The backend driver in Xen 3.x allows guest OS users to cause a denial of service via a kernel thread leak, which prevents the device and guest OS from being shut down or create a zombie domain, causes a hang in zenwatch, or prevents unspecified xm commands from working properly, related to (1) netback, (2) blkback, or (3) blktap.
http://lists.opensuse.org/opensuse-security-announce/2011-01/msg00007.html
http://secunia.com/advisories/42372
http://secunia.com/advisories/42789
http://secunia.com/advisories/43056
http://secunia.com/advisories/46397
http://www.redhat.com/support/errata/RHSA-2011-0004.html
http://www.securitytracker.com/id?1024786
http://www.vmware.com/security/advisories/VMSA-2011-0012.html