An issue was discovered in Xen through 4.9.x. Grant copying code made an implication that any grant pin would be accompanied by a suitable page reference. Other portions of code, however, did not match up with that assumption. When such a grant copy operation is being done on a grant of a dying domain, the assumption turns out wrong. A malicious guest administrator can cause hypervisor memory corruption, most likely resulting in host crash and a Denial of Service. Privilege escalation and information leaks cannot be ruled out.
https://www.debian.org/security/2017/dsa-4050
https://support.citrix.com/article/CTX229057
https://lists.debian.org/debian-lts-announce/2018/10/msg00009.html
http://xenbits.xen.org/xsa/advisory-236.html
http://www.securitytracker.com/id/1039653