Qemu before 1.6.2 block diver for the various disk image formats used by Bochs and for the QCOW version 2 format, are vulnerable to a possible crash caused by signed data types or a logic error while creating QCOW2 snapshots, which leads to incorrectly calling update_refcount() routine.
http://rhn.redhat.com/errata/RHSA-2014-0420.html
http://rhn.redhat.com/errata/RHSA-2014-0421.html
https://bugzilla.redhat.com/show_bug.cgi?id=1078848