The fixup_page_fault function in arch/x86/traps.c in Xen 4.0.1 and earlier on 64-bit platforms, when paravirtualization is enabled, does not verify that kernel mode is used to call the handle_gdt_ldt_mapping_fault function, which allows guest OS users to cause a denial of service (host OS BUG_ON) via a crafted memory access.
https://bugzilla.redhat.com/show_bug.cgi?id=658155
http://www.vmware.com/security/advisories/VMSA-2011-0012.html
http://www.securityfocus.com/archive/1/520102/100/0/threaded
http://www.redhat.com/support/errata/RHSA-2011-0017.html
http://secunia.com/advisories/46397
http://secunia.com/advisories/42884
http://openwall.com/lists/oss-security/2010/11/30/8
http://openwall.com/lists/oss-security/2010/11/30/5
http://lists.xensource.com/archives/html/xen-devel/2010-11/msg01650.html