If you ever tried to scroll a modal page from within Safari on iOS you probably noticed that what is getting scrolled is the parent page content and not the actual modal content.

There is a simple fix for that:

body .ui-dialog .ui-dialog-content{
    -webkit-overflow-scrolling: touch;
}

If you’re not using the built-in modal, the idea is to add the css property to the wrapping element of the iframe.
Let’s say you’re using Skillbuilders’ Modal Plugin, you can use the following:

#cboxLoadedContent{
    -webkit-overflow-scrolling: touch;
}

You can have a look at my Demo Application