How to Fix KVM Error for VirtualBox on Ubuntu 24.04

Recently, after installing an Ubuntu 24.04 Desktop build in VirtualBox running on an Ubuntu 22.04 Desktop host, I encountered an error when trying to run the 24.04 Desktop virtual machine (vm) after the build:

VirtualBox can't operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot (VERR_VMX_IN_VMX_ROOT_MODE).

A quick search for the error message led me to this post on VirtualBox's forum: https://forums.virtualbox.org/viewtopic.php?f=1&t=15720

I didn't dig too deeply into why this happens or how the fix clears it, but the down and dirty version is, at the command line on your Linux host, type:

lsmod | grep kvm

This will tell you the names of the kvm modules. In my case, it looked like this:

tedl@tedl-Gazelle:~$ lsmod | grep kvm
kvm_intel             487424  2
kvm                  1409024  1 kvm_intel
irqbypass              12288  1 kvm

You can disable the kvm kernel extensions from running in root mode by typing:

sudo rmmod [kvm_name]

Replace [kvm_name] with the name obtained with lsmod. In my case, it was:

sudo rmmod kvm_intel
sudo rmmod kvm

I did not have to rebuild my kernel. I restarted VirtualBox and the vm started fine.

If you've run into this error, I hope you find this post helpful and you're able to quickly get your VirtualBox vm's back online!