The compat_iret function in Xen 3.1 through 4.5 iterates the wrong way through a loop, which allows local 32-bit PV guest administrators to cause a denial of service (large loop and system hang) via a hypercall_iret call with EFLAGS.VM set.
http://lists.opensuse.org/opensuse-security-announce/2015-06/msg00004.html
http://lists.opensuse.org/opensuse-security-announce/2015-06/msg00007.html
http://lists.opensuse.org/opensuse-security-announce/2015-06/msg00029.html
http://lists.opensuse.org/opensuse-security-announce/2015-06/msg00030.html
http://lists.opensuse.org/opensuse-security-announce/2015-07/msg00014.html
http://lists.opensuse.org/opensuse-security-announce/2015-09/msg00027.html
http://support.citrix.com/article/CTX201145
http://www.debian.org/security/2015/dsa-3286
http://www.securityfocus.com/bid/75149
http://www.securitytracker.com/id/1032569