[CVE-2026-98684][LOW] smb: client: flush and commit data before querying allocated ranges

[CVE-2026-98684][LOW] smb: client: flush and commit data before querying allocated ranges

[AL-KERNEL]

CVE: CVE-2026-98684
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: smb: client: flush and commit data before querying allocated ranges
Commit: f82242fe3109a3ef72f429540b5bdde08fbfb0e4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f82242fe3109a3ef72f429540b5bdde08fbfb0e4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98684
Analysis date: Sun, 18 Oct 2026 22:30:47 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An SMB allocation-query coherency flaw can cause repeatable zeroing of recently written file data on Windows servers, but the demonstrated impact is limited to corruption of a file the local caller is already authorized to modify.

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

The original Linux kernel CVE announcement for CVE-2026-98684 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98684

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-2026-98684	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98684 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:L/PR:L/UI:N/S:U/C:N/I:H/A:N';CWE-367;CWE-662;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '5.5';DESCR 'A coherency flaw in SMB fallocate emulation can cause valid file contents to be overwritten with zeroes when FSCTL_QUERY_ALLOCATED_RANGES reports recently written data as an unallocated hole. The issue is reproducible against Windows Server 2022 when a write is followed by the allocated range query before the server has committed the data. For the CVSS the PR:L value is used because a local process must have access to the SMB mounted file and invoke the affected fallocate operation. The issue is not directly network reachable by an unrelated remote attacker. The SMB server response is a required condition, but the server already controls the remotely stored file contents. Impact is loss of file integrity and possible permanent data corruption. There is no supported kernel memory corruption, information disclosure, kernel crash, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 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 2.8 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	DANGER DISK INIT 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
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ActionableScore=2
PrimitiveClass=NONE
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. A process with ordinary write access to a file on an existing SMB mount can invoke the affected fallocate operation without host-administrator powers.
* +1 Important filesystem or storage path. The defect causes confirmed and persistent corruption of file contents by replacing recently written data with zeroes.

Negative and non-awarded signals:

* No kernel memory-corruption point. The write targets the requested range of the remote file, not heap or slab memory.
* No integrity bonus. The evidence does not show modification of data that the caller is not already authorized to write.
* No authorization-bypass point. No access-control decision changes from DENY to ALLOW.
* No practical security-exploit-chain point. The demonstrated chain ends in corruption of the operated file, without cross-user access, privilege escalation, or protected-object modification.
* No remote-reachability point. SMB communication is involved, but an unrelated remote host cannot directly invoke the vulnerable local fallocate path.
* No broad-exposure point. SMB is common, but this specific interior-range fallocate emulation path and incoherent Windows response are narrower than ordinary SMB file access.
* No race penalty. The coherency failure is repeatably reproduced under fsx and is not shown to require an uncontrollable one-shot race.
* B5-R does not apply. There is no heap or slab OOB write, retry-accumulated index, or later consumption of corrupted kernel state.
* B3-PENDING does not apply. No write through shared backing into an independently protected file or page is established.
* V33 and V34 do not apply. No attacker-positioned checksum write or privileged downstream consumption of corrupted packet metadata exists.
* B4-PENDING does not apply. There is no missing mandatory authorization state or concrete DENY-to-ALLOW transition.

3. Reachability analysis

An ordinary local process can trigger the behavior if it has write access to a file on an already mounted SMB share and invokes mode 0 or FALLOC_FL_KEEP_SIZE fallocate for an affected interior range. The SMB mount normally represents prior administrative or system setup, but the vulnerable operation itself does not require host UID 0 or an administrative capability once the writable file is exposed.

No namespace capability is required, so the complete trigger is not dependent on namespace-scoped privilege. Windows Server behavior is a prerequisite because its allocated-range query may be incoherent with recent writes. Samba was tested and did not exhibit the vulnerable server-side response. No special hardware or narrow uncontrollable race is required.

4. Primitive and severity interpretation

The exact behavior is a stale-coherency decision followed by a bounded zero-fill of a valid file range. Recently written data remains uncommitted when FSCTL_QUERY_ALLOCATED_RANGES reports the range as a hole, and the SMB client then overwrites the live data with zeroes.

This is persistent file-content corruption, but it is not a heap OOB write, UAF, stale kernel-object use, arbitrary kernel write, COW ownership violation, or authorization bypass. The supplied evidence does not establish corruption of another principals independently protected data because the caller must already be permitted to modify the selected file.

PrimitiveClass is NONE because no meaningful kernel corruption, lifetime, ownership, or security-boundary primitive is established. ImportantCandidate is NO because the demonstrated consequence remains authorized-file data loss without a credible privilege or trust-boundary crossing. AutoDowngradeAllowed is YES because no unresolved Important-preserving primitive remains.

5. One-sentence report phrase

An SMB allocation-query coherency flaw can cause repeatable zeroing of recently written file data on Windows servers, but the demonstrated impact is limited to corruption of a file the local caller is already authorized to modify.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch demonstrates a serious correctness and data-loss defect, but not kernel memory corruption, authorization bypass, cross-owner modification, remote attack, or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: smb: client: flush and commit data before querying allocated ranges
Commit: f82242fe3109a3ef72f429540b5bdde08fbfb0e4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f82242fe3109a3ef72f429540b5bdde08fbfb0e4

Commit description:

The mode 0 / FALLOC_FL_KEEP_SIZE fallocate emulation for small interior
ranges in smb3_simple_fallocate_range() issues
FSCTL_QUERY_ALLOCATED_RANGES and zero-fills any sub-range the server
reports as an unallocated hole, to force block allocation without
changing file contents.

This is unsafe against Windows servers.  Per the Win32 documentation,
FSCTL_QUERY_ALLOCATED_RANGES only reports ranges that *may* contain
nonzero data and is explicitly not coherent with recently written data:

  "a call to FSCTL_QUERY_ALLOCATED_RANGES [after writing to a network
   file] would not necessarily return a correct list of allocated
   regions.  To ensure coherency [...] flush the data to the file"

fsx (generic/363) reproduces the resulting corruption against Windows
Server 2022: a range is written, an uncached read returns the data, yet
the next FSCTL_QUERY_ALLOCATED_RANGES on that range reports it as a whole
hole, so the emulation overwrites live data with zeroes.  A network trace
confirmed the WRITE, the READ returning data and the query returning an
empty range list within microseconds.  Samba does not exhibit this; its
query is coherent with the written data.

Write back the dirty pages, drain any outstanding I/O and issue an SMB2
FLUSH to force the server to commit the data before querying the
allocated ranges, so the query reflects the data actually present.

With this, generic/363 passes against both Windows Server 2022 and Samba
over 100000 fsx operations.

Fixes: 966a3cb ("cifs: improve fallocate emulation")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=f82242fe3109a3ef72f429540b5bdde08fbfb0e4

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

The original Linux kernel CVE announcement for CVE-2026-98684 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98684

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:L/PR:L/UI:N/S:U/C:N/I:H/A:N
  The Best / paranoid CVSS score: 5.5

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).