The handle_mmio function in arch/x86/hvm/io.c in the MMIO operations emulator for Xen 3.3 and 4.x, when running an HVM guest, does not properly reset certain state information between emulation cycles, which allows local guest OS users to cause a denial of service (guest OS crash) via unspecified operations on MMIO regions.
http://www.securityfocus.com/bid/54691
http://www.debian.org/security/2012/dsa-2531
http://security.gentoo.org/glsa/glsa-201309-24.xml
http://secunia.com/advisories/55082
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00018.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00017.html
http://lists.opensuse.org/opensuse-security-announce/2012-08/msg00025.html
http://lists.opensuse.org/opensuse-security-announce/2012-08/msg00024.html