* [CVE-2026-99082][IMPORTANT] KVM: x86: Use active memslots for the per-vCPU MMIO cache
@ 2026-10-27 3:21 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-27 3:21 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99082
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: KVM: x86: Use active memslots for the per-vCPU MMIO cache
Commit: a63f79d9da9468952396af73e1e53330845cf876
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a63f79d9da9468952396af73e1e53330845cf876
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99082
Analysis date: Mon, 26 Oct 2026 23:21:20 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
Wrong-address-space memslot validation can preserve stale MMIO state, bypass KVM write tracking, and leave host shadow translations inconsistent after a VMM-completed RAM write.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99082 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99082
The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.
This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.
The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.
Published priority for this report: IMPORTANT
Manual review required: YES
A detailed explanation of the methodology and priority rules is included
at the end of this message.
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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99082 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99082 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-672;CWE-440;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'KVM used address space 0 memslots when filling and validating the per vCPU MMIO cache even while the vCPU was using the SMM address space. A stale cache entry can misclassify valid guest RAM as MMIO and let VMM completion modify the backing page outside KVM write tracking. If the page contains a nested EPT table, KVM may retain shadow translations derived from obsolete contents, creating a plausible confidentiality, integrity, and availability risk inside the virtualization boundary. For the CVSS the PR:L value reflects that an attacker normally needs control of guest execution or a VM context, while the required SMM cache and memslot update sequence makes AC:H appropriate. No arbitrary host write, UAF, or proven guest to host code execution is demonstrated. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE OOB VIRT HARDWARE LINUS TEST INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; the extra point reflects the specific unknown impact of corrupted translation state consumed by the host MMU.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +1 — Active-state consistency breach: the wrong memslot generation lets an obsolete SMM MMIO-cache entry remain valid.
* +1 — Active semantic-domain transition: valid RAM is reinterpreted as MMIO because validation consults address space 0 rather than the vCPU’s active address space.
* +1 — Secondary structured-state consumption: VMM completion writes the backing page outside KVM write tracking, and host KVM may continue using shadow translations based on obsolete nested-EPT contents.
* +1 — Practical chain: stale cache → false KVM_EXIT_MMIO → userspace completion into RAM → missed write tracking → stale shadow translation.
* +1 — Integrity concern: the patch establishes incorrect host-managed translation state, not merely passive metadata or a WARN.
* -1 — Reduced/delegated privilege: the complete demonstrated actor is a VMM process with `/dev/kvm` and VM-control access, below host administrator but more privileged than guest-only code.
* Paranoid +1 — The corrupted active translation state is consumed by privileged host-MMU processing; the eventual host effect remains unresolved.
No UAF, heap OOB write, arbitrary host write, controlled reclaim, callback control, authorization bypass, or proven guest escape is shown. The WARN is not scored as a kernel crash. No confidentiality or extra availability point is inferred.
3. Reachability analysis
A `/dev/kvm` VMM can create the VM, update SMM memslots, run guest code that fills and exercises the cache, and complete the MMIO exit. Thus the full trigger is available below host-admin powers, without host UID 0 where `/dev/kvm` access is delegated.
A guest alone is not shown able to update host memslots or control VMM exit completion; pure guest-to-host reachability is therefore unproven. No namespace capability governs this path, no special hardware is required, and the sequence is ordered rather than a demonstrated hard race.
4. Primitive and severity reading
The primitive is a CLASS_B active translation/cache consistency corruption, not memory-lifetime corruption. Wrong-address-space generation validation preserves stale MMIO classification, after which privileged KVM/VMM processing modifies RAM without write tracking and leaves host shadow translations stale.
The shown effect is translation integrity failure and a WARN. Host arbitrary memory access, host panic, disclosure, LPE, and VM escape remain speculative. ImportantCandidate is YES because attacker-influenced active state reaches a privileged host-MMU consumer across the delegated VMM/host-kernel boundary, with downstream consequences unresolved. Auto-downgrade is unsafe despite the absence of a proven high-impact exploit.
5. One-sentence report phrase
Wrong-address-space memslot validation can preserve stale MMIO state, bypass KVM write tracking, and leave host shadow translations inconsistent after a VMM-completed RAM write.
6. Manual review advice
MANUAL CHECK REQUIRED
Review the stale shadow-translation consumer and whether a delegated VMM or malicious guest/VMM combination can convert the write-tracking bypass into host-wide corruption or isolation failure.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: x86: Use active memslots for the per-vCPU MMIO cache
Commit: a63f79d9da9468952396af73e1e53330845cf876
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a63f79d9da9468952396af73e1e53330845cf876
Commit description:
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 56f17dd ("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 699023e ("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.
Fixes: 699023e ("KVM: x86: add SMM to the MMU role, support SMRAM address space")
Cc: [email protected]
Signed-off-by: Jinu Kim <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
arch/x86/kvm/x86.h
Diff excerpt:
Not included in this email. See the upstream URL for the full patch.
Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a63f79d9da9468952396af73e1e53330845cf876
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99082 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99082
The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.
The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.
This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.
The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.
Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:
Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7
The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.
CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.
A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.
Conservative ActionableScore: 4
Paranoid ActionableScore: 5
The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.
For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:
ActionableScore below 5 -> MODERATE REGULAR
ActionableScore 5 or more -> MODERATE 7.0
The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.
There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).
KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.
AL-KERNEL base severity for this report: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT
These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).
Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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