[CVE-2026-98471][MODERATE 7.0] KVM: SEV: Do cache maintenance on the source VM during intra-host migration

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-98471
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: SEV: Do cache maintenance on the source VM during intra-host migration
Commit: 37dfb676fafd1eef36fe0bb649607afbb0d5521b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=37dfb676fafd1eef36fe0bb649607afbb0d5521b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98471
Analysis date: Mon, 19 Oct 2026 00:20:53 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Missing source VM cache maintenance during SEV intra-host migration can leave source-only stale or dirty cache lines active after source destruction, creating an unresolved confidentiality and integrity risk across the SEV isolation boundary.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98471 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98471

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: MODERATE 7.0
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-2026-98471	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98471 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both 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:L';CWE-226;CWE-459;CWE-668;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '6.5';DESCR 'KVM SEV intra host migration can leave stale or dirty source VM data in CPU caches because the source VM is converted to a non SEV VM before normal memory reclaim can perform the required cache maintenance. Cache lines associated with memory available only to the source VM may therefore survive after the source VM is destroyed, since the destination VM is not required to have an identical memory configuration. This creates a potential SEV isolation failure involving disclosure of cached guest data, loss of dirty guest writes, or corruption when affected memory is reclaimed or reused. For the CVSS the PR:L value is used because triggering requires access to the relevant KVM VM file descriptors and migration controls, but this access may belong to a restricted virtualization service account rather than a fully privileged host administrator. The issue is local and is not directly network reachable. A guest can influence cached memory contents but normally cannot initiate the encryption context migration without cooperation from host userspace. Impact is potentially broader than denial of service and may include confidentiality and integrity effects across a VM isolation boundary. The exact exploitability depends on cache residency, SEV hardware behavior, source and destination memory layouts, and the ability to control the migration sequence. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 6.5 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. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because a specific evidence based protection or manual review veto was applied and FD false positive review allowed a downgrade. 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 V33_ACTIVE_STATE_BOUNDARY_CANDIDATE and IMPORTANT_CANDIDATE_MANUAL_REVIEW and KP_ONLY_DOWNGRADE_VETO and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 0 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	VIRT INIT SIMPLEFIX HARDWARE  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS V33_ACTIVE_STATE_BOUNDARY_CANDIDATE IMPORTANT_CANDIDATE_MANUAL_REVIEW KP_ONLY_DOWNGRADE_VETO DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=UNRESOLVED
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* No floor or cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Concrete active-state retirement and ownership failure. The source VM is converted to non-SEV state without cache maintenance, allowing source-associated cache lines to remain active after source destruction.
* +1 Active semantic-domain transition. Memory state moves from an SEV-protected source context to a non-SEV or reclaimed context without first retiring the old cache state.
* +1 Cross-boundary isolation risk. The destination VM may not map all memory accessible to the source, so destination-side maintenance cannot cover source-only cache lines.

Paranoid-only signal:

* +1 Plausible confidentiality or integrity consequence. Stale or dirty source-only cache lines can remain after source destruction and may affect later reuse. This is technically credible from the stated cache-maintenance requirement, but no concrete readback, protected victim, or corrupted downstream operation is demonstrated.

Not awarded:

* No UAF, OOB access, arbitrary write, controlled reclaim, callback influence, or refcount corruption is established.
* No reliable host crash or additional availability impact is shown.
* No local-unprivileged bonus was awarded because access to the complete SEV migration sequence is unresolved.
* No host-admin penalty was applied because host-admin-equivalent privilege is not positively established.
* No guest-to-host exploitation bonus was awarded. Guest code can influence cached contents, but guest-only control of the migration ioctl is not established.
* NEW52 B5-R does not apply because there is no retry-accumulated heap OOB write.
* NEW53 B3-PENDING does not apply because no shared file-backed write is established.
* V33 and V34 do not apply because no attacker-influenced structured-state write or downstream privileged consumer is identified.
* B4-PENDING does not apply because no authorization guard changes from DENY to ALLOW.

