Just add:
.modal-content {
height:250px;
overflow:auto;
}
The height can of course be adapted to your personal needs.
Working Example
Update:
It’s actually pretty easy. Just add:
.modal-body {
max-height: calc(100vh - 212px);
overflow-y: auto;
}
to your css.
It uses vh and calc, which also happen to have a good browser support (IE9+).
This is based on this answer. Please read there for more detailed information, I won’t copy his answer.
Working Example