[CVE-2026-97556][LOW] smb: client: avoid leaking refcount when cifs_sb_tlink() fails

[CVE-2026-97556][LOW] smb: client: avoid leaking refcount when cifs_sb_tlink() fails

[AL-KERNEL]

CVE: CVE-2026-97556
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: smb: client: avoid leaking refcount when cifs_sb_tlink() fails
Commit: 9ec991e148368e3207e98134c509a625097f7693
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9ec991e148368e3207e98134c509a625097f7693
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97556
Analysis date: Sat, 26 Sep 2026 14:43:33 -0400
ActionableScore: 2
ActionableScore lower bound: 1
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-2026-97556 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97556

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-2026-97556	LOW	https://www.kernelcve.org/list/?q=CVE-2026-97556 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A missing cifs file reference release in cifs_oplock_break can occur when cifs_sb_tlink fails on a CIFS mount using the multiuser option. The leaked reference can keep CIFS inodes busy and cause unmount failures or VFS Busy inodes warnings after an oplock break is processed. For the CVSS the PR:L is selected because a local user or process must access an existing multiuser CIFS mount and participate in the affected client workflow. The issue is not directly reachable by an unauthenticated network attacker. A remote SMB server can participate in the oplock break flow, but practical triggering also depends on the local multiuser mount configuration and the cifs_sb_tlink failure condition. Impact is primarily local denial of service through resource retention and impaired unmount. No memory corruption primitive or privilege escalation path is indicated. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	DISK LEAK SERVERTOCLIENT LINUS 	YES	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied.

2. Signal breakdown

* +1 Concrete persistent kernel resource/reference leak. The queued oplock break owns a cifsFileInfo reference, but cifs_oplock_break did not call _cifsFileInfo_put when cifs_sb_tlink failed. The missing put is explicit in the patch and commit message.
* +1 Paranoid only. The commit reports busy inodes after unmount and describes the resulting VFS BUG as a crash. This is treated as a plausible availability consequence, but not as a conservative reliable host crash because the failure path was found by code review and was not exercised at runtime.

Important non-awarded signals:

* No Real UAF. The bug is a missing reference drop, not a release followed by a stale use or a second release through a dangling reference.
* No concrete repeated release/refcount mutation. The patch adds the missing first and only required put on the error path. It does not reveal two puts on the same potentially freed reference.
* No controlled reclaim, object replacement, arbitrary write, type confusion, or callback control is established.
* No confidentiality or integrity signal is awarded. Retaining a reference can keep an inode busy but does not expose data or modify protected state.
* No local unprivileged trigger bonus is awarded. The supplied context does not prove that an ordinary unprivileged user can complete the full sequence involving an existing CIFS multiuser mount, an oplock break, cifs_sb_tlink failure, and unmount.
* No remote reachability bonus is awarded. An SMB server may participate in oplock handling, but the vulnerable condition also requires local CIFS multiuser configuration and a local error path. This is not a directly network-triggerable vulnerability in the CVSS or ActionableScore sense.
* No host-admin penalty is applied. The complete privilege boundary for establishing and using the affected mount is not fully demonstrated.
* No hard race penalty is applied. This is an ordered error path, not a demonstrated timing race.
* No niche-subsystem penalty is applied. CIFS multiuser configuration is a prerequisite, but the supplied evidence does not establish that it is attacker-inaccessible or otherwise justifies a negative score.

3. Reachability analysis

The affected path requires a CIFS client mount using the multiuser option. An oplock break must be queued for a CIFS file, and cifs_sb_tlink must subsequently fail while cifs_oplock_break processes that work item. The leaked file reference may then keep an inode or dentry busy until unmount.

A remote SMB server can participate in generating the oplock break, but a network attacker cannot generally reach the vulnerable state without an existing client connection and the required local mount configuration. The supplied material does not establish whether the remote server is attacker controlled, whether an unprivileged local account can use the particular multiuser mount, or whether that account can cause or participate in the required tlink failure.

Host-admin-equivalent privilege is not proven as required for the complete trigger, so HostAdminRequired is UNKNOWN. Below-host-admin reachability is also not fully established, so it is not scored as an ordinary local-unprivileged trigger. No namespace-scoped privilege path is shown.

No special hardware, boot option, sysctl, or race condition is involved. The multiuser mount option is a functional prerequisite. The commit explicitly states that this failure path was found by code review and was not exercised at runtime, reducing confidence in practical trigger reliability.

4. Primitive and severity interpretation

The exact primitive is a persistent CIFS file-reference leak on an error path. cifs_queue_oplock_break acquires a cifsFileInfo reference when it queues work. cifs_oplock_break normally consumes and releases that reference through _cifsFileInfo_put. When cifs_sb_tlink fails, the old code returned through the out path without dropping the reference.

This is not an active stale-object use. It is not a UAF, double free, double put, refcount underflow, or post-lifetime refcount mutation. The patch adds a missing release before the error exit and does not establish that the object is accessed after it is freed.

The demonstrated consequence is resource retention and a busy-inode condition during unmount. The commit reports a VFS BUG and describes a crash after unmount, but the practical reliability and host-wide effect are not demonstrated because the error branch was not runtime-tested. Resource exhaustion or repeated reference accumulation beyond the reported unmount condition is not established.

PrimitiveClass is NONE because no meaningful kernel corruption, stale use, authorization bypass, ownership violation, or security-boundary primitive is established. ImportantCandidate is NO because the available evidence supports a constrained resource leak and local availability issue only. AutoDowngradeAllowed is YES because no unresolved corruption or security-boundary primitive would be discarded by ordinary low-priority handling.

Likely operational severity is Low-like or ordinary Moderate at most. Patch deployment can be deferred relative to memory-corruption, authorization-bypass, and remotely reachable kernel issues, while still being included in normal stable-kernel maintenance.

5. One-sentence report phrase

A missing cifsFileInfo reference release after cifs_sb_tlink failure on a CIFS multiuser mount can retain busy inodes and impair unmount operations, with no demonstrated memory-corruption or privilege-escalation primitive.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a missing reference release and resulting resource retention, but does not establish a UAF, double release, kernel-memory corruption, authorization bypass, or meaningful unresolved exploitation leverage.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: avoid leaking refcount when cifs_sb_tlink() fails
Commit: 9ec991e148368e3207e98134c509a625097f7693
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9ec991e148368e3207e98134c509a625097f7693

Commit description:

cifs_oplock_break() takes over the reference that
cifs_queue_oplock_break() acquired when it queued the work, and drops it
with _cifsFileInfo_put() once the break has been processed.

Only in setups with "-o multiuser", cifs_sb_tlink() may fail, at which
point cifs_oplock_break() returns without putting the file reference,
mirroring the reference leak we already fixed in the companion patch to
cifs_queue_oplock_break().

This would trigger a crash due to busy inodes on the next unmount:

  BUG: Dentry ... still in use (1) [unmount of cifs cifs]
  VFS: Busy inodes after unmount of cifs (cifs)

Drop the reference on that path as well. Doing so before the out label
mirrors the normal path, which also puts the reference before
cifs_done_oplock_break().

Found by Sashiko code review. The failure path was not exercised at
runtime.

Fixes: e8f5f84 ("cifs: fix potential oops in cifs_oplock_break")
Cc: stable@vger.kernel.org
Assisted-by: Kiro:claude-opus-5
Signed-off-by: Bjoern Doebel <doebel@amazon.de>
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/file.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=9ec991e148368e3207e98134c509a625097f7693

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

The original Linux kernel CVE announcement for CVE-2026-97556 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97556

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:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  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).