[CVE-2026-89632][MODERATE 7.0] smb: client: fix use-before-check of ReparseDataLength in reparse_buf_ptr()

[CVE-2026-89632][MODERATE 7.0] smb: client: fix use-before-check of ReparseDataLength in reparse_buf_ptr()

[AL-KERNEL]

CVE: CVE-2026-89632
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: fix use-before-check of ReparseDataLength in reparse_buf_ptr()
Commit: 711cf71300d7992f450600df8864917d3538679f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=711cf71300d7992f450600df8864917d3538679f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89632
Analysis date: Fri, 11 Sep 2026 22:19:15 -0400
ActionableScore: 4
ActionableScore lower bound: 3
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-2026-89632 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89632

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-2026-89632	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89632 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-754;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:L';BEST CVSS score: '7.1';DESCR 'An out-of-bounds read in the SMB client reparse buffer parser can occur because reparse_buf_ptr() reads ReparseDataLength before verifying that OutputCount covers the fixed reparse_data_buffer header. A malicious or compromised SMB server can return a short response with OutputCount smaller than 6, causing the client kernel to read past the received data while processing a reparse point response. For the CVSS the PR:N because the attacker does not need local privileges on the client and only needs the ability to act as or influence the SMB server response. UI:R is used because the client normally has to connect to or access the malicious SMB share. Impact is at least limited confidentiality exposure from a kernel out-of-bounds read and possible denial of service if the invalid read faults or is caught by hardening. No direct privilege escalation primitive is evident because the patch shows a read before validation and not a write or UAF.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)  SKIP CVE-2026-89632 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DISK SIMPLEFIX IMPROVEONLY SERVERTOCLIENT LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative positive signals:

* +2 Remote/network-influenced parser path. A malicious or compromised SMB server can send a malformed response to the Linux SMB client after the client connects or accesses the relevant share.
* +1 Limited kernel OOB read / use-before-check. `reparse_buf_ptr()` reads `ReparseDataLength` before verifying that `count >= sizeof(*buf)`, so `OutputCount < 6` can make the read reach past the received data.

Conservative negative signals:

* No negative privilege penalty. The attacker is the SMB server or a network position influencing the server response, and no host-admin-equivalent privilege on the victim is required for that attacker role.
* No race/timing penalty. This is an ordered parser validation bug, not a timing race.

Paranoid additional signal:

* +1 Possible crash/hardening-sensitive availability concern. The invalid read may fault or be caught by memory hardening/debug instrumentation depending on buffer placement, although the patch does not demonstrate a reliable panic.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* UAF not demonstrated.
* Real lifetime/refcount mutation not demonstrated.
* Confidentiality bonus not awarded because there is no established attacker-visible disclosure path for the bytes read.
* Integrity bonus not awarded because the primitive is read-only.
* Reliable kernel crash bonus not awarded conservatively because the patch establishes an OOB read past received data, not a guaranteed host-wide crash.
* Extra Availability not awarded because no distinct persistent DoS mechanism is shown.
* ImportantCandidate not set because the established primitive is a small read-before-bounds-check issue without demonstrated corruption, privilege escalation, or security-boundary bypass.

3. Reachability analysis

The evaluated attacker is a malicious SMB server, compromised SMB server, or a network attacker capable of influencing SMB server responses. The victim must have the Linux SMB client connect to or access the relevant SMB share and process a reparse point response.

For the attacker role, host administrator privileges on the victim are not required. User interaction or prior client-side mount/access is normally needed, but that is different from attacker privilege. Kernel CIFS/SMB mounts may often be created by privileged local configuration, but the malformed response itself is supplied by the remote server side.

Namespace-scoped privilege is not relevant here. No CAP_NET_ADMIN, BPF, KVM, or namespace-local control-plane operation is part of the trigger. No special hardware requirement is indicated. No race is involved, and the trigger is repeatable by returning a short `OutputCount` in the relevant response.

4. Primitive and severity interpretation

The exact primitive is a read-before-validation leading to a small OOB read from the SMB client response buffer. The bug is not a UAF, not a double release, not an OOB write, and not a controlled memory corruption primitive.

This is best classified as `CLASS_C`: invalid parser behavior with possible crash or limited internal overread, but without an independently useful exploitation primitive. The demonstrated consequence is reading bytes at offset 4-5 before proving that the fixed header is present. A speculative consequence is local kernel faulting or limited internal data exposure, but no attacker-visible leak, privilege escalation, or kernel write is established.

ImportantCandidate is `NO` because the patch supports an OOB read but not an Important-grade primitive. AutoDowngradeAllowed is `YES` because the issue can be handled as borderline/actionable Moderate triage unless additional evidence shows disclosure, reliable crash, or a stronger downstream effect.

Likely operational severity is Moderate-like: higher than a purely local validation bug because it is network-influenced by an SMB server, but below Important because no memory corruption write, UAF, boundary bypass, or LPE path is established.

5. One-sentence report phrase

A malicious SMB server can return a short reparse point response that makes the Linux SMB client read ReparseDataLength before validating the fixed header length, causing a small kernel OOB read with possible DoS but no demonstrated write, UAF, or privilege escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a network-influenced kernel client parser OOB read, so it should be reviewed for any downstream disclosure or reliable crash path, but the supplied patch does not establish a strong corruption or privilege-escalation primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: fix use-before-check of ReparseDataLength in reparse_buf_ptr()
Commit: 711cf71300d7992f450600df8864917d3538679f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=711cf71300d7992f450600df8864917d3538679f

Commit description:

reparse_buf_ptr() reads buf->ReparseDataLength before checking that
count covers the full fixed header:

    buf = (struct reparse_data_buffer *)((u8 *)io + off);
    len = sizeof(*buf);                          /* 8 bytes */
    rdlen = le16_to_cpu(buf->ReparseDataLength); /* offset 4, 2 bytes */

    if (count < len || count < rdlen + len)      /* check comes after */

struct reparse_data_buffer has ReparseDataLength at offset 4.  If a
server returns OutputCount < 6, the read at offset 4-5 reaches past
the end of the received data.  The off+count bounds against iov_len
were already validated, but that does not protect against count being
smaller than sizeof(*buf).

Split the check: verify count >= sizeof(*buf) before reading
ReparseDataLength, then verify count covers the data region.

Fixes: a158bb6 ("smb: client: optimise reparse point querying")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-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/smb2inode.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=711cf71300d7992f450600df8864917d3538679f

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

The original Linux kernel CVE announcement for CVE-2026-89632 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89632

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:L/I:N/A:L
  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: 3
  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).