VirtualBox 5.1.8 installation didn't install kernel extensions. How do I fix this? [macOS 10.13]

1. When I installed VirtualBox it said that the installation "failed" but I still got VirtualBox.app in my applications folder.
2. When I try to boot windows 7 VM, it says "Kernel drivers not installed (rc = -1908).
3. Online solutions are saying I should "restart the host". I checked my StartupItems folder and it's empty.

So I guess the installer failed to install kernel extensions.

So there's this thing called SIP which prevents installing kernel extensions. Should I disable it for now?

What do I do? I need a Windows virtual machine in 8 hours...

High Sierra comes with a new security feature: Secure Kernel Extension Loading, which blocks kernel extension loading. The implementation appears to be questionable.

To allow the loading of some non-Apple kernel extension you have to either allow execution in Preferences > Security & Privacy > General:

or disable the security feature with a Terminal command (not recommended):

sudo spctl --master-disable

The solution worked for me on High Sierra. Trick was to do one install run that will definitely fail, but give you the prompt on the General Security tab to Allow Oracle Extensions. Click the Allow option. Then re-run the install again, should result in a successful install.