd));
+
+	/* Checks unprivileged enforcement without no_new_privs. */
+	drop_caps(_metadata);
+	ASSERT_EQ(-1, landlock_restrict_self(-1, -1));
+	ASSERT_EQ(EPERM, errno);
+	ASSERT_EQ(-1, landlock_restrict_self(-1, 0));
+	ASSERT_EQ(EPERM, errno);
+	ASSERT_EQ(-1, landlock_restrict_self(ruleset_fd, 0));
+	ASSERT_EQ(EPERM, errno);
+
 	ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
 
+	/* Checks invalid flags. */
+	ASSERT_EQ(-1, landlock_restrict_self(-1, -1));
+	ASSERT_EQ(EINVAL, errno);
+
+	/* Checks invalid ruleset FD. */
 	ASSERT_EQ(-1, landlock_restrict_self(-1, 0));
 	ASSERT_EQ(EBADF, errno);
-}
 
-TEST(unpriv_enforce_without_no_new_privs)
-{
-	int err;
-
-	drop_caps(_metadata);
-	err = landlock_restrict_self(-1, 0);
-	ASSERT_EQ(EPERM, errno);
-	ASSERT_EQ(err, -1);
+	/* Checks valid call. */
+	ASSERT_EQ(0, landlock_restrict_self(ruleset_fd, 0));
+	ASSERT_EQ(0, close(ruleset_fd));
 }
 
 TEST(ruleset_fd_io)