This comment isn't really correct all, it's not possible to do that with the architecture of X. The best you can get is XSecurity and its related extensions, anything more than that is going to break the protocol. There is quite a good reason that Qubes has not even tried to get X to do this. The main issues are that the protocol is sequential and you can't reorder requests which makes modern polkit-style mechanisms practically infeasible, and also you can't really fail anything because errors are usually treated as fatal in Xlib and most clients don't bother with error handling in Xlib because it is complicated and badly implemented. More about that here: https://news.ycombinator.com/item?id=21267275
BTW none of the issues occur in D-Bus, that was designed to fit these use cases which is why it typically gets used for "privileged" APIs on Linux. Fixing this issue in X11 would probably entail throwing out the protocol entirely and making something new that works more like D-Bus. Maybe you could avoid that by hacking Xlib and making another X server with an elaborate set of heuristics but that won't work for everything and also seems like a really bad way to do security. At best you probably rewrite the whole X server to end up with something roughly equivalent to XWayland.
BTW none of the issues occur in D-Bus, that was designed to fit these use cases which is why it typically gets used for "privileged" APIs on Linux. Fixing this issue in X11 would probably entail throwing out the protocol entirely and making something new that works more like D-Bus. Maybe you could avoid that by hacking Xlib and making another X server with an elaborate set of heuristics but that won't work for everything and also seems like a really bad way to do security. At best you probably rewrite the whole X server to end up with something roughly equivalent to XWayland.