CVE-2020-15565 – This vulnerability in Xen hypervisor allows x86... (How to Fix)

Q: What is the severity of CVE-2020-15565?

CVE-2020-15565 has a CVSS score of 8.8 (HIGH). This vulnerability in Xen hypervisor allows x86 Intel HVM guest OS users to potentially cause host OS denial of service or gain privileges due to insufficient cache write-back when splitting large page mappings. Only x86 Intel systems running Xen with HVM guests using hardware assisted paging and passed-through PCI devices are affected. AMD and ARM systems are not vulnerable.

📋 TL;DR

This vulnerability in Xen hypervisor allows x86 Intel HVM guest OS users to potentially cause host OS denial of service or gain privileges due to insufficient cache write-back when splitting large page mappings. Only x86 Intel systems running Xen with HVM guests using hardware assisted paging and passed-through PCI devices are affected. AMD and ARM systems are not vulnerable.

💻 Affected Systems

Products:

Xen Hypervisor

Versions: Xen 3.2 through 4.13.x

Operating Systems: All operating systems running Xen hypervisor

Default Config Vulnerable: ✅ No

Notes: Only affects x86 Intel systems with HVM guests using HAP, passed-through PCI devices, and page table sharing enabled (enabled by default only if Xen considers IOMMU and CPU large page size support compatible).

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Privilege escalation allowing guest to gain host-level access, potentially compromising the entire virtualization infrastructure and all other guests.

🟠

Likely Case

Host crash leading to denial of service affecting all virtual machines on the host.

🟢

If Mitigated

No impact if proper patching or workarounds are applied, or if affected configurations are not used.

🌐 Internet-Facing: MEDIUM - Requires specific guest configuration and passed-through PCI devices, but could affect cloud environments.

🏢 Internal Only: HIGH - Virtualization infrastructure is typically internal, and successful exploitation could compromise entire virtual environments.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: HIGH

Exploitation requires specific guest configuration and passed-through PCI devices. The vulnerability involves complex memory management operations.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Patches available for affected versions - see XSA-321 advisory

Vendor Advisory: http://xenbits.xen.org/xsa/advisory-321.html

Restart Required: Yes

Instructions:

1. Review XSA-321 advisory. 2. Apply appropriate patches for your Xen version. 3. Reboot hypervisor host. 4. Verify patch application.

🔧 Temporary Workarounds

Disable page table sharing

linux

Prevent the vulnerable code path by disabling page table sharing between IOMMU and CPU

Add 'iommu=no-shared-pt' to Xen boot parameters

Disable PCI passthrough

all

Remove the required condition for exploitation by disabling PCI device passthrough

Remove PCI passthrough configurations from affected guests

🧯 If You Can't Patch

Apply workaround to disable page table sharing via Xen boot parameters
Disable PCI passthrough for all HVM guests on affected systems
Isolate affected virtualization hosts from critical networks

🔍 How to Verify

Check if Vulnerable:

Check Xen version with 'xl info' or 'xm info' and verify if running affected version (3.2-4.13.x) on x86 Intel hardware with HVM guests using PCI passthrough

Check Version:

xl info | grep xen_version  OR  xm info | grep xen_version

Verify Fix Applied:

Verify Xen version is patched per XSA-321, check that 'iommu=no-shared-pt' is in boot parameters if using workaround, and confirm no guests have PCI passthrough enabled

📡 Detection & Monitoring

Log Indicators:

Xen hypervisor crashes or instability
Unexpected guest behavior with PCI passthrough devices
Memory management errors in Xen logs

Network Indicators:

Unusual network traffic from virtualization hosts
Guest-to-guest communication anomalies

SIEM Query:

source="xen.log" AND ("crash" OR "panic" OR "IOMMU" OR "page table")

📊 Metadata

CVE ID: CVE-2020-15565

CVSS v3 Score: 8.8 (HIGH)

CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

CWE: CWE-400

Published: July 7, 2020

Last Updated: November 21, 2024

🔗 References

http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00024.html Mailing List,Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00031.html Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/07/07/4 Mailing List,Third Party Advisory
http://xenbits.xen.org/xsa/advisory-321.html Patch,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/MXESCOVI7AVRNC7HEAMFM7PMEO6D3AUH/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/VB3QJJZV23Z2IDYEMIHELWYSQBUEW6JP/
https://security.gentoo.org/glsa/202007-02 Third Party Advisory
https://www.debian.org/security/2020/dsa-4723 Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00024.html Mailing List,Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00031.html Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/07/07/4 Mailing List,Third Party Advisory
http://xenbits.xen.org/xsa/advisory-321.html Patch,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/MXESCOVI7AVRNC7HEAMFM7PMEO6D3AUH/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/VB3QJJZV23Z2IDYEMIHELWYSQBUEW6JP/
https://security.gentoo.org/glsa/202007-02 Third Party Advisory
https://www.debian.org/security/2020/dsa-4723 Third Party Advisory

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