[CVE-2026-97557][MODERATE 7.0] smb: client: avoid leaking refcount in cifs_queue_oplock_break()

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

CVE: CVE-2026-97557
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: avoid leaking refcount in cifs_queue_oplock_break()
Commit: af0193bbf1ac8c0f67f972998b0cc629d6116cf6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af0193bbf1ac8c0f67f972998b0cc629d6116cf6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97557
Analysis date: Sat, 26 Sep 2026 15:00:28 -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.

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

The original Linux kernel CVE announcement for CVE-2026-97557 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97557

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-97557	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97557 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'A reference leak in cifs_queue_oplock_break occurs when a second oplock break arrives while the first oplock break work item is already queued. queue_work then returns false, but the code has already acquired another cifsFileInfo reference that no worker will release. A slow responding or malicious SMB server may repeatedly create this condition and retain file references until CIFS unmount fails with busy dentries or reaches a kernel BUG path. For the CVSS the PR:N is used because a remote attacker controlling the SMB server does not need local code execution on the client after the client has mounted the attacker controlled share. The issue is network reachable through an active SMB client connection, but requires the victim to use the affected CIFS client against a server controlled or influenced by the attacker. Impact is primarily resource exhaustion and denial of service. Confidentiality and integrity impact are not supported because this patch identifies a leaked reference and does not demonstrate a memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	DISK LEAK SERVERTOCLIENT LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
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 was applied. The issue is a concrete persistent reference leak with a later kernel BUG or unmount failure, but it does not establish memory corruption, stale-object use, double release, or a security-boundary bypass.

2. Signal breakdown

* +2 Remote reachable / directly network-triggerable. A remote SMB server can send repeated oplock break notifications to an already connected CIFS client. The trigger requires an active CIFS mount to the server, but no local code execution by the remote attacker after that connection exists.
* +1 Concrete persistent kernel resource/reference leak. `cifsFileInfo_get()` increments the `cifsFileInfo` reference count even when `queue_work()` returns false. No second work item is queued and therefore no matching `_cifsFileInfo_put()` occurs for that increment.
* +1 Reliable kernel crash / strong DoS. The commit documents that later CIFS unmount can encounter busy dentries and inodes and reach `kernel BUG at fs/super.c:777`.

Non-awarded signals:

* No Real UAF signal. The bug leaks an additional valid reference. It does not drop the final reference, free `cifsFileInfo`, or subsequently dereference a freed object.
* No concrete repeated release/refcount mutation. There is no first `put` that may destroy the object followed by a second `put` on the same dangling reference. The defect is the opposite direction: an unmatched `get`.
* No controlled reclaim, object replacement, arbitrary write, callback control, information disclosure, integrity impact, or privilege escalation is established.
* No additional Availability point. The documented kernel BUG and unmount failure describe the same availability consequence and are not counted twice.
* No hard-race penalty. The condition requires a second oplock notification while work remains queued, but a slow or malicious server can materially influence this state. The supplied evidence does not establish a narrow uncontrollable timing window.
* No niche-subsystem penalty. CIFS is not universally enabled or mounted, but the patch does not establish that the affected deployment is unusually rare or inaccessible once a CIFS mount exists.

3. Reachability analysis

The relevant attacker is a remote SMB server, or an actor able to influence the SMB server response stream used by an existing CIFS client mount. The server can cause repeated oplock break notifications while the first `cifs_oplock_break` work item remains queued. A slow-response condition is explicitly identified in the commit.

The victim must already have mounted an SMB share from the attacker-controlled or attacker-influenced server. Creating a CIFS mount often involves local mount administration or prior system configuration, but the supplied patch does not establish the exact privilege boundary for every deployment. Therefore, `HostAdminRequired=UNKNOWN` is appropriate. The attacker does not need host administrative rights merely to transmit protocol messages after the connection exists.

No evidence shows that the complete mount and oplock path can be created solely with attacker-controlled namespace-scoped privileges. Therefore, `NamespaceScopedPrivilege=UNKNOWN`.

4. Primitive and severity interpretation

The exact primitive is an unmatched `cifsFileInfo` reference increment. When `queue_work()` returns false because the work item is already pending, the second reference remains retained permanently because no additional worker will execute the matching `_cifsFileInfo_put()`.

This is not an active structured-object UAF and not fixed stale storage. It is a resource and reference ownership leak. There is no first lifetime operation that may free the object, no surviving dangling pointer, and no second write-like refcount decrement through freed storage.

The demonstrated consequence is leaked CIFS file references, busy dentries or inodes during later unmount, failed unmount, and a possible kernel BUG path. A stronger consequence such as host-wide resource exhaustion may be possible after sustained abuse, but the patch does not quantify leak rate, persistence across reconnects, or a practical route to arbitrary memory pressure. Memory corruption and privilege escalation are speculative and unsupported.

The issue is classified as `CLASS_C` because the established practical security consequence is denial of service or invalid filesystem shutdown behavior. `ImportantCandidate=NO` because no meaningful corruption or security-boundary primitive remains unresolved. `AutoDowngradeAllowed=YES` because the patch establishes a leak rather than a UAF, repeated release, controlled corruption, or authorization bypass.

Operationally, this should be tracked as a network-influenced CIFS client availability issue. It merits review where clients mount shares from untrusted or weakly trusted SMB servers.

5. One-sentence report phrase

A remote SMB server can cause cifs_queue_oplock_break to leak cifsFileInfo references by sending repeated oplock breaks while work is already queued, potentially causing CIFS unmount failures or a kernel BUG denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is remotely influenced through an active SMB client connection and can reach a kernel BUG during unmount, but the available evidence supports a reference leak and DoS only, not memory corruption or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: avoid leaking refcount in cifs_queue_oplock_break()
Commit: af0193bbf1ac8c0f67f972998b0cc629d6116cf6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af0193bbf1ac8c0f67f972998b0cc629d6116cf6

Commit description:

cifs_queue_oplock_break() unconditionally takes a reference on the
target file before queueing cifs_oplock_break(). Only that work item
decreases the reference counter again.

If another oplock break arrives while that work is still queued,
queue_work() will return false and not queue this second work item. As a
result, we will never reach the point to drop the file reference again
and are leaking this reference. This can be triggered when interacting
with a slow-responding server.

As a result, later unmount operations for this file system will fail with

  BUG: Dentry ... still in use (1) [unmount of cifs cifs]
  VFS: Busy inodes after unmount of cifs (cifs)
  kernel BUG at fs/super.c:777!

Fix this by only incrementing the reference count if the work has been
queued successfully. Taking it after queue_work() is safe because all
three callers hold tcon->open_file_lock across the call and
_cifsFileInfo_put() decrements under that same lock, so a worker that
starts the handler in the window cannot drop the reference before it has
been taken.

Fixes: b98749c ("CIFS: keep FileInfo handle live during 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/super.c
  fs/smb/client/misc.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=af0193bbf1ac8c0f67f972998b0cc629d6116cf6

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

The original Linux kernel CVE announcement for CVE-2026-97557 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97557

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:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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).