Running Ubuntu 17.10 as a VirtualBox guest works fine except for accessing files located on the host.
Only vboxguest and vboxvideo modules are available and loaded.
vboxsf module is missing.
Load the Guest Additions ISO image and let run the installer.
On a fresh new install, you may have this error :
Make sure to install the build-essential package :
apt install build-essential
One completed, reboot your system to make sure the right modules are loaded : vboxguest, vboxvideo, vboxsf
Once logged in, access to a directory located on your host may be denied.
Be sure that your user account is part of the vboxsf local group.
Add yourself :
usermod -aG vboxsf your_login
Log out and log in again. This should be good now