[CVE-2026-98447][MODERATE 7.0] xfs: fix wild memcpy access when formatting ondisk rtrefcount btree roots

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

CVE: CVE-2026-98447
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfs: fix wild memcpy access when formatting ondisk rtrefcount btree roots
Commit: d46492fb8018d12d00651b06b9d240e747110b38
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d46492fb8018d12d00651b06b9d240e747110b38
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98447
Analysis date: Mon, 19 Oct 2026 01:47:46 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
An oversized memcpy in XFS realtime refcount btree root conversion performs a confirmed out-of-bounds write beyond structured filesystem buffers, creating credible kernel or metadata corruption that requires manual exploitation and reachability analysis.

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

The original Linux kernel CVE announcement for CVE-2026-98447 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98447

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98447	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98447 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 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-787;CWE-125;CWE-119;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An oversized memcpy in the XFS realtime refcount btree root conversion copies twice the valid key array size. This produces an out of bounds read and an out of bounds write that can corrupt memory located beyond the source and destination buffers. The issue can be reached while formatting or converting an ondisk realtime refcount btree root on an affected XFS filesystem. For the CVSS the PR:L reflects a local user or process that can perform filesystem operations capable of changing the affected realtime refcount metadata after the filesystem has already been configured and mounted. AC:H reflects the requirement for a specific XFS realtime configuration and a suitable btree root state. The issue is not directly network reachable. Impact is at least a local denial of service through kernel or filesystem failure and may include confidentiality and integrity impact because a concrete kernel memory corruption primitive is present. Privilege escalation is plausible but not demonstrated and the exact overwrite control requires manual source and layout analysis. 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 2.7 and ActionableScore 3 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	DANGER DISK SIMPLEFIX LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

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

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative floor: 4 -> 5 because the patch establishes a concrete kernel out-of-bounds write whose adjacent target and exploitation leverage remain unresolved.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Concrete kernel memory corruption. Both conversion directions use an oversized `memcpy`, producing an actual out-of-bounds write beyond the intended btree-root buffers.
* +1 Important filesystem path. The affected code converts XFS realtime refcount btree roots between in-memory and on-disk representations.
* +1 Reliable strong DoS concern. A write beyond an allocated or formatted root buffer can realistically cause an assertion, filesystem shutdown, metadata failure, or kernel fault.
* +1 Integrity concern. The erroneous copy can overwrite adjacent kernel or filesystem metadata rather than merely reading it.
* -1 Unusual and difficult-to-instantiate configuration. The vulnerable path requires the new XFS realtime refcount feature and corresponding filesystem layout, which normally requires prior filesystem provisioning and mounting.
* Total: 2 + 1 + 1 + 1 - 1 = 4.

Paranoid difference:

* +1 Unresolved exploitation leverage. The copied length depends on `numrecs`, and the source contains structured btree keys. The exact allocation layout may permit corruption of adjacent active state, but neither the target object nor useful payload control is established.
* The conservative floor and paranoid arithmetic both produce the final score of 5.

Important non-awarded signals:

* No UAF, stale lifetime operation, repeated release, or refcount mutation is present.
* No arbitrary or directed write is demonstrated.
* No controlled adjacent victim, callback, credential target, or controlled reclaim is demonstrated.
* No confidentiality point is awarded because the OOB read has no established disclosure path.
* No local-unprivileged bonus is awarded because the supplied diff does not establish that an ordinary user can force the complete root-formatting transition.
* No remote-reachability bonus is awarded.
* B5-R does not apply. There is no retry-accumulated index or offset.
* B3-PENDING does not apply. No shared-backing or COW write across an authority boundary is established.
* V33 active-state cross-boundary promotion is not established. A privileged consumer and concrete guest, namespace, or delegated-interface boundary are not identified.
* B4-PENDING does not apply because this is not an authorization decision defect.

3. Reachability analysis

The affected functions run when XFS converts a realtime refcount btree root between its on-disk and in-memory forms. The vulnerable filesystem must use the realtime refcount feature introduced by the referenced XFS change.

Creating, configuring, and mounting such a filesystem ordinarily involves a privileged actor. However, the supplied code does not resolve whether normal operations by a non-administrative user on an already mounted affected filesystem can force the relevant btree-root conversion and record count. Therefore `HostAdminRequired=UNKNOWN` rather than assuming either host administration or ordinary-user reachability.

No namespace-scoped capability path is shown. No special physical hardware is required. This is not a race and has no timing penalty. It is not directly network-triggerable, although a network file service could theoretically expose filesystem operations only after separate privileged export configuration.

4. Primitive and severity interpretation

The exact primitive is a length-error OOB read followed by an OOB write in each affected conversion direction:

* `xfs_rtrefcountbt_from_disk` copies twice the valid key-array size into an in-memory btree-root buffer.
* `xfs_rtrefcountbt_to_disk` copies twice the valid key-array size into an on-disk root-format buffer.

The commit explicitly confirms memory corruption beyond the buffers. This is broader than an invalid read or warning-only defect. It is classified as `CLASS_B` because concrete corruption exists, but the supplied evidence does not establish meaningful attacker control over the adjacent target, a directed write, or a completed security-boundary exploit.

Demonstrated consequences are corruption of adjacent memory or metadata and a credible filesystem or kernel DoS. Privilege escalation and sensitive-data disclosure remain speculative. Manual review should inspect allocation layouts surrounding `rblock` and `dblock`, the maximum validated `bb_numrecs`, whether the following pointer copy repairs any overwritten region, and whether ordinary writable-file operations can force the conversion repeatedly.

`ImportantCandidate=YES` preserves the direct OOB write for exploitation review. `AutoDowngradeAllowed=NO` because automatically treating the issue as crash-only would discard unresolved kernel-memory and filesystem-metadata corruption leverage.

5. One-sentence report phrase

An oversized memcpy in XFS realtime refcount btree root conversion performs a confirmed out-of-bounds write beyond structured filesystem buffers, creating credible kernel or metadata corruption that requires manual exploitation and reachability analysis.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a real kernel OOB write, while the adjacent victim layout, attacker control over the root state, below-host-admin triggerability, and consequences beyond filesystem or kernel failure remain unresolved.

7. Call-site confidence

Call-site confidence: medium

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

Patch: xfs: fix wild memcpy access when formatting ondisk rtrefcount btree roots
Commit: d46492fb8018d12d00651b06b9d240e747110b38
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d46492fb8018d12d00651b06b9d240e747110b38

Commit description:

LOLLM noticed that the inode btree root formatting methods copy too many
bytes -- there's only one set of keys in node blocks, not two.  This
causes memory corruption of whatever's beyond the buffers.

Cc: stable@vger.kernel.org # v6.14
Fixes: f0415af ("xfs: wire up a new metafile type for the realtime refcount")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/xfs/libxfs/xfs_rtrefcount_btree.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=d46492fb8018d12d00651b06b9d240e747110b38

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

The original Linux kernel CVE announcement for CVE-2026-98447 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98447

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: 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: 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).