3. Reachability analysis

The complete trigger is an SEV intra-host migration performed through `sev_vm_move_enc_context_from()`. Three principals must remain separate:

* A malicious guest can influence guest memory and which data become resident or dirty in CPU caches. It cannot ordinarily invoke the host-side encryption-context migration by itself.
* A local VMM or process holding the relevant KVM VM file descriptors can request migration and select the source VM through `source_fd`. The supplied evidence does not establish whether that process must have host-admin-equivalent powers.
* A host administrator may provision the SEV VMs and VMM environment, but the patch does not prove that host root must perform every trigger operation.

The practical attacker model is therefore a local VMM or a compound guest-plus-VMM sequence, not a guest-only or network trigger. No user or network namespace capability path is involved. AMD SEV hardware and active intra-host SEV migration are prerequisites. No race or narrow timing window is required, although cache residency and differing source and destination memslots affect whether security-relevant stale state remains.

4. Primitive and severity interpretation

The exact primitive is failure to retire security-sensitive CPU cache state belonging to the source SEV VM before the source is converted to non-SEV state and destroyed. This is active hardware-backed state rather than a freed heap object, so `RealUAF=NO`.

The source and destination may have different memslots. Consequently, source-only memory can leave cache lines that destination-side maintenance will never cover. The source-to-non-SEV transition also prevents normal SEV reclaim paths from supplying the missing maintenance.

This establishes a Class B active-state and isolation-consistency primitive. It does not establish arbitrary host memory access, a host crash, privilege escalation, or a demonstrated guest-to-host compromise. Disclosure of cached plaintext or corruption caused by dirty cache lines remains a technically plausible but unresolved consequence.

`ImportantCandidate=YES` because stale SEV-protected state survives destruction across a virtualization isolation transition. `AutoDowngradeAllowed=NO` because classifying this only as cleanup bookkeeping could discard a meaningful unresolved confidentiality and integrity boundary issue. Operationally it is an Actionable Moderate issue that requires Important-vs-Moderate review rather than automatic closure.

5. One-sentence report phrase

Missing source VM cache maintenance during SEV intra-host migration can leave source-only stale or dirty cache lines active after source destruction, creating an unresolved confidentiality and integrity risk across the SEV isolation boundary.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a security-sensitive cache retirement failure across an SEV ownership transition, while the downstream exposure and required VMM privileges remain unresolved.

7. Call-site confidence

Call-site confidence: medium

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UPSTREAM PATCH SUMMARY
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Patch: KVM: SEV: Do cache maintenance on the source VM during intra-host migration
Commit: 37dfb676fafd1eef36fe0bb649607afbb0d5521b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=37dfb676fafd1eef36fe0bb649607afbb0d5521b

Commit description:

Manually perform cache maintenance on the source VM during intra-host
migration to ensure no stale data is left in CPU caches after the VM is
destroyed.  Because the source VM is "converted" to a non-SEV VM, KVM's
memory reclaim flows won't trigger cache maintenance, e.g. when all guest
memory is reclaimed in response to detaching from the mmu_notifier.

Note, relying on the destination VM to do cache maintenance isn't an option
as KVM doesn't require identical guest memory configurations, i.e. the
source VM may have access to memory that the destination VM does not.
Enforcing equivalent memory configurations is infeasible, as it would
require a *deep* comparison of memslots, e.g. to verify that not only are
the memslot identical, but what the memslots point at is also identical.

Fixes: b566393 ("KVM: SEV: Add support for SEV intra host migration")
Cc: stable@vger.kernel.org
Reported-by: Stefan Teodorescu <fane@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260923163721.1584779-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/svm/sev.c

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=37dfb676fafd1eef36fe0bb649607afbb0d5521b

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-98471 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98471

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L
  The Best / paranoid CVSS score: 6.5

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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).