you need to enable it, once way described here:
https://wiki.archlinux.org/index.php/Xorg#Ctrl.2BAlt.2BBackspace_does_not_work
** Changed in: xorg (Ubuntu)
Status: New => Invalid
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https://bugs.launchpad.net/bugs/1031604
Title:
cntrl-alt-del does not kill Xorg
Status in “xorg” package in Ubuntu:
Invalid
Bug description:
Setting the traditional cntrl-alt-del to kill X in the gnome-control-
center (Keyboard Layout/Options/Key sequence to kill the X server)
does not allow killing of X while logged in, despite that the sequence
does kill the log-in display prior to entering a desktop session.
ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: xorg 1:7.6+12ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-27.43-generic 3.2.21
Uname: Linux 3.2.0-27-generic i686
ApportVersion: 2.0.1-0ubuntu11
Architecture: i386
Date: Wed Aug 1 00:19:37 2012
DistroCodename: precise
DistroVariant: ubuntu
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
SourcePackage: xorg
UpgradeStatus: Upgraded to precise on 2012-03-23 (130 days ago)
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