* [CVE-2026-98975][IMPORTANT] xfs: stop exchanging reflink flags when exchanging mappings [ Upstream
@ 2026-10-27 7:04 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-27 7:04 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98975
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: xfs: stop exchanging reflink flags when exchanging mappings [ Upstream
Commit: 1fa2f3a7ef7ee0f2503a4b0afe24bc8c9d0820be
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1fa2f3a7ef7ee0f2503a4b0afe24bc8c9d0820be
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98975
Analysis date: Tue, 27 Oct 2026 03:04:28 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A local low-privilege XFS mapping exchange can incorrectly clear reflink state, causing a later authorized write to one file to bypass CoW and corrupt another file sharing the block.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98975 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98975
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-98975 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-98975 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT 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:L/PR:L/UI:N/S:U/C:N/I:H/A:H';CWE-284;*CWE-664;CWE-682;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '7.1';DESCR 'XFS could clear or transfer the reflink flag even when an inode still owned shared blocks outside its current disk size. A later write could bypass CoW and modify the shared physical block in place, corrupting another file that references the same extent. For the CVSS the PR:L value reflects that reliable exploitation requires a local user or process able to perform the mapping exchange and write to a participating file. This is a concrete cross file integrity violation rather than a crash only issue. Confidentiality impact is not established because the bug does not provide a read or disclosure primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7.1 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. 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 TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 2 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 DANGER DISK INIT LOG LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; no additional speculative effects scored.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +1 Local below-host-admin trigger: supplied evidence grounds a local PR:L actor able to perform the mapping exchange and later write.
* +1 Dirty-Pipe-like shared-storage corruption: writing one file modifies a shared physical block still referenced by another file because CoW is incorrectly bypassed.
* +1 Integrity impact: the commit explicitly confirms corruption of the other file, not merely stale metadata or a possible crash.
* +1 Practical exploit chain: exchange or swap mappings → reflink flag cleared/transferred incorrectly → later write proceeds in place → other file changes.
* +1 Important filesystem path: XFS reflink/COW ownership and extent-sharing semantics protect independent file contents.
* No COW ownership-bypass +2: the evidence does not establish that the attacker lacks permission to the second file or identify a protected victim.
* No confidentiality, kernel-memory corruption, UAF, crash, arbitrary write, LPE, or persistent disk-corruption claim.
* No B5-R retry-accumulated OOB write, B3 shared-backing skb write, or B4 authorization-decision bypass applies.
3. Reachability analysis
The supplied summary establishes a local reduced-privilege actor able to invoke the exchange and write a participating file; host UID 0 or host-administrator capabilities are not required. No namespace capability is involved. The actor controls the operation sequence and write data, but the evidence does not establish control over an unauthorized victim file or cross-user setup. No special hardware, race, boot option, or prior host provisioning is shown.
4. Primitive and severity reading
The primitive is corruption of active XFS reflink/COW ownership state: an inode retains shared blocks but loses the reflink flag, so the privileged write path later treats shared storage as exclusive and writes in place. This is CLASS_B non-memory cross-file integrity corruption. The changed bytes in the other file are proven; a cross-user permission bypass is unresolved. ImportantCandidate is YES under the cross-file integrity/active-state profile because a concrete corrupt state is consumed by the later CoW write path. Auto-downgrade is disallowed despite the absence of arbitrary write or LPE.
5. One-sentence report phrase
A local low-privilege XFS mapping exchange can incorrectly clear reflink state, causing a later authorized write to one file to bypass CoW and corrupt another file sharing the block.
6. Manual review advice
MANUAL CHECK REQUIRED
Confirm whether realistic exchange setups permit the second file to become separately owned or permission-protected, which would strengthen this into a full COW ownership bypass.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: xfs: stop exchanging reflink flags when exchanging mappings [ Upstream
Commit: 1fa2f3a7ef7ee0f2503a4b0afe24bc8c9d0820be
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1fa2f3a7ef7ee0f2503a4b0afe24bc8c9d0820be
Changed files:
fs/xfs/libxfs/xfs_exchmaps.c
fs/xfs/libxfs/xfs_log_format.h
fs/xfs/xfs_bmap_util.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=1fa2f3a7ef7ee0f2503a4b0afe24bc8c9d0820be
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98975 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98975
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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