[CVE-2026-98685][MODERATE REGULAR] smb: client: discard post-EOF pagecache when extending a file via clone range

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

CVE: CVE-2026-98685
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: smb: client: discard post-EOF pagecache when extending a file via clone range
Commit: 15bb9f1d3e967e328649e92a1451e60a0dc66118
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15bb9f1d3e967e328649e92a1451e60a0dc66118
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98685
Analysis date: Sat, 24 Oct 2026 09:33:42 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
CIFS clone-range extension can expose attacker-dirtied bytes retained past the old EOF because the straddling page-cache folio was not invalidated.

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

The original Linux kernel CVE announcement for CVE-2026-98685 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98685

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 REGULAR
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-2026-98685	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98685 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=1 LEGACY=MODERATE RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';CWE-459;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '3.6';DESCR 'An SMB client pagecache consistency flaw can leave the folio crossing the old EOF cached when cifs_remap_file_range extends a file. Data dirtied past EOF through mmap can then become visible after i_size grows and may replace expected cloned content for local readers. For the CVSS the PR:L value is used because reliable triggering requires a local process with permission to mmap or modify the destination file and invoke clone range. AC:H reflects the required ordering and page alignment conditions. The demonstrated primitive is stale attacker influenced file data rather than UAF OOB access or kernel memory corruption. Integrity impact is supported. Limited confidentiality impact is retained only in the paranoid vector because visibility to another reader depends on file sharing and permission boundaries. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 3.6 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. 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_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE.';NO MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	READ DISK SIMPLEFIX SERVERTOCLIENT LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 4.
* Paranoid score: 4; speculative cross-user disclosure was not scored.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Local unprivileged trigger: a process permitted to mmap/modify the destination and invoke clone range can exercise the sequence without host-administrator power.
* +1 Practical integrity chain: post-EOF mmap dirtiness survives the incomplete invalidation, clone extends `i_size`, and the stale bytes become visible as file content.
* +1 Integrity plausible: stale attacker-influenced bytes can replace content expected from the clone operation.
* +1 Important filesystem/storage path: the defect affects CIFS page-cache invalidation and clone/remap file-content consistency.
* No confidentiality point: no protected prior data or concrete cross-user disclosure is shown.
* No ownership/COW-breach point: the evidence does not show modification of a separately permissioned file or backing object.
* No UAF, OOB write, arbitrary write, authorization bypass, kernel crash, or privilege escalation is shown.
* NEW52/B5-R and NEW53/B3-PENDING do not apply: this is stale page-cache consumption, not retry-accumulated heap overwrite or a missing writable-isolation shared-backing write.

3. Reachability analysis

The grounded attacker is a local process with ordinary access sufficient to mmap or modify the destination file and request clone/remap. No UID 0, capability, namespace-specific privilege, special hardware, or prior host-admin setup is shown. Triggering requires page-boundary and operation-order conditions, but the evidence does not establish an uncontrollable race warranting a timing penalty.

4. Primitive and severity reading

The primitive is attacker-influenced stale page-cache data becoming active file content after an EOF-extending clone. It is a constrained CLASS_B semantic file-integrity defect, not kernel-memory corruption. The patch-supported effect is incorrect local file content; cross-user disclosure, protected-file corruption, persistence to the SMB server, and privilege escalation remain unproven. ImportantCandidate is NO because no ownership or authority boundary is demonstrated. Auto-downgrade is allowed because the impact remains confined to a file the attacker can already modify.

5. One-sentence report phrase

CIFS clone-range extension can expose attacker-dirtied bytes retained past the old EOF because the straddling page-cache folio was not invalidated.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm product-specific clone permissions and whether a separately trusted reader relies on the destination file, but no Important-preserving primitive is presently shown.

7. Call-site confidence

Call-site confidence: high

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

Patch: smb: client: discard post-EOF pagecache when extending a file via clone range
Commit: 15bb9f1d3e967e328649e92a1451e60a0dc66118
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15bb9f1d3e967e328649e92a1451e60a0dc66118

Commit description:

cifs_remap_file_range() discarded the pagecache only for the folios
overlapping the destination region, so when a clone extends the file the
folio straddling the old EOF - which may hold data dirtied past EOF
through an mmap - was never dropped and became visible as file content
once the file was extended.

Start the discard at the smaller of fstart and the old EOF.  i_size is
still old here, so nothing past the old EOF is flushed to the server.

Fixes: c54fc3a ("cifs: Fix flushing, invalidation and file size with FICLONE")
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/cifsfs.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=15bb9f1d3e967e328649e92a1451e60a0dc66118

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

The original Linux kernel CVE announcement for CVE-2026-98685 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98685

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:L/I:L/A:N
  The Best / paranoid CVSS score: 3.6

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: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).