CVE-2021-28701
📋 TL;DR
CVE-2021-28701 is a race condition vulnerability in Xen's grant table v2 status page handling that allows guest VMs to retain access to freed memory pages. This affects Xen hypervisor systems running paravirtualized (PV) guests. The vulnerability could lead to information disclosure or privilege escalation within the hypervisor context.
💻 Affected Systems
- Xen Hypervisor
📦 What is this software?
Fedora by Fedoraproject
Fedora by Fedoraproject
Fedora by Fedoraproject
Xen by Xen
⚠️ Risk & Real-World Impact
Worst Case
Guest VM could read sensitive data from reallocated memory pages, potentially leading to hypervisor compromise or data leakage between VMs.
Likely Case
Information disclosure where guest VMs access stale data from freed pages, potentially exposing sensitive information from other VMs or the hypervisor.
If Mitigated
With proper isolation controls and updated hypervisor, risk is limited to denial of service at worst.
🎯 Exploit Status
Exploitation requires guest VM access and precise timing to trigger the race condition during grant table version switching.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Xen 4.14.0, 4.13.1, 4.12.4, and later versions
Vendor Advisory: http://xenbits.xen.org/xsa/advisory-384.html
Restart Required: Yes
Instructions:
1. Update Xen hypervisor to patched version. 2. Reboot hypervisor host. 3. Verify all VMs are using updated grant table handling.
🔧 Temporary Workarounds
Disable grant table v2
linuxForce all guests to use grant table v1 only
xl vm-create configfile.cfg 'gnttab_version=1'
Edit VM config to include: gnttab_version = 1
🧯 If You Can't Patch
- Isolate affected Xen hosts from critical infrastructure
- Monitor for unusual guest VM memory access patterns
🔍 How to Verify
Check if Vulnerable:
Check Xen version with: xl info | grep xen_versionCheck Version:
xl info | grep xen_versionVerify Fix Applied:
Verify Xen version is 4.14.0+, 4.13.1+, or 4.12.4+📡 Detection & Monitoring
Log Indicators:
- Xen hypervisor logs showing grant table version switching
- Unexpected memory access patterns in Xen debug logs
Network Indicators:
- Not network exploitable - hypervisor local vulnerability
SIEM Query:
Search for Xen hypervisor logs containing 'grant table' and 'version switch' events with error codes🔗 References
- http://www.openwall.com/lists/oss-security/2021/09/08/2
- http://xenbits.xen.org/xsa/advisory-384.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/3HEHUIUWSSMCQGQY3GWX4J2SZGYP5W2Z/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CEHZLIR5DFYYQBH55AERWHLO54OFU42C/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/L4MI3MQAPGILCLXBGQWPZHGE3ALSO4ZU/
- https://security.gentoo.org/glsa/202208-23
- https://www.debian.org/security/2021/dsa-4977
- https://xenbits.xenproject.org/xsa/advisory-384.txt
- http://www.openwall.com/lists/oss-security/2021/09/08/2
- http://xenbits.xen.org/xsa/advisory-384.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/3HEHUIUWSSMCQGQY3GWX4J2SZGYP5W2Z/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CEHZLIR5DFYYQBH55AERWHLO54OFU42C/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/L4MI3MQAPGILCLXBGQWPZHGE3ALSO4ZU/
- https://security.gentoo.org/glsa/202208-23
- https://www.debian.org/security/2021/dsa-4977
- https://xenbits.xenproject.org/xsa/advisory-384.txt
