[CVE-2026-99177][MODERATE 7.0] KVM: SEV: Fix page dirtying in sev_gmem_post_populate()

[CVE-2026-99177][MODERATE 7.0] KVM: SEV: Fix page dirtying in sev_gmem_post_populate()

[AL-KERNEL]

CVE: CVE-2026-99177
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: SEV: Fix page dirtying in sev_gmem_post_populate()
Commit: 184382f9852d433af21de96c334ecf43b8414ed8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=184382f9852d433af21de96c334ecf43b8414ed8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99177
Analysis date: Fri, 30 Oct 2026 10:41:40 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
KVM SEV can race source-mapping teardown with unlocked page dirtying, allowing `set_page_dirty()` to consume stale mapping/a_ops state and potentially crash or corrupt the host kernel.

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

The original Linux kernel CVE announcement for CVE-2026-99177 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99177

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-2026-99177	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99177 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 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:H';CWE-362;*CWE-416;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in KVM SEV sev_gmem_post_populate can occur because set_page_dirty is called for a page obtained through get_user_pages_fast lacking a page lock. The GUP reference keeps the page object alive but does not necessarily stabilize its mapping while another thread removes or truncates the source mapping. set_page_dirty can inspect the mapping and its a_ops after the mapping lifetime changes, making this a potential use after free rather than only an incorrect dirty accounting issue. For the CVSS the PR:L value is selected because triggering requires a local process with access to KVM SEV VM management and control over the source mapping. AC:H reflects the need to win a mapping lifetime race. Impact is at least a host denial of service and may include confidentiality or integrity impact if the stale mapping access is exploitable. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. 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_KPANIC 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 2.6 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. 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 SIMPLEFIX LEAK HARDWARE LINUS KPANIC MEMORY  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=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 4. REAL UAF FLOOR 4 changes 2 -> 4.
* Raw paranoid score: 4.
* Paranoid score: 5. REAL UAF FLOOR 5 changes 4 -> 5 because the stronger reading has attacker-driven mapping retirement followed by structured mapping/a_ops consumption.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 real lifetime violation: the GUP reference preserves `struct page`, but does not stabilize `page->mapping`; unlocked `set_page_dirty()` can consume mapping state while it is removed.
* +1 plausible host-kernel crash: stale mapping or address-space operations can cause a fatal dereference.
* -1 hard race: successful triggering requires concurrency between dirtying and source-mapping removal/truncation; the supplied hints characterize this as AC:H.
* Paranoid +1 attacker-influenced lifetime transition: the VMM process is described as controlling both the KVM operation and source mapping.
* Paranoid +1 structured stale-object leverage: `set_page_dirty()` may inspect the stale mapping and its `a_ops`, materially beyond passive dirty accounting.
* No controlled reclaim, attacker-shaped replacement, arbitrary write, callback hijack, disclosure, or LPE is shown. No separate confidentiality/integrity or extra availability point is awarded.
* This is not B5-R, B3-PENDING, or an authorization bypass.

3. Reachability analysis

The grounded actor is a local VMM process with KVM SEV VM-management access and control of the source mapping, not guest code alone. The patch does not establish whether `/dev/kvm`/SEV access is delegated below host administration, requires host UID 0, or is namespace-scoped. The trigger therefore remains privilege-unknown rather than proven unprivileged. SEV-capable configuration is required, but the evidence does not justify a rare-hardware penalty. Repeated race attempts appear possible, although reliable race control is not shown.

4. Primitive and severity reading

This is an active structured-object lifetime mismatch: a live page reference survives, while its mapping association can disappear before `set_page_dirty()` consumes mapping and address-space-operation state. It is CLASS_B, not fixed stale storage and not a proven CLASS_A corrupt primitive. The shown effect is potential stale structured access and host DoS; useful post-reuse behavior remains unknown.

ImportantCandidate is YES because attacker-influenced mapping retirement can feed stale active mapping state into a privileged host-kernel consumer, while the delegated VMM/host trust boundary and downstream leverage remain unresolved. Auto-downgrade is disallowed pending review, but guest escape, arbitrary host access, and Important severity are not proven.

5. One-sentence report phrase

KVM SEV can race source-mapping teardown with unlocked page dirtying, allowing `set_page_dirty()` to consume stale mapping/a_ops state and potentially crash or corrupt the host kernel.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm the complete VMM privilege boundary and inspect whether stale mapping/a_ops reuse permits effects beyond a host crash.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: SEV: Fix page dirtying in sev_gmem_post_populate()
Commit: 184382f9852d433af21de96c334ecf43b8414ed8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=184382f9852d433af21de96c334ecf43b8414ed8

Commit description:

set_page_dirty() requires that the caller holds some kind of lock to
ensure that the page's mapping does not concurrently go away.
That is not the case for a random page we got from get_user_pages_fast(),
so use set_page_dirty_lock().

Fixes: 97cd21d ("KVM: SEV: Mark source page dirty when writing back CPUID data on failure")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://patch.msgid.link/20260810-x86-kvm-setpagedirty-v1-1-85f180892d4f@google.com
[sean: tag for stable]
Signed-off-by: Sean Christopherson <seanjc@google.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=184382f9852d433af21de96c334ecf43b8414ed8

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

The original Linux kernel CVE announcement for CVE-2026-99177 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99177

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: 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).