[CVE-2026-89640][LOW] cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0

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

CVE: CVE-2026-89640
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0
Commit: b098f5e5858797827666e6cd73033f52fc39b5f6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b098f5e5858797827666e6cd73033f52fc39b5f6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89640
Analysis date: Sat, 12 Sep 2026 02:01:02 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89640 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89640

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89640	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89640 CHECK WITH IMPACT FROM ORIG NN 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:L/A:L';CWE-191;*CWE-20;CWE-367;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.4';DESCR 'A CIFS clone range operation with len set to zero can underflow the effective length when the source offset is beyond the source file size. This can corrupt the ByteCount sent in the FSCTL_DUPLICATE_EXTENTS_TO_FILE request and can invert the local writeback range used before the clone operation. For the CVSS the PR:L is used because a local user or process with access to the mounted CIFS share and suitable file permissions can request the remap operation without needing full administrator privileges. The issue is not directly network reachable against the Linux client because the trigger is a local filesystem operation, although it affects an SMB protocol request sent to the server. Impact is mainly a low integrity risk from an invalid clone range and a possible low availability impact from malformed or failed clone processing, with no supported path to kernel memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	OOB DISK SIMPLEFIX IMPROVEONLY SERVERTOCLIENT LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

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

2. Signal breakdown

Positive signals:

* +1 Local below-host-admin trigger. A local user or process with access to an existing CIFS mount and suitable file permissions can request a clone or remap operation.
* +1 Important filesystem/storage path. The bug affects CIFS remap/clone range handling and the SMB FSCTL_DUPLICATE_EXTENTS_TO_FILE request construction.

Non-awarded signals:

* +0 Real UAF not awarded. No stale object, post-free use, double release, refcount misuse, or RCU lifetime violation is established.
* +0 Strong corruption not awarded. The patch shows loff_t underflow and invalid range construction, not attacker-controlled kernel memory corruption.
* +0 Confidentiality not awarded. No disclosure path or read-back of protected kernel or file data is established.
* +0 Integrity bonus not separately awarded. A malformed clone range may affect operation semantics, but the evidence does not establish unauthorized modification of another principal's protected data.
* +0 Reliable kernel crash not awarded. The commit describes malformed ByteCount and inverted writeback range, but not a demonstrated kernel panic.
* +0 Remote reachability not awarded. The vulnerable operation is triggered through a local filesystem operation on the Linux CIFS client.
* +0 Security boundary bypass not awarded. No COW/shared-block ownership violation, permission bypass, or protected-file overwrite primitive is established.
* +0 Manual-review memory corruption signal not awarded. There is no UAF, OOB write, arbitrary write, controlled reclaim, or LPE evidence.

Negative signals:

* No negative score applied. The CIFS mount is a prerequisite, but the analysis does not rely on a rare configuration or proven host-admin-only trigger for the vulnerable operation once the mount exists.

3. Reachability analysis

The evaluated attacker is a local user or local process on a system where a CIFS filesystem is already mounted. The attacker needs access to files on that mount and enough file permissions to invoke the remap or clone operation. This is below host-admin in normal multi-user filesystem semantics, so HostAdminRequired is NO for the complete vulnerable operation after the mount exists.

The issue is not directly network-triggerable against the Linux client. Although the malformed FSCTL_DUPLICATE_EXTENTS_TO_FILE request is sent over SMB to a server, the attacker triggers it locally through the client filesystem API. There is prior environmental setup because a CIFS mount must exist, but that is a deployment condition rather than proof that exploitation itself requires host administrator privileges. No namespace-scoped capability requirement is involved.

No special hardware, race timing, or complex asynchronous lifetime condition is required. The TOCTOU and 32-bit torn-read note is relevant to why i_size_read is used consistently, but the main trigger described here is len == 0 with off greater than source file size.

4. Primitive and severity interpretation

The exact primitive is an integer underflow in effective clone length calculation:

off > i_size with len == 0 produces a negative loff_t length.

That invalid length can corrupt the SMB ByteCount field and invert the range passed to filemap_write_and_wait_range before the later bounds check rejects the operation. This is an invalid range / input validation flaw, not a kernel memory lifetime bug.

PrimitiveClass is NONE because no meaningful kernel corruption, UAF, ownership violation, authorization bypass, or security-boundary primitive is established. The demonstrated consequence is malformed request construction and incorrect local range handling. The plausible operational impact is low: failed clone processing, confusing server-side request handling, or limited filesystem operation disruption. A stronger consequence such as privilege escalation, arbitrary kernel write, protected file overwrite, or information disclosure is speculative and not supported by the supplied patch.

ImportantCandidate is NO because there is no Class A primitive and no Class B lifetime/corruption primitive requiring Important-vs-Moderate review. AutoDowngradeAllowed is YES because the issue is a bounded local filesystem input validation/integer underflow problem without unresolved memory-corruption or boundary-bypass leverage.

5. One-sentence report phrase

A CIFS clone-to-EOF operation can underflow its computed length when the source offset is beyond EOF, causing malformed SMB duplicate-extent request parameters and invalid local range handling, but the evidence supports only low operational impact without kernel memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is an integer underflow and validation-order bug with no UAF, OOB write, authorization bypass, COW ownership violation, demonstrated crash, or LPE primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0
Commit: b098f5e5858797827666e6cd73033f52fc39b5f6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b098f5e5858797827666e6cd73033f52fc39b5f6

Commit description:

With len == 0 (clone to EOF), the effective length is computed as:

    len = src_inode->i_size - off;

If off > i_size, this is a negative loff_t, corrupting the ByteCount
in the FSCTL_DUPLICATE_EXTENTS_TO_FILE request and inverting the range
in filemap_write_and_wait_range().  The existing off >= i_size check
fires only after the ioctl has already been sent.

Snapshot i_size_read() once for both the bounds check and the length
calculation, eliminating the TOCTOU and 32-bit torn-read risk.  Reject
off > src_size with -EINVAL.  Treat off == src_size as a no-op,
consistent with __generic_remap_file_range_prep().

Fixes: 04b38d6 ("vfs: pull btrfs clone API to vfs layer")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.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=b098f5e5858797827666e6cd73033f52fc39b5f6

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

The original Linux kernel CVE announcement for CVE-2026-89640 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89640

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:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 4.4

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).