[CVE-2026-89630][MODERATE 7.0] smb: client: restore the data_offset bound in is_valid_oplock_break()

[CVE-2026-89630][MODERATE 7.0] smb: client: restore the data_offset bound in is_valid_oplock_break()

[AL-KERNEL]

CVE: CVE-2026-89630
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: restore the data_offset bound in is_valid_oplock_break()
Commit: 5b16a1967a01ad4496a7206a87c3eb16f1df2b05
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5b16a1967a01ad4496a7206a87c3eb16f1df2b05
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89630
Analysis date: Sat, 12 Sep 2026 04:34:39 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89630 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89630

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 7.0
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-2026-89630	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89630 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H';CWE-125;*CWE-20;CWE-190;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'An out of bounds read in the SMB client oplock break validation path can occur because is_valid_oplock_break uses an ineffective DataOffset bound after the RFC1002 preamble accounting changed. A malicious or compromised SMB server can send a crafted SMB1 notification or oplock related response with an invalid DataOffset, causing the Linux client kernel to read beyond the received SMB message and potentially crash. For the CVSS the PR:N because the attacker does not need privileges on the victim and only needs control of or ability to impersonate an SMB server that the client communicates with. UI:R is used because the client normally has to mount or connect to the malicious server or an already configured share must point to a compromised endpoint. The issue is network reachable from server to client. Impact is at least denial of service via kernel crash, with limited confidentiality impact considered only in the paranoid score due to the out of bounds read primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	DISK SIMPLEFIX IMPROVEONLY SERVERTOCLIENT LINUS KPANIC 	YES	NO	checked

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

ActionableScore=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap changed the score.

2. Signal breakdown

Positive signals:

* +2 Remote reachable / network-triggerable. A malicious or compromised SMB server can send a crafted SMB1 oplock or notification response to a Linux SMB client that communicates with it.
* +1 Weak read primitive / OOB read candidate. The broken DataOffset bound allows the client to accept offsets beyond the received SMB message.
* +1 Reliable kernel crash / strong DoS concern. The earlier bound was explicitly meant to prevent a read overflow, and unchecked kernel parsing of an attacker-controlled offset can plausibly lead to an Oops or kernel crash.

Negative signals:

* No negative privilege penalty. Host-admin-equivalent privilege is not required by the remote SMB server attacker.
* No race/timing penalty. This is a deterministic bounds-check regression, not a timing race.
* No legacy SMB1 penalty applied. The path is SMB1-specific, but the supplied evidence does not establish a required rare deployment constraint strong enough to subtract points.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* UAF not demonstrated.
* Confidentiality not awarded because no read-back or disclosure path is shown.
* Integrity not awarded because no unauthorized write or state modification primitive is shown.
* Extra Availability not awarded because the same crash is already counted once.
* ImportantCandidate not set because the established primitive is an OOB read / crash risk without a demonstrated disclosure, write, lifetime corruption, or security-boundary bypass.

Repeated release / refcount analysis:

* First release can end lifetime: not applicable.
* Exact same reference survives: not applicable.
* Second lifetime/refcount mutation exists: no.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The likely attacker is a malicious, compromised, or impersonated SMB server. The victim side is the Linux SMB client. The vulnerable path is reached when the client processes an SMB1 oplock break or notification-related response.

The attacker does not need local code execution or privileges on the victim. User interaction or prior configuration is still relevant because the client normally must connect to, mount, or already have an active session with the SMB server. This supports a network server-to-client model with user interaction or prior trust, not a pure unsolicited Internet packet model.

HostAdminRequired=NO because the attacker does not need host-admin-equivalent privilege on the victim to send the malformed server response once the client communicates with the server.

NamespaceScopedPrivilege=NO because namespaces are not material to this trigger.

No special hardware, sysctl, boot option, or race condition is indicated. The main deployment limitation is that the affected path is SMB1 client handling.

4. Primitive and severity interpretation

The exact primitive is an attacker-influenced out-of-bounds read in SMB client validation. The expression used for the message length became effectively zero after RFC1002 preamble accounting changed. A later subtraction wrapped, making the DataOffset bound ineffective and allowing invalid 32-bit offsets to pass validation.

This is not an active structured UAF and not fixed stale storage. It is a bounds-check / integer-underflow style parsing bug leading to an invalid read. The demonstrated and most defensible consequence is client kernel DoS through crash. A limited confidentiality concern is theoretically possible for OOB reads in general, but no concrete disclosure path is established in the supplied patch or context.

PrimitiveClass=CLASS_C because the established behavior is primarily invalid read / crash behavior, without an independently useful corruption, ownership, lifetime, authorization, or write primitive.

ImportantCandidate=NO because the evidence does not establish controlled corruption, LPE, information disclosure, privileged-resource access, or a security-boundary bypass.

AutoDowngradeAllowed=YES because this is a Class C network parser DoS/OOB-read candidate with no established C/I consequence. Manual review is still recommended because it is network-reachable from a malicious server and touches kernel client-side parsing.

Likely operational severity: actionable enough for manual triage, but not an Important-class candidate based on the supplied evidence alone.

5. One-sentence report phrase

A malicious SMB server can exploit an ineffective DataOffset bound in the SMB1 client oplock break validation path to trigger an out of bounds read and potentially crash the Linux client kernel.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network-reachable from a malicious or compromised SMB server and involves attacker-controlled kernel parsing, but the established primitive is an OOB read / DoS risk without proven disclosure, write, UAF, or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: restore the data_offset bound in is_valid_oplock_break()
Commit: 5b16a1967a01ad4496a7206a87c3eb16f1df2b05
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5b16a1967a01ad4496a7206a87c3eb16f1df2b05

Commit description:

Commit 83bfbd0 ("cifs: Remove the RFC1002 header from smb_hdr")
changed the quantity this bound is measured against.  It used to be
srv->total_read minus the 4-byte RFC1002 preamble that total_read then
included, so it was the SMB message length.  The same commit stopped
counting the preamble, and the mechanical substitution to
srv->total_read - srv->pdu_size left an expression that is identically
zero: standard_receive3() reads MID_HEADER_SIZE() bytes and then exactly
pdu_length - MID_HEADER_SIZE() more, adding both to total_read.

len is therefore 0, the subtraction below it wraps, and no __u32
DataOffset can exceed the result, so the check from commit 097f586
("cifs: read overflow in is_valid_oplock_break()") no longer rejects
anything.  Use total_read, which is now the message length on its own.

Fixes: 83bfbd0 ("cifs: Remove the RFC1002 header from smb_hdr")
Cc: stable@kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/smb1misc.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=5b16a1967a01ad4496a7206a87c3eb16f1df2b05

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

The original Linux kernel CVE announcement for CVE-2026-89630 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89630

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:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/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: 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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).