That exists: look for 'trusted path'. It was a feature of compartmented mode workstation (CMW) operating systems like Trusted Solaris and lives on in the requirement to use Ctrl-Alt-Del to call up the Windows login prompt. In Trusted Solaris (TSOL) it was a dedicated area of the screen—along the bottom—where no user mode process was allowed to write; the OS displayed a special symbol there (sort of like the padlock in a web browser) when the user was interacting directly with the OS. Some CMW systems even implemented that functionality in hardware, electronically compositing windows from different physical frame buffers to the video display. Ctrl-Alt-Del is actually in hardware, too (or it used to be); the keyboard interface on the first IBM PC detected that specific key combination and toggled the reset line on the CPU (or maybe it was an interrupt; I forget). Every subsequent PC-compatible machine, to this day, has the same functionality built in to the hardware, on the A20 reset line. It's mostly vestigial today.
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