[CVE-2026-98507][MODERATE 7.0] mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish [ Upstream

[CVE-2026-98507][MODERATE 7.0] mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98507
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish [ Upstream
Commit: a500df43cde075f6ea4b8bae495d89f2c35b3e16
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a500df43cde075f6ea4b8bae495d89f2c35b3e16
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98507
Analysis date: Sun, 18 Oct 2026 22:13:33 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A local unprivileged process may race concurrent page faults so that a partially initialized anon_vma exposes a freed stale root, causing mismatched rwsem operations, permanent task hangs, and unresolved active kernel state corruption.

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

The original Linux kernel CVE announcement for CVE-2026-98507 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98507

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
======================================================================

CVE-2026-98507	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98507 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 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:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in __anon_vma_prepare can publish vma anon_vma before initialization of the anon_vma root field becomes visible to another CPU. A concurrent page fault can then obtain a stale root from reused SLAB_TYPESAFE_BY_RCU memory, lock the old root, and unlock a different root after the initialization becomes visible. This creates a use after free candidate and can corrupt rwsem state, permanently lock an anon_vma, and hang tasks. For the CVSS the PR:L is selected because an unprivileged local process can normally create adjacent VMAs, change VMA access flags with mprotect, and trigger concurrent page faults. AC:H is selected because successful triggering depends on a narrow race window and weak memory ordering. The issue is not network reachable. Impact is at least local denial of service and the explicit stale root UAF path may permit confidentiality or integrity impact if reused memory can be influenced, so privilege escalation risk requires manual review. 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 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 has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP. The independent weaker Aegis shadow evaluated CVSS 3.2 and ActionableScore 4 with primitive class CLASS_B 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	WRITE OOB LOCK INIT RACE UAF TEST KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Applied conservative floor: REAL UAF FLOOR 4 changes 3 to 4.
* Raw paranoid score: 4
* Paranoid score: 5
* Applied paranoid floor: REAL UAF FLOOR 5 changes 4 to 5 because the stale active object is used for write-like rwsem operations, not merely dereferenced during cleanup.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF and lifetime violation. A reused SLAB_TYPESAFE_BY_RCU anon_vma can expose a stale root pointing to an object whose lifetime has ended.
* +1 Local unprivileged trigger. An ordinary process can create adjacent VMAs, alter its own VMA flags with mprotect, and cause concurrent page faults.
* +1 Strong DoS. The mismatched down_write and up_write operations can permanently lock one anon_vma and corrupt another rwsem count, producing indefinitely blocked tasks.
* -1 Hard timing and ordering requirement. Triggering requires concurrent faults, publication before initialization becomes visible, reuse containing an old root, and weak memory ordering. Production reproduction was only two to five times per month.

Paranoid-only signal:

* +1 Weak exploitation concern. The stale path performs write-like locking operations against old and new rwsem objects, producing active structured-state corruption rather than an immediate invalid read or guaranteed fault.

Important non-awarded signals:

* Controlled reclaim or attacker selection of the stale root target is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Structured stale-object leverage is not separately awarded because attacker influence over replacement contents is unresolved.
* Strong LPE leverage, confidentiality impact, and integrity impact beyond the demonstrated lock corruption are not awarded.
* No extra Availability point is awarded because the permanent lock and resulting DoS are the same consequence.
* No repeated release, put, free, or refcount operation exists. The dedicated repeated-release signal and its paranoid bonus do not apply.
* Remote reachability is not established.
* NEW52 B5-R does not apply because there is no heap OOB write or retry-accumulated index.
* NEW53 B3-PENDING does not apply because no shared file-backed storage write is involved.
* NEW54 B4-PENDING does not apply because no authorization decision or mandatory security guard is bypassed.
* V32, V33, and V34 do not apply because this is not checksum metadata corruption and no guest, namespace, or delegated networking trust boundary is involved.

3. Reachability analysis

The evaluated attacker is an ordinary local process operating on its own address space. Creating mappings, changing eligible VMA access flags with mprotect, and triggering page faults do not require UID 0 or host-administrator capabilities.

No namespace capability is required, so the trigger is below host-admin privilege but is not specifically namespace-scoped. There is no network trigger, device prerequisite, boot parameter, or special physical hardware requirement. The demonstrated occurrence was on arm64, where the relevant store ordering is meaningful.

KSM was enabled in the reproduced Linux 5.10 environment and increased the number of adjacent unmerged but anon_vma-compatible VMAs because that tree lacked a later VMA-merging fix. The root cause does not prove that KSM is mandatory on every affected version, but KSM materially improves reachability in the documented environment.

The race is difficult because it depends on exact concurrent page faults, slab reuse, and visibility ordering. Repeated attempts are possible, but the supplied production frequency supports retaining the timing penalty.

4. Primitive and severity interpretation

The exact primitive is publication of a partially initialized active structured object. Thread B can observe the newly published anon_vma while still reading a stale root retained from previously freed SLAB_TYPESAFE_BY_RCU storage. It then executes down_write against the old root and up_write against the newly visible root.

This is a real UAF or post-lifetime stale-object use, not merely an invalid pointer hypothesis. The stale object is active structured state because its root selects a live rwsem and the stale path modifies lock state. The use occurs in the ordinary page-fault and anon_vma preparation path, not in cleanup or destruction.

There is no concrete repeated release or refcount mutation. There is also no demonstrated controlled reclaim, arbitrary write, information disclosure, callback influence, credential modification, or completed privilege-escalation chain. Therefore the issue remains CLASS_B rather than CLASS_A.

The demonstrated consequence is persistent lock-state corruption and local denial of service. The unresolved consequence is whether allocator shaping could make the stale root and resulting rwsem operations useful beyond DoS. This unresolved active-object corruption supports ImportantCandidate=YES and mandatory review, but it does not prove Important severity or LPE.

AutoDowngradeAllowed=NO because automatic crash-only classification would discard the explicit UAF and the fact that stale state controls write-like operations on two different kernel synchronization objects.

5. One-sentence report phrase

A local unprivileged process may race concurrent page faults so that a partially initialized anon_vma exposes a freed stale root, causing mismatched rwsem operations, permanent task hangs, and unresolved active kernel state corruption.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a real active-object UAF and write-like corruption of rwsem state reachable below host-admin privilege, while control over reuse and consequences beyond DoS remain unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish [ Upstream
Commit: a500df43cde075f6ea4b8bae495d89f2c35b3e16
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a500df43cde075f6ea4b8bae495d89f2c35b3e16

Changed files:
  mm/rmap.c
  mm/vma.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=a500df43cde075f6ea4b8bae495d89f2c35b3e16

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

The original Linux kernel CVE announcement for CVE-2026-98507 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98507

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:N/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).