The memory_exchange function in common/memory.c in Xen 3.2.x through 4.6.x does not properly hand back pages to a domain, which might allow guest OS administrators to cause a denial of service (host crash) via unspecified vectors related to domain teardown.
https://security.gentoo.org/glsa/201604-03
http://xenbits.xen.org/xsa/advisory-159.html
http://www.securitytracker.com/id/1034391
http://www.securityfocus.com/bid/79038