I doubt CD-ROMs would be used within Whonix and the CD-ROM kernel driver has had some vulnerabilities before such as CVE-2018-11506.
How to exploit, how to make the kernel load the module?
Irrelevant in a VM without cd-rom drive?
A VM without cd-rom drive won’t have the module. Won’t need the config therefore?
and a VM with cd-rom drive should work out of the box because the drive was manually added?
I just tested it and live mode works fine with squashfs blacklisted.
It can easily be removed for things that need CD-ROMs. I imagine most would be using USBs instead anyway.
No, it’s loaded by default whether you add it or not. Run lsmod and you’ll see the cdrom and sr_mod modules already loaded.
lsmod | grep cd
Shows nothing for me in a Qubes VM.
How could an attacker make the kernel load such a module?
Similar as above:
No module load -> no exploitation attempt using that module possible.
Or are there other ways a non-root user may cause loading such a module?
Live mode does not use squashfs, but the host/iso does which will fail if the module is not loaded.
Instead of blacklisting you could also skip compiling them into the custom kernel. I guess the cloud kernel from the debian repo won’t have most of those and other stuff. But I also could not get it to work.
Why just stick to non-root?
Because root access means game over if malware gained root. The attacker has way too many ways to compromise the system in persistent ways. Root should be allowed to load the module / mount any disk.
Modprobing stuff would be one of the options for a kernel exploit. Certainly not the only one but at least one options less for the attacker.
Edit: We also have apparmor which limits what a user, including root, can do.
Larger discussion. Created Untrusted Root - does it make sense to try to improve security by restricting root? for it.
It’s there for me in Virtualbox. Virtualbox might add CD-ROM support by default while Qubes only adds the bare minimum. If so, is there a way to disable it in the Virtualbox ovas by default?
An attacker could attempt to mount something containing the filesystem which will load the module.
Things like thunar-volman and udisks allow unprivileged mounting.
Root can just remove the file that blacklists these modules anyway.
Ubuntu’s default blacklisted modules: https://git.launchpad.net/ubuntu/+source/kmod/tree/debian/modprobe.d?h=ubuntu/disco
As per https://security.stackexchange.com/questions/119712/methods-root-can-use-to-elevate-itself-to-kernel-mode, we should disable CPU MSRs.
MSRs are only exposed when the msr module is loaded so we can blacklist that module to prevent them from being abused.
Does this break virt-detect?
Good point but even if it does, that is only required by two features.
Non-essential features. [1] could be implemented in another way.
Kicksecure has both, VM and host packages:
Whonix doesn’t have that yet and it’s difficult to make the conversion for existing users. (Since this is the “top level” meta package.)
[2] might be lost but I guess that is a feature reduction we can endure.
No. MSRs have nothing to do with that.
I’m pretty sure they do because when i blocked access to them using libvirt it broke hypervisor detection, but @Patrick explained it’s not a big deal.
No, you can even test it yourself by running systemd-detect-virt and seeing the msr module isn’t even loaded.
Good to know. Then go for it 
Merged.