Использование overflow: auto;
до .modal-full
.modal-dialog {
position: fixed;
margin: 0;
width: 100%;
height: 100%;
padding: 0;
}
.modal-full {
min-width: 100%;
overflow:auto;
}
.modal-full .modal-content {
min-height: 100vh;
}
<div class="modal-dialog modal-full">
<div class="modal-content">
<div class="modal-header">
<h4 class="modal-title" id="myModalLabel">Test</h4>
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
</div>
<div class="modal-body">
<h6>Text in a modal</h6>
<p>Duis mollis, est non commodo luctus, nisi erat porttitor ligula.</p>
<hr>
<h6>Overflowing text to show scroll behavior</h6>
<p>Cras mattis consectetur purus sit amet fermentum. Cras justo odio, dapibus ac facilisis in, egestas eget quam. Morbi leo risus, porta ac consectetur ac, vestibulum at eros.</p>
<p>Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Vivamus sagittis lacus vel augue laoreet rutrum faucibus dolor auctor.</p>
<p>Aenean lacinia bibendum nulla sed consectetur. Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Donec sed odio dui. Donec ullamcorper nulla non metus auctor fringilla.</p>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-light" data-dismiss="modal">Close</button>
<button type="button" class="btn btn-primary">Save changes</button>
</div>
</div><!-- /.modal-content -->
</div><!-- /.modal-dialog -->