Use a negative value for the fourth length which defines the spread distance. This is often overlooked, but supported by all major browsers. See this Fiddle for the result.
div {
background: red;
height: 100px;
width: 200px;
-moz-box-shadow: inset 0 -10px 10px -10px #000000;
-webkit-box-shadow: inset 0 -10px 10px -10px #000000;
box-shadow: inset 0 -10px 10px -10px #000000;
}<div></div>