An issue was discovered in Xen through 4.9.x allowing x86 PV guest OS users to execute arbitrary code on the host OS because of a race condition that can cause a stale TLB entry.
https://xenbits.xen.org/xsa/advisory-241.html
https://www.debian.org/security/2017/dsa-4050
https://support.citrix.com/article/CTX228867
https://security.gentoo.org/glsa/201801-14
https://lists.debian.org/debian-lts-announce/2018/10/msg00009.html
https://lists.debian.org/debian-lts-announce/2017/11/msg00027.html