Use-after-free vulnerability in QEMU in Xen 4.5.x and earlier does not completely unplug emulated block devices, which allows local HVM guest users to gain privileges by unplugging a block device twice.
http://lists.fedoraproject.org/pipermail/package-announce/2015-September/165373.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-September/167792.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-September/167820.html
http://www.securityfocus.com/bid/76152