[CVE-2026-98718][IMPORTANT] KVM: SEV: Do cache maintenance on the source VM *before* clearing SEV state

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

CVE: CVE-2026-98718
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: KVM: SEV: Do cache maintenance on the source VM *before* clearing SEV state
Commit: 8b7d4f68fef96cebe0b6dbb6e5607772337f7369
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8b7d4f68fef96cebe0b6dbb6e5607772337f7369
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98718
Analysis date: Sat, 24 Oct 2026 04:21:22 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A race in SEV migration allows late cache writeback to overwrite a reclaimed page after it has been reassigned to another host owner.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-98718 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98718

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-2026-98718	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98718 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 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-362;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H';BEST CVSS score: '7';DESCR 'A race in KVM SEV intra-host migration allowed the source VM to lose its SEV active state before cache maintenance completed. A concurrent unmap or reclaim could then free and reallocate a page without the normal cache flush, after which WBNOINVD could write stale guest ciphertext over data belonging to the new page owner. This is a concrete cross-owner memory integrity violation and may corrupt kernel or process memory, cause a host crash, or under a favorable allocation potentially support host privilege escalation. For the CVSS the PR:L value reflects that reliable triggering normally requires access to KVM or a VMM service account that can coordinate SEV migration and memory teardown. AC:H reflects the tiny race window between clearing the SEV state and completing reclaim or page reuse. The issue is not directly network reachable and a guest lacking control over migration is not by itself a proven trigger. 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 1.9 and ActionableScore 4 with primitive class CLASS_B 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	VIRT SIMPLEFIX HARDWARE INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4
* Conservative score: 5; manual-review safety floor applied for a concrete strong cross-owner post-lifetime write.
* Raw paranoid score: 4
* Paranoid score: 5; same safety floor, with no additional speculative exploit points.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Concrete post-lifetime cross-owner write: after reclaim and reallocation, WBNOINVD can write stale guest ciphertext over the new page owner’s data.
* +1 — Active-state retirement/synchronization failure: `src->active` was cleared before cache maintenance, causing concurrent reclaim to skip its required flush.
* +1 — Integrity impact: data belonging to a distinct host page owner is actually overwritten.
* +1 — Practical chain: migration state change → reclaim/reallocation → late cache writeback → new-owner corruption is explicitly described.
* +1 — Plausible host crash if the reallocated page contains kernel or process-critical state.
* -1 — Complete trigger is restricted to a below-host-admin KVM/VMM service actor able to coordinate migration and memory teardown.
* -1 — The commit identifies a tiny reclaim/reallocation race window.
* No arbitrary target, controlled allocation, controlled reclaim, confidentiality impact, or LPE is shown.
* No extra availability point: the possible host crash is counted once.

3. Reachability analysis

The supported attacker is a local KVM/VMM service actor with access to the VM migration and memory-unmap lifecycle, not ordinary guest code. A guest alone cannot initiate the required host-side migration and teardown sequence. The supplied evidence supports reduced, below-host-administrator VMM privilege, but no namespace-capability path. AMD SEV hardware and intra-host migration are prerequisites. The attacker can coordinate the relevant operations, but host page reuse within the tiny window remains timing-dependent and the eventual victim allocation is uncontrolled.

4. Primitive and severity reading

This is stale post-lifetime physical-page/cache state rather than a structured-object UAF: guest-owned dirty cache contents remain operational after the page has been freed and reassigned. The subsequent hardware cache flush performs a real write into storage owned by another host principal. That concrete cross-owner overwrite is CLASS_A even though its payload and target are constrained.

Shown effects are new-owner data corruption and plausible host failure. Targeted kernel corruption, disclosure, privilege escalation, and guest escape remain unproven. ImportantCandidate is YES because a real host ownership-boundary corruption primitive would be lost by automatic downgrade.

5. One-sentence report phrase

A race in SEV migration allows late cache writeback to overwrite a reclaimed page after it has been reassigned to another host owner.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm practical VMM privilege, race reliability, allocator targeting, and whether affected deployments permit corruption of security-sensitive host pages.

7. Call-site confidence

Call-site confidence: high

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: KVM: SEV: Do cache maintenance on the source VM *before* clearing SEV state
Commit: 8b7d4f68fef96cebe0b6dbb6e5607772337f7369
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8b7d4f68fef96cebe0b6dbb6e5607772337f7369

Commit description:

Explicitly flush caches after intra-host migration before clearing "SEV
active" on the source VM, as doing cache maintenance afterwards creates a
tiny window where memory reclaim could return memory to the host without
performing a cache flush, e.g. as pointed out by Sashiko:

  CPU1 in sev_migrate_from():
    src->active = false;

  CPU2 running concurrent unmap:
    Since active is false, the automatic cache flush in
    sev_guest_memory_reclaimed is skipped.
    The host frees and reallocates the page.

  CPU1 in sev_migrate_from():
    sev_writeback_caches(src_kvm);
    Executes a hardware cache flush (wbnoinvd), which writes the guest's old
    dirty ciphertext over the new page owner's data.

Fixes: 93de2a6 ("KVM: SEV: Do cache maintenance on the source VM during intra-host migration")
Cc: stable@vger.kernel.org
Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260923165304.1662E1F000FF@smtp.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260928154644.2559454-3-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=8b7d4f68fef96cebe0b6dbb6e5607772337f7369

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

The original Linux kernel CVE announcement for CVE-2026-98718 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98718

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:N/I:H/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: unknown
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).