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Response snapshot Oct 27, 2026, 01:26 UTC

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VulnerabilityCVE-2026-99082

Linux kernel: KVM: x86: Use active memslots for the per-vCPU MMIO cache

In the Linux kernel, the following vulnerability has been resolved: KVM: x86: Use active memslots for the per-vCPU MMIO cache KVM tags the per-vCPU MMIO cache with a memslot generation so that a memslot update invalidates cached MMIO information.

At a glance

Exploited
Not known
Severity
8.8 High (CVSS 3.1)
Chance of exploitation
0.2% in the next 30 days
Sources
1 source

What to do

Check the vendor advisory for affected versions and fixes.

Read NVD CVE-2026-99082 (opens a new tab)

What happened

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Use active memslots for the per-vCPU MMIO cache

KVM tags the per-vCPU MMIO cache with a memslot generation so that a memslot update invalidates cached MMIO information. Both the fill and validation paths use kvm_memslots(), which unconditionally selects address space 0, even when the vCPU is running in SMM and using address space 1.

Commit 56f17dd3fbc4 ("kvm: x86: fix stale mmio cache bug") added the memslot generation to the cache key so that a memslot update could not leave a stale entry valid. When commit 699023e23965 ("KVM: x86: add SMM to the MMU role, support SMRAM address space") added the SMM address space, these helpers were not converted to use the active memslots.

Continue reading

Consequently, an update to the SMM memslots can leave an entry from the old SMM address space apparently valid after the vCPU returns to the normal address space. A guest can then cause an access to valid RAM at the same GFN to be returned to userspace as KVM_EXIT_MMIO.

Completing that exit through the VMM's RAM address space writes the backing page without going through KVM's write-tracking path. If the page backs a nested EPT table, KVM can continue using shadow translations derived from the old contents. The WARN_ON_ONCE() in kvm_mmu_write_protect_fault() catches this invalid cache and memslot combination.

Use the vCPU's active memslots when caching and validating the entry. Memslot generations are unique across address spaces, so the generation also distinguishes the normal and SMM views. This matches the MMIO SPTE cache, which already uses kvm_vcpu_memslots().

On an unpatched current-mainline kernel, a regression test that fills the cache in SMM and updates only the SMM memslots fails the intended RAM write and triggers the kvm_mmu_write_protect_fault() warning. With this change, the same test completes the RAM write without a warning. The x86/smm_test, set_memory_region_test, and memslot_modification_stress_test selftests also pass.

Scores and how they were measured

CVSS base severity

8.8

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.2%

Scored Percentile 4.7%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    NVD CVE-2026-99082 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  2. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-99082, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 24). Linux kernel: KVM: x86: Use active memslots for the per-vCPU MMIO cache (CVE-2026-99082). https://kevwire.com/record/CVE-2026-99082

MLA

"Linux kernel: KVM: x86: Use active memslots for the per-vCPU MMIO cache (CVE-2026-99082)." KEVWire, 24 Oct. 2026, kevwire.com/record/CVE-2026-99082.

Chicago

KEVWire. "Linux kernel: KVM: x86: Use active memslots for the per-vCPU MMIO cache (CVE-2026-99082)." Last modified October 24, 2026. https://kevwire.com/record/CVE-2026-99082.

Evidence summary (plain text)

Linux kernel: KVM: x86: Use active memslots for the per-vCPU MMIO cache (Oct 23, 2026).
In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Use active memslots for the per-vCPU MMIO cache

KVM tags the per-vCPU MMIO cache with a memslot generation so that a
memslot update invalidates cached MMIO information.  Both the fill and
validation paths use kvm_memslots(), which unconditionally selects address
space 0, even when the vCPU is running in SMM and using address space 1.

Commit 56f17dd3fbc4 ("kvm: x86: fix stale mmio cache bug") added the
memslot generation to the cache key so that a memslot update could not
leave a stale entry valid.  When commit 699023e23965 ("KVM: x86: add SMM
to the MMU role, support SMRAM address space") added the SMM address space,
these helpers were not converted to use the active memslots.

Consequently, an update to the SMM memslots can leave an entry from the old
SMM address space apparently valid after the vCPU returns to the normal
address space.  A guest can then cause an access to valid RAM at the same
GFN to be returned to userspace as KVM_EXIT_MMIO.

Completing that exit through the VMM's RAM address space writes the backing
page without going through KVM's write-tracking path.  If the page backs a
nested EPT table, KVM can continue using shadow translations derived from
the old contents.  The WARN_ON_ONCE() in kvm_mmu_write_protect_fault()
catches this invalid cache and memslot combination.

Use the vCPU's active memslots when caching and validating the entry.
Memslot generations are unique across address spaces, so the generation
also distinguishes the normal and SMM views.  This matches the MMIO SPTE
cache, which already uses kvm_vcpu_memslots().

On an unpatched current-mainline kernel, a regression test that fills the
cache in SMM and updates only the SMM memslots fails the intended RAM write
and triggers the kvm_mmu_write_protect_fault() warning.  With this change,
the same test completes the RAM write without a warning.  The x86/smm_test,
set_memory_region_test, and memslot_modification_stress_test selftests also
pass.
CVSS 8.8 (version 3.1; evidence as of Oct 24, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-99082).
EPSS 0.2% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-99082&date=2026-10-26).
Sources:
NVD CVE-2026-99082: https://nvd.nist.gov/vuln/detail/CVE-2026-99082
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?pubStartDate=2026-10-12T13:35:55.142Z&pubEndDate=2026-10-26T13:35:55.142Z&resultsPerPage=500&startIndex=9000
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-99082&date=2026-10-26
Record ID: CVE-2026-99082.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

NVD status: Received. Publication in the NVD is not evidence of exploitation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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