The grant-table feature in Xen through 4.8.x has a race condition leading to a double free, which allows guest OS users to cause a denial of service (memory consumption), or possibly obtain sensitive information or gain privileges, aka XSA-218 bug 2.
https://xenbits.xen.org/xsa/advisory-218.html
https://security.gentoo.org/glsa/201710-17
https://security.gentoo.org/glsa/201708-03
http://www.securitytracker.com/id/1038722