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CVE Record

CVE-2025-40300: x86/vmscape: Add conditional IBPB mitigation

In the Linux kernel, the following vulnerability has been resolved: x86/vmscape: Add conditional IBPB mitigation VMSCAPE is a vulnerability that exploits insufficient branch predictor isolation between a guest and a userspace hypervisor (like QEMU). Existing mitigations already protect kernel/KVM from a malicious guest. Userspace can additionally be protected by flushing the branch predictors after a VMexit. Since it is the userspace that consumes the poisoned branch predictors, conditionally issue an IBPB after a VMexit and before returning to userspace. Workloads that frequently switch between hypervisor and userspace will incur the most overhead from the new IBPB. This new IBPB is not integrated with the existing IBPB sites. For instance, a task can use the existing speculation control prctl() to get an IBPB at context switch time. With this implementation, the IBPB is doubled up: one at context switch and another before running userspace. The intent is to integrate and optimize these cases post-embargo. [ dhansen: elaborate on suboptimal IBPB solution ]

UnknownCVSS not scoredNot KEV-listed Updated
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Glexia's Take moderate

Analyst readout for executives and security teams

Plain-English summary

CVE-2025-40300 is a Linux kernel issue affecting virtualization isolation. A malicious guest may influence CPU branch prediction used by a userspace hypervisor such as QEMU. The kernel fix adds a protective branch-predictor flush when returning from a VM exit. Business impact is mainly for organizations running untrusted or multi-tenant virtual machines.

Executive priority

Treat as a targeted virtualization-host priority, not a broad emergency. Patch shared or untrusted VM infrastructure promptly, while coordinating performance testing because the mitigation can add overhead in some workloads.

Technical view

VMSCAPE involves insufficient branch predictor isolation between a guest and userspace hypervisor. The resolved Linux change conditionally issues IBPB after VMexit and before returning to userspace. The CVE notes existing mitigations already protect kernel/KVM, while this change protects userspace consumers of poisoned predictors. VMexit-heavy workloads may see overhead.

Likely exposure

Exposure is most relevant on Linux virtualization hosts using userspace hypervisors like QEMU, especially where guests are untrusted or multi-tenant. General Linux systems not hosting VMs are less likely to be exposed based on the provided description.

Exploitation context

The provided sources do not show active exploitation, and the CVE is not listed as KEV. Exploitation would be specialized: a malicious guest attempting a branch-prediction side channel against userspace hypervisor context after VM exits.

Researcher notes

The fix is intentionally conditional and not integrated with existing IBPB sites. The CVE text notes possible duplicated IBPB with speculation-control prctl context-switch behavior, with optimization intended post-embargo. Affected-version details in the source bundle are incomplete and should be validated against vendor kernels.

Mitigation direction

  • Apply vendor Linux kernel updates that include the VMSCAPE conditional IBPB mitigation.
  • Prioritize multi-tenant virtualization hosts and systems running untrusted guest workloads.
  • Review Debian LTS advisories if using affected Debian kernel packages.
  • Plan performance testing for VMexit-heavy workloads after applying the mitigation.
  • For unsupported kernels, consult the OS or kernel vendor for backport guidance.

Validation and detection

  • Inventory Linux hosts running KVM/QEMU or similar userspace hypervisor stacks.
  • Compare installed kernel builds against vendor advisories and referenced stable commits.
  • Confirm the kernel includes the conditional IBPB after VMexit mitigation.
  • Check distribution security notices for package-specific fixed versions.
  • Monitor virtualization performance after patching, especially frequent VMexit workloads.
Prepared
Confidence
medium
Sources
6

Public sources used

Based on public source material and reviewed before publication.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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Vulnerability profile CVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0 CVSS vectors
3 Timeline events
3 ADP providers
22 Source links

SSVC decision data

CISA-ADP CISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reserved CVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE published CVE Program

    The CVE record was published.

  3. CVE updated CVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CVE CVE Program Container
siemens-SADP ADP container
CISA-ADP CISA ADP Vulnrichment
other:ssvc

Source materials

Affected products

Products and packages named in the record

Vendor Product Version / package Status
Linux Linux 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, 15d45071523d89b3fb7372e2135fbd72f6af9506, c51f1e5f57cca88d8d5894b6fad1638f643a99d0, 4b3870c343a82cd2df7192cc5149c87205dcc611, 3.16.57, 4.4.168 unaffected
Linux Linux 4.16, 0, 5.10.244, 5.15.193, 6.1.152, 6.6.106, 6.12.47, 6.16.7, 6.17 affected
Weakness

CWE details

No CWE listed

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