[CVE-2026-98683][LOW] smb: client: flush dirty data before zeroing a range [ Upstream

[CVE-2026-98683][LOW] smb: client: flush dirty data before zeroing a range [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98683
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: smb: client: flush dirty data before zeroing a range [ Upstream
Commit: cecf4114069fc19629d284710fd783d56bbca862
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cecf4114069fc19629d284710fd783d56bbca862
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98683
Analysis date: Mon, 19 Oct 2026 06:30:56 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An SMB client writeback ordering bug can restore stale data after an interior zero range operation, but the demonstrated effect is limited to incorrect contents in a file the caller is already authorized to modify.

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

The original Linux kernel CVE announcement for CVE-2026-98683 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98683

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: LOW
Manual review required: NO

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-2026-98683	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98683 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT LOW AEGIS_AS=2 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N';CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.7';DESCR 'An SMB client cache coherency bug can allow dirty data to be written back after smb3_zero_range has already issued FSCTL_SET_ZERO_DATA, causing stale file contents to reappear inside a range that should contain zeros. The condition affects an interior range when a dirty folio overlaps that range and is not flushed before page cache invalidation and the server side zero operation. For the CVSS the PR:L value is selected because a local user or process must be able to invoke the zero range operation and modify the affected file on a mounted SMB share. The issue is not directly network reachable even though SMB uses a network server, because the vulnerable operation is initiated through the local filesystem interface. The demonstrated impact is loss of file data integrity rather than a kernel crash or memory corruption. The paranoid score uses I:H only for a still defensible case where stale writeback alters security sensitive or shared file contents. No UAF, OOB access, information disclosure, or privilege escalation primitive is supported by this code path. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.3 and ActionableScore 2 with primitive class CLASS_C and manual review NO. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	DISK INIT SIMPLEFIX SERVERTOCLIENT LINUS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Local unprivileged trigger. An ordinary process with write access to a file on an existing SMB mount can invoke FALLOC_FL_ZERO_RANGE without host-administrator privileges.
* +1 Important filesystem or storage path. The defect affects SMB page-cache writeback ordering and the resulting file contents stored on the server.

Negative signals:

* No negative points. The random manifestation reported by generic/363 does not establish a sufficiently narrow or uncontrollable race for a timing penalty.

Important non-awarded signals:

* No UAF, heap OOB write, arbitrary write, refcount error, or kernel-memory corruption is established.
* Integrity impact against another principal or a protected file is not established. The caller already needs permission to modify the affected file.
* The stale writeback affects the same requested file and range. No cross-file, COW, shared-backing, or cross-authority ownership violation is shown.
* No kernel crash, host-wide availability loss, information disclosure, privilege escalation, or authorization bypass is established.
* B5-R does not apply because there is no retry-accumulated heap OOB write or later consumption of corrupted heap state.
* B3-PENDING does not apply because the patch does not establish modification of independently permissioned shared or file-backed storage through missing writable isolation.
* V32, V33, and V34 do not apply because no checksum-placement write, corrupted network header, privileged downstream packet consumer, or networking trust boundary exists.
* B4-PENDING does not apply because no mandatory security guard changes from DENY to ALLOW.

3. Reachability analysis

The vulnerable operation is reached through a local filesystem request against an already mounted SMB share. A user needs write access to the target file but does not need host UID 0 or an administrative capability to invoke the operation.

Creation of the SMB mount may require prior administrative setup, but that setup is not part of each trigger once a writable share has been exposed to ordinary users. No namespace capability is involved in the vulnerable operation, and the complete trigger does not depend on special hardware, a boot option, or a privileged control-plane API.

The stale-data result requires a dirty folio overlapping an interior zero range and writeback occurring after FSCTL_SET_ZERO_DATA. The test reports random reproduction, but repeated file operations can recreate the relevant cache state. The supplied evidence does not show a narrow one-shot race that warrants a scoring penalty.

4. Primitive and severity interpretation

The exact defect is incorrect ordering between dirty page-cache writeback, cache invalidation, and server-side range zeroing. A valid dirty folio can be written back after the server has zeroed the range, restoring stale bytes where zeros are expected.

This is not a stale object after lifetime termination. The folio remains valid, and no freed kernel object is accessed. There is no repeated release, refcount mutation, OOB access, or attacker-directed kernel-memory target.

PrimitiveClass is CLASS_C because the demonstrated effect is invalid file state caused by a cache-coherency and operation-ordering defect, without an independently useful kernel-corruption or authorization primitive. The practical consequence is corruption of the expected contents of a file the caller can already modify.

A stronger cross-user integrity consequence would require evidence that the operation can modify independently protected data or bypass another principal's permissions. That mechanism is not present in the supplied patch. ImportantCandidate is therefore NO, and automatic downgrade is allowed.

5. One-sentence report phrase

An SMB client writeback ordering bug can restore stale data after an interior zero range operation, but the demonstrated effect is limited to incorrect contents in a file the caller is already authorized to modify.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a file-data consistency defect but no kernel-memory corruption, cross-authority write, authorization bypass, crash, disclosure, or privilege-escalation primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: smb: client: flush dirty data before zeroing a range [ Upstream
Commit: cecf4114069fc19629d284710fd783d56bbca862
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cecf4114069fc19629d284710fd783d56bbca862

Changed files:
  fs/smb/client/smb2ops.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=cecf4114069fc19629d284710fd783d56bbca862

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

The original Linux kernel CVE announcement for CVE-2026-98683 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98683

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:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N
  The Best / paranoid CVSS score: 4.7

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: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).