[CVE-2026-98974][IMPORTANT] xfs: fix exchange-range-to-eof file size exchange [ Upstream

[CVE-2026-98974][IMPORTANT] xfs: fix exchange-range-to-eof file size exchange [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98974
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: xfs: fix exchange-range-to-eof file size exchange [ Upstream
Commit: b361d32b884f8ddcb9bad5d99b69eece648f8e43
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b361d32b884f8ddcb9bad5d99b69eece648f8e43
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98974
Analysis date: Mon, 26 Oct 2026 21:44:39 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
XFS TO_EOF exchange mishandling can clear reflink protection and let a local user modify a readable but non-writable file through shared physical blocks.

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

The original Linux kernel CVE announcement for CVE-2026-98974 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98974

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

CVE-2026-98974	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98974 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT IMPORTANT AEGIS_AS=4 LEGACY=IMPORTANT RESULT=SAME (final reconciliation agrees; DECISION: both IMPORTANT)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H';CWE-682;CWE-284;CWE-693;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '7.1';DESCR 'Incorrect TO_EOF size handling in XFS exchange range can leave shared written blocks beyond the apparent EOF. A later exchange can then incorrectly clear the reflink flag, after which an extending write modifies a physical block that is still shared with another file. This is a concrete cross file integrity violation and can let a local attacker alter a readable but non writable file through attacker controlled files on the same reflink enabled XFS filesystem. For the CVSS the PR:L value is selected because exploitation requires a local user or process that can create and modify files, issue the relevant clone and exchange range operations, and read the victim file used as the reflink source. No race is required and the demonstrated sequence is deterministic once filesystem and permission prerequisites are met. Privilege escalation is plausible if the corrupted peer is a trusted file later consumed by a privileged component, while confidentiality impact is not established because the attacker already needs read access to clone the source data. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7.1 and the production ActionableScore returned 7 with recommendation Strong Important candidate. 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. final reconciliation stage IMPORTANT to IMPORTANT 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 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 KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 6.5 and ActionableScore 4 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	MAYBE	DANGER DISK INIT LOG LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=7
PrimitiveClass=CLASS_A
RealUAF=NO
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: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7.
* Paranoid score: 7; no additional speculative effects awarded.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 Dirty-Pipe-like ownership/COW breach: an attacker-authorized write to `file1` modifies block 100 still shared with the separately permissioned `peer`.
* +1 Integrity plausible: the demonstrated sequence changes contents of a readable but non-writable peer file.
* +1 Local unprivileged trigger: the complete PoC uses ordinary file, clone, exchange-range, and write operations without host-admin powers.
* +1 Practical exploit chain: incorrect TO_EOF sizes → shared blocks beyond EOF → erroneous reflink-flag clearing → write without COW → peer modification.
* +1 Filesystem metadata corruption reachable locally: inode size and reflink ownership state become inconsistent.
* +1 Important filesystem path: XFS reflink/COW mapping and ownership invariants are directly violated.

Not awarded: confidentiality, kernel crash, memory corruption, arbitrary kernel write, LPE, or persistent disk corruption. The same ownership breach is not counted again as strong exploit leverage.

3. Reachability analysis

A local user needs readable access to the source peer and writable attacker-controlled files on the same XFS filesystem with reflink and exchange-range support. The user can invoke the full deterministic sequence; no race, namespace capability, UID 0, or host-admin operation is shown as necessary. Filesystem feature availability is a deployment prerequisite, not per-trigger privileged setup.

4. Primitive and severity reading

The primitive is a concrete cross-file COW/ownership violation, not a UAF or kernel-memory overwrite. Incorrect file-size exchange leaves a shared extent beyond apparent EOF; a later exchange clears the reflink flag, and an extending write modifies storage still owned by another file. This is CLASS_A because protected shared-file corruption is demonstrated. Privilege escalation through a trusted consumer remains unproven, but the cross-authority integrity effect itself supports Important candidacy and forbids automatic downgrade. B4-PENDING does not apply because no authorization predicate changes DENY to ALLOW.

5. One-sentence report phrase

XFS TO_EOF exchange mishandling can clear reflink protection and let a local user modify a readable but non-writable file through shared physical blocks.

6. Manual review advice

MANUAL CHECK REQUIRED

The demonstrated cross-file ownership/COW breach is Important-preserving even without proven LPE or arbitrary kernel write.

7. Call-site confidence

Call-site confidence: high

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

Patch: xfs: fix exchange-range-to-eof file size exchange [ Upstream
Commit: b361d32b884f8ddcb9bad5d99b69eece648f8e43
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b361d32b884f8ddcb9bad5d99b69eece648f8e43

Changed files:
  fs/xfs/libxfs/xfs_exchmaps.c
  fs/xfs/xfs_exchrange.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=b361d32b884f8ddcb9bad5d99b69eece648f8e43

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

The original Linux kernel CVE announcement for CVE-2026-98974 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98974

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:L/PR:L/UI:N/S:U/C:N/I:H/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 7.1

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: 7

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).