The grant-table feature in Xen through 4.8.x provides false mapping information in certain cases of concurrent unmap calls, which allows backend attackers to obtain sensitive information or gain privileges, aka XSA-218 bug 1.
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