Monitor locked to low-res when using EFI/Secure Boot on Coffee Lake

[jimav]

Booting with EFI, as opposed to legacy BIOS booting, somehow prevents detecting the resolutions supported by a VESA-compliant monitor; instead a single low resolution mode is used, and no other choices are available in the "Displays" setting dialog.

At least that's what happens using built-in Intel graphics on a i7-8700K / Z370 (Coffee Lake), with a 1920x1200 DVI monitor.

This effectively prevents enabling Secure Boot with Ubuntu on this platform.

I don't know if older hardware has the same problem (this is the first UEFI-capable system I've owned)

The problem is visible when booting from the installer DVD in "try Ubuntu" mode (the install DVDs are set up to boot either way). If you boot in EFI mode, the desktop comes up at 1024x768. But if you boot in legacy BIOS mode, it comes up normally (1920x1200 in my case). Ditto for installed systems.

STEPS TO REPRODUCE:

1. Go out and buy a new Coffee Lake system with ASUS Z370-A PRIME motherboard and i7-8700K cpu
2. Enable Secure Boot in the "bios" (disable legacy booting)
3. Boot the 17.10 or 16.04 Ubuntu Desktop DVD in "try Ubuntu" mode

RESULTS: Resolution abnormally low, and can not be changed

4. Turn off Secure Boot and enable legacy or "Compatibility" support (CSM in ASUS-speak).
5. Boot the same Desktop installer DVD

RESULTS: Normal resolution

Installing in legacy BIOS mode is a work-around, but preventing use of Secure Boot is a security problem (the system is potentially vulnerable to bootstrap malware)