More Information
login_head handles authentication, registering, resetting passwords, forgot password,
and other user handling.

The login_head filter can be used to filter the logo image on the WordPress login page. By default this logo is of WordPress.

Note: this is not the only possible use of this filter. It can be used to add anything to the section on the login page.

Basic Examples
Where “wpdocs_custom_function_name” is the function to be called when the content is being retrieved.

add_filter( 'login_head', 'wpdocs_custom_function_name' );

In the below example the default logo is changed to custom logo, using CSS.

function wpdocs_my_custom_login_logo() {
    echo '<style type="text/css">
	h1 a { background-image: url(http://example.com/your-logo.png) !important; margin: 0 auto; }
	</style>';
}
add_filter( 'login_head', 'wpdocs_my_custom_login_logo' );

To validate the login details before user logins, you can use the below function:
(p.s. At first, you may have inserted some extra fields using ‘login_form’ action hook.)

function wpdocs_ref_access() {
    global $error;
    if ( empty( $_POST['custom_field_name'] ) ) {
        $error  = 'Restricted area, please login to continue.';
    }
}
add_action( 'login_head', 'wpdocs_ref_access' );