The grant-table feature in Xen through 4.8.x mishandles MMIO region grant references, which allows guest OS users to cause a denial of service (loss of grant trackability), aka XSA-224 bug 3.
https://xenbits.xen.org/xsa/advisory-224.html
https://security.gentoo.org/glsa/201710-17
https://security.gentoo.org/glsa/201708-03