Description of Problem

A number of security issues have been identified in certain CPU hardware that may allow unprivileged code running on a CPU core to infer the value of memory data belonging to other processes, virtual machines or the hypervisor that are, or have recently been, running on the same CPU core.

These issues have the following identifiers:

• CVE-2018-12126: Microarchitectural Store Buffer Data Sampling

• CVE-2018-12127: Microarchitectural Load Port Data Sampling

• CVE-2018-12130: Microarchitectural Fill Buffer Data Sampling

• CVE-2019-11091: Microarchitectural Data Sampling Uncacheable Memory

Although these are not vulnerabilities in the Citrix Hypervisor (formerly Citrix XenServer) product, this bulletin and associated hotfixes provides assistance in mitigating these CPU issues.

Mitigating Factors

Customers with AMD CPUs are believed to be unaffected by these issues.

Some Intel CPUs are believed to be unaffected by these issues. A list of affected Intel CPUs is expected to be made available at https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00233.html

Identification of the specific CPU(s) present on a Citrix Hypervisor machine may be obtained by typing the command

grep “model name” /proc/cpuinfo

in the Dom0 console.

What Customers Should Do

Full mitigation of these issues for systems with vulnerable CPUs requires all of:

Updates to Citrix Hypervisor
Updates to the CPU microcode
Disabling CPU hyper-threading (also known as simultaneous multi-threading)

In addition, updates to guest operating systems may be required to protect guest VMs from code running within that same VM. Guest VMs will need to be stopped and started (rather than rebooted) to fully mitigate these issues within the guest VM. Customers are advised to follow their operating system provider’s recommendations. Likewise, updates to the host system firmware (“BIOS updates”) may be required and Citrix recommends that you follow the guidance of your hardware vendor for any updates that they may provide.

Updates to Citrix Hypervisor

Citrix has released hotfixes that contain mitigations for these CPU issues. These hotfixes can be found on the Citrix website at the following locations:

Citrix Hypervisor 8.0: CTX250041 – https://support.citrix.com/article/CTX250041

Citrix XenServer 7.6: CTX250040 – https://support.citrix.com/article/CTX250040

Citrix XenServer 7.1 LTSR CU2: CTX250039 – https://support.citrix.com/article/CTX250039

Citrix XenServer 7.0: CTX250038 – https://support.citrix.com/article/CTX250038

Updates to the CPU microcode

The hotfixes released with this bulletin contain microcode for all supported CPU models for which Intel has presently made updates available. This microcode will be automatically applied each time the system boots. Any further microcode updates may be installed by means of system firmware updates (“BIOS updates”) and Citrix strongly recommends that you follow the guidance of your hardware vendor for any updates that they may provide.

CPUs that are vulnerable to these issues, and for which the CPU manufacturer has not provided microcode updates, will not have full mitigation of these issues.

Once the hotfix has been applied, customers with vulnerable CPUs can determine if the microcode required to mitigate these issues has been loaded into the CPU by typing the command

xl dmesg | grep “Hardware features:”

in the Dom0 console shortly after the host has rebooted to apply the hotfix. If the output includes the text MD_CLEAR, updated microcode is present.

Disabling CPU hyper-threading

Mitigation of these issues requires disabling hyper-threading on vulnerable CPUs. Customers should evaluate their workload and determine if the mitigation of disabling hyper-threading is required in their environment, and to understand the performance impact of this mitigation. Citrix recommends disabling hyper-threading in deployments with untrusted workloads. The following document provides the steps to disable hyper-threading via the Xen command line: https://support.citrix.com/article/CTX237190

Note that disabling hyper-threading will result in the number of available pCPUs being reduced and is likely to adversely impact performance. The following document covers additional issues that may be encountered in environments where customers have over-provisioned or pinned pCPUs (for example when hyper-threads are disabled): https://support.citrix.com/article/CTX236977

What Citrix Is Doing

Citrix is notifying customers and channel partners about this potential security issue. This article is also available from the Citrix Knowledge Center at http://support.citrix.com/.

Obtaining Support on This Issue

If you require technical assistance with this issue, please contact Citrix Technical Support. Contact details for Citrix Technical Support are available at https://www.citrix.com/support/open-a-support-case.html.

Reporting Security Vulnerabilities

Citrix welcomes input regarding the security of its products and considers any and all potential vulnerabilities seriously. For guidance on how to report security-related issues to Citrix, please see the following document: CTX081743 – Reporting Security Issues to Citrix

Changelog

Date	Change
14th May 2019	Initial publication
16th May 2019	Added additional hotfixes and included guidance on restarting guest VMs