GNTTABOP_swap_grant_ref in Xen 4.2 through 4.5 does not check the grant table operation version, which allows local guest domains to cause a denial of service (NULL pointer dereference) via a hypercall without a GNTTABOP_setup_table or GNTTABOP_set_version.
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/msg00030.html
https://security.gentoo.org/glsa/201604-03
https://support.citrix.com/article/CTX206006
http://support.citrix.com/article/CTX201145
http://www.debian.org/security/2015/dsa-3286