[CVE-2026-64388][LOW] smb/client: fix chown/chgrp with SMB3 POSIX Extensions

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

CVE: CVE-2026-64388
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: smb/client: fix chown/chgrp with SMB3 POSIX Extensions
Commit: 550cfb8a81181331d4d0f76ab75ee58a0bf41e3e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=550cfb8a81181331d4d0f76ab75ee58a0bf41e3e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64388
Analysis date: Sat, 25 Jul 2026 13:58:06 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
SMB3 POSIX Extensions mounts could ignore chown/chgrp UID/GID updates unless specific CIFS ACL flags were enabled, causing limited ownership metadata integrity mismatch on the server without evidence of memory corruption or direct remote kernel attack.

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

The original Linux kernel CVE announcement for CVE-2026-64388 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64388

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64388	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:N';CWE-732;CWE-284;CWE-670;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '3.3';DESCR 'SMB3 POSIX Extensions mounts could ignore chown/chgrp ownership updates unless CIFS ACL or mode from SID mount flags were also enabled. This can leave UID/GID metadata unchanged on the server even though local policy or administrative workflow expects ownership changes to be applied. For the CVSS the PR:L is used for the paranoid score because a local user who owns files may be able to exercise chgrp or related metadata update paths on a mounted share, while full chown usually requires administrative capability. The issue is not directly network reachable as an attack against the kernel and does not indicate memory corruption. Impact is limited integrity policy bypass due to stale ownership or group metadata.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	DISK SIMPLEFIX SERVERTOCLIENT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Paranoid scoring signals:

* Local unprivileged trigger: +1
  A local user may be able to exercise chgrp or related ownership metadata paths on an already mounted SMB3 POSIX Extensions share, depending on file ownership and mount policy.

* Filesystem/storage metadata integrity issue reachable through local filesystem operations: +1
  The bug affects UID/GID metadata propagation to the SMB server, so it can leave ownership or group information stale despite local operations expecting it to change.

* Integrity impact plausible: +1
  The practical impact is limited integrity policy mismatch: ownership/group metadata may remain unchanged on the server.

Negative or limiting signals:

* No memory corruption: 0
  The patch is a conditional logic fix in cifs_setattr_nounix(). It does not show UAF, OOB, double free, type confusion, arbitrary write, refcount misuse, or page-cache corruption.

* Not network-triggerable as a kernel attack: 0
  Although SMB is a network filesystem, the vulnerable operation is a local client-side setattr/chown/chgrp path, not remote packet parsing.

* Configuration-dependent SMB3 POSIX Extensions path: -1
  The issue requires SMB3 POSIX Extensions and a mount mode where CIFS ACL or mode-from-SID flags were not already enabled.

Effective paranoid score after limiting factors: 2. Conservative score is 0 because typical chown setup and mount configuration are administrator-controlled and the issue is mainly correctness/policy mismatch.

## 3. Reachability analysis

The bug is triggered through local filesystem metadata operations on an SMB3 POSIX Extensions mount. Full chown usually requires root or CAP_CHOWN, while chgrp may be reachable by a lower-privileged local user under normal Unix ownership/group rules. Namespaces or containers do not by themselves make this a host kernel compromise, but delegated SMB mounts or service-managed shares could make the metadata mismatch visible to non-root users.

The path is not a default remote attack surface against the kernel. A remote SMB server does not appear to gain a direct memory corruption primitive from this patch. Realistic exploitation depends on a system already using SMB3 POSIX Extensions and relying on chown/chgrp to enforce ownership policy on the server.

Call-site confidence: high, because the relevant cifs_setattr_nounix() conditional and downstream id_mode_to_cifs_acl() call are visible in the patch.

## 4. Severity interpretation

This behaves like a low-severity filesystem permission/metadata correctness issue, not an Important-class kernel vulnerability. Theoretical impact is stale UID/GID state on the server and possible limited integrity policy bypass. Realistic exploitation evidence does not show memory corruption, privilege escalation, sensitive file theft, kernel crash, or network-triggered compromise.

## 5. One-sentence report phrase

SMB3 POSIX Extensions mounts could ignore chown/chgrp UID/GID updates unless specific CIFS ACL flags were enabled, causing limited ownership metadata integrity mismatch on the server without evidence of memory corruption or direct remote kernel attack.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for urgent security triage because the issue is a low-impact logic bug with limited integrity impact, no memory corruption, no LPE primitive, and no direct remote kernel trigger.

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

Patch: smb/client: fix chown/chgrp with SMB3 POSIX Extensions
Commit: 550cfb8a81181331d4d0f76ab75ee58a0bf41e3e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=550cfb8a81181331d4d0f76ab75ee58a0bf41e3e

Commit description:

Ownership (chown) and group (chgrp) modifications were being ignored when
mounting with SMB3 POSIX Extensions unless CIFS_MOUNT_CIFS_ACL or
CIFS_MOUNT_MODE_FROM_SID were also explicitly set.

Fix this by checking for posix_extensions in cifs_setattr_nounix() when
updating UID and GID, ensuring that id_mode_to_cifs_acl() is called to map
and set the ownership/group information on the server.

Cc: stable@vger.kernel.org
Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/inode.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=550cfb8a81181331d4d0f76ab75ee58a0bf41e3e

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

The original Linux kernel CVE announcement for CVE-2026-64388 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64388

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:H/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:N
  The Best / paranoid CVSS score: 3.3

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: 0
  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).