The do_hvm_op function in xen/arch/x86/hvm/hvm.c in Xen 4.2.x on the x86_32 platform does not prevent HVM_PARAM_NESTEDHVM (aka nested virtualization) operations, which allows guest OS users to cause a denial of service (long-duration page mappings and host OS crash) by leveraging administrative access to an HVM guest in a domain with a large number of VCPUs.
http://xenbits.xen.org/gitweb/?p=xen.git%3Ba=commit%3Bh=d60d7082289a74e44b3dc8f67df46c3404ca08bf
http://security.gentoo.org/glsa/glsa-201309-24.xml