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FedoraOperating system · Fedoraproject

CVE-2020-25598

MEDIUM · 5.5 CVSS v3.1 Published 2020-09-23
Fix available
A fix is available. Upgrade to after 4.14.0 or later.
See remediation →
57/100
Remediation priority · Elevated
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
An issue was discovered in Xen 4.14.x. There is a missing unlock in the XENMEM_acquire_resource error path. The RCU (Read, Copy, Update) mechanism is a synchronisation primitive. A buggy error path in the XENMEM_acquire_resource exits without releasing an RCU reference, which is conceptually similar to forgetting to unlock a spinlock. A buggy or malicious HVM stubdomain can cause an RCU reference to be leaked. This causes subsequent administration operations, (e.g., CPU offline) to livelock, resulting in a host Denial of Service. The buggy codepath has been present since Xen 4.12. Xen 4.14 and later are vulnerable to the DoS. The side effects are believed to be benign on Xen 4.12 and 4.13, but patches are provided nevertheless. The vulnerability can generally only be exploited by x86 HVM VMs, as these are generally the only type of VM that have a Qemu stubdomain. x86 PV and PVH domains, as well as ARM VMs, typically don't use a stubdomain. Only VMs using HVM stubdomains can exploit the vulnerability. VMs using PV stubdomains, or with emulators running in dom0, cannot exploit the vulnerability.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In Xen 4.14.x, the XENMEM_acquire_resource hypercall has a missing RCU (Read-Copy-Update) unlock in an error path. When this error path is taken, an RCU reference is not released—similar to forgetting to unlock a spinlock. A malicious or buggy HVM stubdomain can trigger this leak, causing subsequent host administration operations (like CPU offline) to livelock, resulting in a Denial of Service. Only x86 HVM VMs with stubdomains can exploit this; PV domains, PVH, and ARM VMs are not vulnerable.

MitigationApply the Xen security patch to address the missing RCU unlock, or upgrade to a fixed Xen version. As a workaround, avoid using HVM stubdomains (use PV stubdomains or run emulators in dom0 instead).

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FedoraOperating system
Affected:= 31= 32= 33
XenOperating system
Affected:>= 4.12.0, <= 4.14.0
LeapOperating system
Affected:= 15.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Xen version
    Run 'xl info' or 'xm info' to display Xen hypervisor version information
    Affected if Version is >= 4.12.0 and <= 4.14.0 (e.g., 4.13.x)
  2. Confirm x86 architecture
    Check the host architecture via 'uname -m' or 'xl info' - look for x86_64 or i686
    Affected if System is x86 (x86_64 or i686); ARM systems are not affected
  3. Check for active HVM domains with stubdomains
    Run 'xl list' to list all VMs, then for each HVM domain check stubdomain status via 'xl debug-keys R' or review domain config for 'stubdomain' or 'device_model_stubdomain' settings
    Affected if Any HVM domains are configured with stubdomains enabled; PV, PVH, and ARM VMs are not vulnerable
  4. Verify stubdomain processes exist
    Run 'ps aux | grep stubdomain' or check for qemu processes with stubdomain naming patterns on the host
    Affected if Stubdomain processes (qemu-* stubs) are running for HVM guests
  5. Monitor for livelock symptoms
    Attempt a non-critical CPU offline operation (if possible) or check for hung scheduler operations via 'xl cpupool-list' or 'xl cpu-list'; look for operations that hang without completing
    Affected if CPU management operations hang or exhibit livelock behavior after HVM stubdomain activity

You are affected if your Xen version is between 4.12.0 and 4.14.0 AND you are running x86 HVM virtual machines with stubdomains enabled, which can trigger the RCU unlock leak leading to livelock during host administration operations.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Upgrade available Upgrade to a release after 4.14.0
Interim mitigation

Apply the Xen security patch to address the missing RCU unlock, or upgrade to a fixed Xen version. As a workaround, avoid using HVM stubdomains (use PV stubdomains or run emulators in dom0 instead).

Fix this in Fedora Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2020-25598 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data