The acceleration support for the "REP MOVS" instruction in Xen 4.4.x, 3.2.x, and earlier lacks properly bounds checking for memory mapped I/O (MMIO) emulated in the hypervisor, which allows local HVM guests to cause a denial of service (host crash) via unspecified vectors.
https://security.gentoo.org/glsa/201504-04
http://xenbits.xenproject.org/xsa/advisory-112.html
http://www.securityfocus.com/bid/71331
http://www.debian.org/security/2015/dsa-3140
http://support.citrix.com/article/CTX201794
http://support.citrix.com/article/CTX200288
http://secunia.com/advisories/62672
http://secunia.com/advisories/59949
http://rhn.redhat.com/errata/RHSA-2015-0783.html
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00010.html
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00005.html
http://kb.juniper.net/InfoCenter/index?page=content&id=JSA10705