An issue was discovered in Xen through 4.11.x on AMD x86 platforms, possibly allowing guest OS users to gain host OS privileges because small IOMMU mappings are unsafely combined into larger ones.
https://xenbits.xen.org/xsa/advisory-275.html
https://www.debian.org/security/2019/dsa-4369
https://support.citrix.com/article/CTX239432
https://lists.debian.org/debian-lts-announce/2019/10/msg00008.html
http://www.securityfocus.com/bid/106182
http://lists.opensuse.org/opensuse-security-announce/2019-04/msg00072.html