[CVE-2026-64136][MODERATE 7.0] smb: client: protect tc_count increment in smb2_find_smb_sess_tcon_unlocked()

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

CVE: CVE-2026-64136
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: protect tc_count increment in smb2_find_smb_sess_tcon_unlocked()
Commit: 7df1df6f40c0720d30206aa35c0343b962350e0d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7df1df6f40c0720d30206aa35c0343b962350e0d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64136
Analysis date: Sun, 19 Jul 2026 23:20:02 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the SMB client tcon lookup path increments `tc_count` without `tc_lock`, allowing corrupted lifetime accounting that can plausibly cause kernel crash or a weak UAF candidate when SMB activity races with teardown paths.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64136 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64136

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
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CVE-2026-64136	MODERATE	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:R/S:U/C:H/I:H/A:H';CWE-362;CWE-667;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A race in the SMB client tcon lookup path can increment tc_count without holding tcon tc_lock in smb2_find_smb_sess_tcon_unlocked(). This can corrupt the tcon lifetime accounting when concurrent reference drops or teardown paths update the same counter under the lock. The minimum expected impact is a kernel crash or hang from inconsistent references. Because tc_count controls object lifetime, the bug is also a use-after-free candidate if a lost or reordered reference allows a tcon object to be released while another path still uses it. For the CVSS the PR:N is used for the paranoid network server scenario because the attacker may only need control of the SMB server or network path after the client connects, not a local account on the victim. UI:R is kept because the client normally must be configured or induced to connect to the attacker controlled or compromised SMB server. Impact is at least denial of service and in the paranoid interpretation may include confidentiality or integrity impact due to refcount based memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	LOCK DISK SIMPLEFIX SERVERTOCLIENT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	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

Conservative signals:

* Local unprivileged trigger: +1. A local user may be able to generate SMB client activity through an already mounted share, even if creating the mount normally requires admin privileges.
* Memory corruption, weak/indirect corruption candidate: +1. The unlocked increment affects `tc_count`, which is a lifetime/reference counter, but the patch does not show attacker-controlled reclaim, overwrite, callback control, or type confusion.
* Real lifetime corruption: +1. A race on a tcon reference counter can cause incorrect object lifetime accounting.
* Reliable kernel crash / strong DoS: +1. Refcount imbalance in SMB client connection objects can plausibly lead to stale reference use, premature release, hang, or crash.
* Broad/common deployed subsystem: +1. SMB/CIFS client is a commonly deployed network filesystem client, although not always mounted by default.
* Hard or unreliable race: -1. Exploitation depends on a timing window between reference acquisition and concurrent teardown/drop paths.

Paranoid additional interpretation:

* Remote reachable / network-triggerable: +2 instead of only local triggering. A malicious or compromised SMB server may influence SMB responses and connection timing after the client connects, but this still normally requires user/admin action or existing configuration to connect the client.
* Race-UAF downgrade applies. This should remain a weak memory-corruption candidate rather than strong UAF because the patch only shows missing locking around a refcount increment, not controlled reclaim or write-after-free.

## 3. Reachability analysis

The most conservative trigger is local activity against an existing SMB mount. Mount creation usually requires privileges, but once mounted, ordinary local file operations may generate SMB traffic and tcon lookup paths.

A paranoid network scenario is possible if the victim client connects to an attacker-controlled or compromised SMB server. That server may influence protocol responses and timing, but this is not equivalent to unauthenticated Internet-reachable server-side attack surface because client connection or configuration is required.

Namespaces and containers may lower practical privileges if SMB mounts are delegated or exposed into containers. In normal host deployments, setup is privileged, but use of an existing mount can be less privileged.

Call-site confidence: medium. The patch shows the lifetime counter update but not all callers and teardown paths. The helper name, SMB client context, and `tc_count` semantics are enough to require lifetime-focused review.

## 4. Severity interpretation

This is not an ordinary low-risk locking cleanup because the missing lock protects a reference/lifetime counter. The realistic impact is most likely DoS from corrupted tcon lifetime accounting, stale references, or teardown races.

Theoretical memory-corruption potential exists because an incorrect reference count can become a UAF candidate. However, the patch does not demonstrate attacker-controlled reclaim, object replacement, callback dispatch, arbitrary write, or a reliable privilege-escalation primitive. Therefore it fits Actionable Moderate rather than a strong Important classification.

## 5. One-sentence report phrase

A race in the SMB client tcon lookup path increments `tc_count` without `tc_lock`, allowing corrupted lifetime accounting that can plausibly cause kernel crash or a weak UAF candidate when SMB activity races with teardown paths.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a race on a lifetime reference counter in a network filesystem client, with plausible stale-reference or UAF behavior, and a possible malicious SMB server influence model even though a strong exploitation primitive is not proven by the patch.

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

Patch: smb: client: protect tc_count increment in smb2_find_smb_sess_tcon_unlocked()
Commit: 7df1df6f40c0720d30206aa35c0343b962350e0d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7df1df6f40c0720d30206aa35c0343b962350e0d

Commit description:

Commit 96c4af4 ("cifs: Fix locking usage for tcon fields")
refactored cifs code to change cifs_tcp_ses_lock for tc_lock around
tc_count changes.

There was missing lock around tc_count increment inside
smb2_find_smb_sess_tcon_unlocked().

Cc: stable@vger.kernel.org
Fixes: 96c4af4 ("cifs: Fix locking usage for tcon fields")
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Henrique Carvalho <henrique.carvalho@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/smb2transport.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=7df1df6f40c0720d30206aa35c0343b962350e0d

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

The original Linux kernel CVE announcement for CVE-2026-64136 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64136

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:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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).