Multiple vulnerabilities have been discovered in the Xen hypervisor: CVE-2017-17563 Jan Beulich discovered that an incorrect reference count overflow check in x86 shadow mode may result in denial of service or privilege escalation. CVE-2017-17564 Jan Beulich discovered that improper x86 shadow mode reference count error handling may result in denial of service or privilege escalation. CVE-2017-17565 Jan Beulich discovered that an incomplete bug check in x86 log-dirty handling may result in denial of service. CVE-2017-17566 Jan Beulich discovered that x86 PV guests may gain access to internally used pages which could result in denial of service or potential privilege escalation. In addition this update ships the Comet shim to address the Meltdown class of vulnerabilities for guests with legacy PV kernels. In addition, the package provides the Xen PTI stage 1 mitigation which is built-in and enabled by default on Intel systems, but can be disabled with `xpti=false" on the hypervisor command line Please refer to the following URL for more details on how to configure individual mitigation strategies: https://xenbits.xen.org/xsa/advisory-254.html Additional information can also be found in README.pti and README.comet.