CVE-2020-25596 Explained : Impact and Mitigation

An issue was discovered in Xen through 4.14.x where x86 PV guest kernels can experience denial of service via SYSENTER. This vulnerability affects all versions of Xen from 3.2 onwards, with only x86 systems that support the SYSENTER instruction in 64-bit mode being vulnerable. This CVE is believed to impact Intel, Centaur, and Shanghai CPUs.

Understanding CVE-2020-25596

Xen vulnerability leading to denial of service via SYSENTER.

What is CVE-2020-25596?

Xen through 4.14.x is susceptible to a denial of service vulnerability via SYSENTER instruction in x86 PV guest kernels.
The issue arises due to incorrect delivery of a #GP fault, causing a fatal fault in the guest kernel.
Only x86 systems supporting SYSENTER in 64-bit mode are vulnerable, affecting Intel, Centaur, and Shanghai CPUs.

The Impact of CVE-2020-25596

Malicious or buggy userspace can crash the guest kernel, resulting in a VM Denial of Service.
Only x86 PV guests can exploit the vulnerability; x86 PVH / HVM guests are not affected.

Technical Details of CVE-2020-25596

Xen vulnerability details.

Vulnerability Description

Xen's sanitization path injects a #GP fault, incorrectly delivering it twice to the guest, leading to a fatal fault in the guest kernel.

Affected Systems and Versions

All versions of Xen from 3.2 onwards are vulnerable.
Only x86 systems supporting the SYSENTER instruction in 64-bit mode are impacted.

Exploitation Mechanism

Malicious or buggy userspace can exploit the vulnerability to crash the guest kernel, causing a VM Denial of Service.

Mitigation and Prevention

Protecting systems from CVE-2020-25596.

Immediate Steps to Take

Apply patches provided by Xen to mitigate the vulnerability.
Monitor for any unusual system behavior that could indicate exploitation.

Long-Term Security Practices

Regularly update Xen and other software to prevent known vulnerabilities.
Implement strict access controls to limit potential attack surfaces.

Patching and Updates

Stay informed about security advisories and apply patches promptly to secure systems.