[CVE-2026-46139][MODERATE 7.0] smb: client: use kzalloc to zero-initialize security descriptor buffer

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

CVE: CVE-2026-46139
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: use kzalloc to zero-initialize security descriptor buffer
Commit: 4c3ed344a970aad51388ac3b0145b98318f0e21f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c3ed344a970aad51388ac3b0145b98318f0e21f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46139
Analysis date: Thu, 28 May 2026 12:59:09 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
The SMB client used an uninitialized security descriptor buffer, allowing a 2 byte ACL reserved field to contain kernel heap data that may be sent to an SMB server and may also cause chmod or ACL updates to fail.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46139 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46139

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: 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
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CVE-2026-46139	MODERATE	CHECK	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:L';CWE-908;*CWE-457;*CWE-200;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:N/A:L';BEST CVSS score: '5.4';DESCR 'SMB client ACL construction used kmalloc() for a security descriptor buffer and did not initialize the 2 byte ACL reserved field after num_aces was split into a le16 field and a reserved field. As a result, small amounts of uninitialized kernel heap data could be placed into the on wire security descriptor and sent to the SMB server, and non zero garbage can also make Samba reject chmod with EINVAL. For the CVSS the PR:N is used in the paranoid remote server model because a malicious server needs no local account on the client, but victim interaction or an existing SMB mount and ACL update operation are required. The issue is network exposed only in the client to server direction and does not provide a write primitive or privilege escalation path. Impact is mainly low confidentiality exposure and limited availability impact for chmod or ACL updates.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	DISK SIMPLEFIX SERVERTOCLIENT KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote/network-exposed confidentiality path: **+2**. The uninitialized bytes are serialized into an SMB security descriptor and can be observed by the SMB server.
* Local unprivileged trigger: **+1**. If an SMB mount already exists and permissions allow it, a local user may trigger ACL/chmod updates without needing full admin privileges.
* Confidentiality impact plausible: **+1**. The bug can leak 2 bytes of uninitialized kernel heap data to the server.
* Important filesystem/storage path: **+1** for paranoid score. The issue is in SMB client ACL/security descriptor handling.
* No memory corruption: **+0**. This is uninitialized data exposure, not OOB write, UAF, double-free, or heap overwrite.
* No LPE signal: **+0**. The patch does not show a privilege escalation primitive.
* Availability is limited: **+0** conservative. Samba may reject the descriptor and chmod may fail, but this is operation-level failure rather than strong system-wide DoS.

## 3. Reachability analysis

The vulnerable path is reached when the Linux SMB client builds an ACL security descriptor, for example during chmod or ACL update operations. A malicious or untrusted SMB server can receive the on-wire descriptor and observe the uninitialized reserved field, but usually needs the victim to mount the share and perform an ACL-changing operation. Namespaces do not materially change the remote-server leak model, but local triggering may be possible by an unprivileged user if the SMB mount is already available and writable. The path is not packet-processing RCE and is not automatically triggered by arbitrary inbound network traffic.

## 4. Severity interpretation

This is best treated as **Actionable Moderate**, not Important. The realistic impact is a very small kernel heap information leak plus possible ACL/chmod failure. There is no demonstrated write primitive, no memory lifetime bug, and no credible privilege escalation path from the patch context. The reason not to auto-close is that the leak crosses a network trust boundary from SMB client to server.

## 5. One-sentence report phrase

The SMB client used an uninitialized security descriptor buffer, allowing a 2 byte ACL reserved field to contain kernel heap data that may be sent to an SMB server and may also cause chmod or ACL updates to fail.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Manual review is recommended because this is a remote-observable kernel information leak across an SMB client/server boundary, although the leak is very small and there is no memory corruption or LPE evidence.

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

Patch: smb: client: use kzalloc to zero-initialize security descriptor buffer
Commit: 4c3ed344a970aad51388ac3b0145b98318f0e21f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c3ed344a970aad51388ac3b0145b98318f0e21f

Commit description:

Commit 62e7dd0 ("smb: common: change the data type of num_aces
to le16") split struct smb_acl's __le32 num_aces field into __le16
num_aces and __le16 reserved. The reserved field corresponds to Sbz2
in the MS-DTYP ACL wire format, which must be zero [1].

When building an ACL descriptor in build_sec_desc(), we are using a
kmalloc()'ed descriptor buffer and writing the fields explicitly using
le16() writes now. This never writes to the 2 byte reserved field,
leaving it as uninitialized heap data.

When the reserved field happens to contain non-zero slab garbage,
Samba rejects the security descriptor with "ndr_pull_security_descriptor
failed: Range Error", causing chmod to fail with EINVAL.

Change kmalloc() to kzalloc() to ensure the entire buffer is
zero-initialized.

Fixes: 62e7dd0 ("smb: common: change the data type of num_aces to le16")
Cc: stable@vger.kernel.org

Signed-off-by: Bjoern Doebel <doebel@amazon.de>
Assisted-by: Kiro:claude-opus-4.6
[1] https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-dtyp/20233ed8-a6c6-4097-aafa-dd545ed24428
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/cifsacl.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=4c3ed344a970aad51388ac3b0145b98318f0e21f

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

The original Linux kernel CVE announcement for CVE-2026-46139 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46139

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:H/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:L
  The Best / paranoid CVSS score: 5.4

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: 4
  Paranoid ActionableScore: 5

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).