CVE-2018-12891 Explained : Impact and Mitigation

A problem has been discovered in Xen versions up to 4.10.x, where certain operations related to the para-virtualized (PV) Memory Management Unit (MMU) may experience significant delays. This vulnerability allows a malicious guest to exploit bypasses in preemption checks, potentially leading to a denial of service (DoS) attack on the host system.

Understanding CVE-2018-12891

This CVE identifies a critical vulnerability in Xen virtualization software that affects x86 systems running multi-vCPU para-virtualized guests.

What is CVE-2018-12891?

Xen versions up to 4.10.x are susceptible to a flaw where certain PV MMU operations can be manipulated by a malicious guest to bypass preemption checks, potentially causing a DoS attack on the host system.

The Impact of CVE-2018-12891

Malicious or faulty para-virtualized guests can launch a DoS attack, affecting the entire host system.
The attack can prevent the use of a physical CPU for an undetermined period, impacting system availability.
Vulnerable Xen versions from 3.4 onwards are at risk, with broader attack possibilities in versions 3.3 and earlier.
Only x86 systems are affected, while ARM systems remain unaffected.
Multi-vCPU x86 para-virtualized guests can exploit this vulnerability.

Technical Details of CVE-2018-12891

Xen software versions up to 4.10.x are vulnerable to this issue due to bypasses in preemption checks.

Vulnerability Description

Certain PV MMU operations can be manipulated by a malicious guest to bypass preemption checks.
This manipulation can lead to an unbounded number of iterations, causing a DoS attack on the host system.

Affected Systems and Versions

All Xen versions from 3.4 onwards are vulnerable to this issue.
Xen versions 3.3 and earlier are at risk of a wider class of attacks due to the absence of preemption checks.

Exploitation Mechanism

Malicious guests can exploit bypasses in preemption checks by manipulating their own page table contents.
This allows the bypasses to be used repeatedly without limit, potentially causing a DoS attack.

Mitigation and Prevention

Xen users should take immediate steps to mitigate the risks posed by CVE-2018-12891.

Immediate Steps to Take

Apply security patches provided by Xen to address this vulnerability.
Monitor Xen security advisories for updates and follow best practices for secure virtualization.

Long-Term Security Practices

Regularly update Xen software to the latest secure versions.
Implement network segmentation and access controls to limit the impact of potential attacks.

Patching and Updates

Stay informed about security updates and patches released by Xen.
Prioritize the installation of patches to protect systems from known vulnerabilities.