Can't upgrade extension pack as normal user from GUI on Ubuntu 18.04 (due to missing gksu?)

[Nate Eldredge]

On Ubuntu 18.04, it is no longer possible to upgrade the extension pack from the VirtualBox manager GUI when running as an unprivileged user. This used to work under Ubuntu 17.10 and earlier versions (you were prompted for a password to gain root privileges).

After accepting the license, I get the following error message:

Failed to install the Extension Pack /home/nate/.config/VirtualBox/Oracle_VM_VirtualBox_Extension_Pack-5.2.14.vbox-extpack.

The installer failed with exit code 127: Error creating textual authentication agent: Error opening current controlling terminal for the process (`/dev/tty'): No such device or address.

Result Code: NS_ERROR_FAILURE (0x80004005)
Component: ExtPackManagerWrap
Interface: IExtPackManager {edba9d10-45d8-b440-1712-46ac0c9bc4c5}

I think the root cause of the error is that Ubuntu 18.04 no longer includes gksu (see ​Ubuntu bug 1740618). Looking at the source to VBoxExtPackHelperApp, it tries to execute gksu to elevate privileges. If that doesn't work it tries to run pkexec, which here doesn't work either, I guess because it doesn't have a tty to talk to. The "Error creating textual authentication agent" message is apparently from pkexec.

A workaround is to install the extension pack manually with VBoxManage, or to run the VirtualBox GUI as root under sudo (but this is a bit annoying for me, because root has a different ~/.config/VirtualBox which points to a bunch of stale VMs and produces more warnings). However, from the symptoms above, it would be hard for a new user to know to try this.

VirtualBox 5.2.14 installed from ​http://download.virtualbox.org/virtualbox/debian. The package file is virtualbox-5.2_5.2.14-123301~Ubuntu~bionic_amd64.deb.

(Not sure which component applies to the GUI manager, or if this is "installer". Have marked as component GUI for now but feel free to change.)