Here is the correct shortcode function
function section_shortcode_func( $atts, $content = null ){
$atts = shortcode_atts( array( 'bgimage' => '' ), $atts);
$bgimage = isset( $atts['bgimage'] ) ? $atts['bgimage'] : '';
ob_start();
echo "<div class="section" style="background-image: url($bgimage)">";
return ob_get_clean();
}
Now you can call it this way
[section bgimage="xxx.xxx/wp-content/uploads/2017/10/xxx.jpg"]