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CVE Record

CVE-2021-46977: KVM: VMX: Disable preemption when probing user return MSRs

In the Linux kernel, the following vulnerability has been resolved: KVM: VMX: Disable preemption when probing user return MSRs Disable preemption when probing a user return MSR via RDSMR/WRMSR. If the MSR holds a different value per logical CPU, the WRMSR could corrupt the host's value if KVM is preempted between the RDMSR and WRMSR, and then rescheduled on a different CPU. Opportunistically land the helper in common x86, SVM will use the helper in a future commit.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysisunknown

Security readout for executives and security teams

Plain-English summary

CVE-2021-46977 is a Linux kernel KVM issue in Intel VMX handling. A timing window could let KVM restore a CPU-specific register value on the wrong logical CPU, corrupting host state. The public record does not provide CVSS, confirmed impact, or active exploitation evidence.

Executive priority

Treat this as a virtualization platform maintenance item, not a confirmed emergency. Patch KVM hypervisors through normal kernel update processes, with higher priority for shared, multi-tenant, or security-sensitive environments.

Technical view

The flaw occurs when KVM probes user return MSRs with RDMSR/WRMSR while preemption remains enabled. If scheduling moves execution to another logical CPU between the read and write, a per-CPU MSR value can be written back incorrectly. Upstream stable commits resolve this by disabling preemption during the probe.

Likely exposure

Exposure is most relevant to Linux systems running KVM with VMX, typically Intel-based virtualization hosts. The source lists Linux kernel versions including 5.5, 5.10.38, 5.11.22, 5.12.5, and 5.13 as affected, but distribution backports must be checked.

Exploitation context

The bundle does not cite KEV listing, exploit availability, or active exploitation. The vulnerability is a kernel race/CPU scheduling flaw; practical impact and triggerability are not fully described in the provided public sources.

Researcher notes

The key evidence is the upstream fix description: preemption between RDMSR and WRMSR can move execution across logical CPUs and corrupt host MSR state. Severity and exploitation details are incomplete, so avoid asserting privilege escalation or guest-to-host escape without more evidence.

Mitigation direction

  • Check Linux distribution advisories for CVE-2021-46977 coverage.
  • Update affected virtualization hosts to kernels containing the referenced stable KVM fix.
  • Prioritize internet-facing or multi-tenant KVM hypervisors first.
  • Use normal maintenance windows if no additional exploit evidence exists.

Validation and detection

  • Inventory Linux hosts running KVM with Intel VMX enabled.
  • Compare installed kernel builds against vendor advisories or upstream stable fix commits.
  • Review kernel changelogs for the referenced KVM VMX preemption fix.
  • Confirm patched kernels are booted, not merely installed.
Prepared
Confidence
medium
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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ATT&CK lookup starting points

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CVE-2021-46977 mapping review

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Vulnerability profileCVE 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.

0CVSS vectors
3Timeline events
2ADP providers
5Source links

SSVC decision data

CISA-ADPCISA 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 reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

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

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux4be5341026246870818e28b53202b001426a5aec, 4be5341026246870818e28b53202b001426a5aec, 4be5341026246870818e28b53202b001426a5aec, 4be5341026246870818e28b53202b001426a5aecunaffected
LinuxLinux5.5, 0, 5.10.38, 5.11.22, 5.12.5, 5.13affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.