Bug#703979: Cleanup after install wipes /target if umount failed (e.g. shell open in /target)

[Ben Armstrong <synrg@sanctuary.nslug.ns.ca>]

Package: debian-installer-launcher
Version: 15
Severity: grave

If umount is prevented for any reason, such as the user opening a shell in
/lib/live/installer/target and then leaving it open, while exiting the
installer, then the cleanup of /lib/live/installer after the install will
remove the /target! It will also do damage to any bind-mounted filesystems
under /lib/live/installer, (e.g. /lib/live/installer/dev), causing harm to the
running live system.

If any of the 'umount -f' commands performed in plugins/live fails in the
cleanup after the installer exits, it should be considered fatal and no further
cleanup should be attempted, or at the very least the cleanup needs to not
cross filesystems so that nothing but the installer itself is removed.

Thanks go to Kragen Javier Sitaker, who worked with me on irc to help identify
this problem. He initially found it using one of our 6.0.7 images, so the bug
is probably present in all existing versions of debian-installer-launcher.

Ben