[CVE-2026-89636][MODERATE 7.0] smb: client: clear ce->tgthint in free_tgts()

[CVE-2026-89636][MODERATE 7.0] smb: client: clear ce->tgthint in free_tgts()

[AL-KERNEL]

CVE: CVE-2026-89636
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: clear ce->tgthint in free_tgts()
Commit: 7507bd1885643d0461a6017767492450af1ce2a3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7507bd1885643d0461a6017767492450af1ce2a3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89636
Analysis date: Fri, 11 Sep 2026 23:08:25 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
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-2026-89636 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89636

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-2026-89636	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89636 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-672;*CWE-416;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A use-after-free can occur in the SMB client DFS cache because free_tgts() frees cached DFS target entries but leaves ce tgthint pointing to one of the freed cache_dfs_tgt objects. A malicious or compromised SMB DFS server, or an attacker able to influence DFS referral traffic, may be able to drive target cache refresh or cleanup paths and later cause the stale target hint to be used. For the CVSS the PR:N because the attacker can be remote and does not need a local account on the client system in the server side attack model. The base score keeps AC:H and UI:R because the client usually must be mounted or directed to the affected DFS namespace and the vulnerable state depends on DFS cache behavior. The paranoid score treats this as a network reachable kernel use-after-free with possible memory corruption impact. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact, so this should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	DISK UAF SERVERTOCLIENT LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap changed the score.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap changed the score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation. The commit directly states that `free_tgts()` frees `cache_dfs_tgt` structures while `ce->tgthint` may still point to one of those freed objects, and later use results in use-after-free.
* +2 Network client attack surface. In the SMB DFS client model, a malicious or compromised SMB/DFS server can plausibly influence DFS referral target cache state that reaches this client-side code after the client connects to the DFS namespace.
* +1 Reliable kernel DoS concern. A stale kernel pointer to a freed DFS target object can realistically produce a kernel fault or crash when dereferenced later.

Paranoid additional signal:

* +1 Structured stale-object exploitation leverage. `cache_dfs_tgt` is not just fixed stale storage. It is an active DFS target object used as a target hint, and its contents and lifecycle can plausibly be influenced by server-provided DFS referral behavior. This creates unresolved exploitation leverage beyond a shallow cleanup-only stale dereference.

Non-awarded signals:

* +2 Strong exploitation leverage not awarded. Controlled reclaim, attacker-shaped replacement object, arbitrary read/write, callback control, or demonstrated privilege escalation is not established.
* Concrete repeated release/refcount +1 not awarded. The patch shows stale dereference/use after free, not a second concrete `put`/`free`/refcount mutation on the exact same dangling reference.
* Paranoid repeated-refcount +1 not awarded. There is no concrete second lifetime/refcount mutation through freed storage.
* Extra Availability +1 not awarded. The same UAF crash/DoS is already counted once.
* Confidentiality and Integrity bonuses not awarded. C/I impact is possible in a worst-case UAF interpretation, but the supplied evidence does not establish disclosure, arbitrary write, controlled reclaim, or a concrete privilege-escalation primitive.
* Race/timing penalty not applied. The supplied patch does not establish a narrow or hard-to-repeat race as the main condition.
* Host-admin penalty not applied. Host-admin-equivalent privilege is not positively established for the attacker in the malicious SMB/DFS server model.
* Rare subsystem penalty not applied. SMB/CIFS DFS is configuration-dependent, but not enough evidence is supplied to treat it as rare and attacker-unreachable.

Repeated-release/refcount check:

* First release can end lifetime: yes, `kfree(t)` frees each `cache_dfs_tgt`.
* Exact same reference survives: yes, `ce->tgthint` may still point to one freed `cache_dfs_tgt`.
* Second operation is a real lifetime/refcount mutation: no, the shown bug is later use of the stale pointer, not a second free/refcount mutation.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: not needed. Conservative raw score is already 5.

3. Reachability analysis

The most relevant attacker model is a malicious or compromised SMB/DFS server, or an attacker able to influence DFS referral traffic for a client that is using DFS. The attacker does not need a local account on the victim in that server-side model, but the complete setup of the SMB mount or DFS namespace may involve prior local or administrative configuration that is not fully resolved by the supplied patch.

HostAdminRequired is therefore UNKNOWN rather than YES or NO. The evidence does not prove that host-admin privilege is required for the complete vulnerable path, but it also does not prove that an ordinary local user can independently create the full setup.

NamespaceScopedPrivilege is NO because no namespace-scoped capability path is indicated.

The issue is not triggered by arbitrary inbound packets to a listening service on the victim. It is client-side exposure after the victim SMB client communicates with an SMB/DFS endpoint. There is no special hardware prerequisite. No narrow race is established from the supplied patch.

4. Primitive and severity interpretation

The exact primitive is a stale post-lifetime pointer in the SMB client DFS cache. `free_tgts()` frees all target entries but leaves `ce->tgthint` pointing to a freed `cache_dfs_tgt`. If later DFS cache logic uses that target hint, the kernel may dereference a freed object.

This is an active structured-object UAF, not fixed stale DMA or fixed async buffer storage. The stale object represents DFS target state and may affect target selection or referral processing. The visible patch does not show the later call sites, so confidence in the exact downstream behavior is medium, but the commit directly establishes the UAF.

This is not an ordinary repeated-release/refcount case. It is category A in the prompt’s distinction for stale pointer use, not category B second lifetime/refcount mutation through the same reference.

PrimitiveClass is CLASS_B because a real kernel lifetime/corruption primitive exists, but no controlled reclaim, arbitrary write, callback hijack, demonstrated LPE, or concrete C/I primitive is established. Demonstrated consequence is at least denial of service via kernel crash. Speculative consequence is broader UAF exploitation, potentially affecting confidentiality or integrity, but that is not proven.

ImportantCandidate is YES because this is a network-client reachable active-object kernel UAF with attacker-influenced DFS cache state and unresolved exploitation leverage. AutoDowngradeAllowed is NO because automatically closing it as ordinary Moderate would risk discarding a meaningful unresolved kernel lifetime primitive in a network client path.

Likely operational severity is Actionable Moderate, with Strong Important candidate review justified if product exposure includes SMB DFS mounts to untrusted or semi-trusted servers.

5. One-sentence report phrase

A stale DFS target hint in the SMB client cache can point to a freed cache_dfs_tgt object after target cleanup, allowing a malicious or compromised SMB DFS server to potentially trigger a kernel use-after-free and crash the client system.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real kernel UAF in SMB client DFS cache state with plausible remote server-side influence, active structured object use, and unresolved exploitation leverage beyond a simple NULL dereference.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: clear ce->tgthint in free_tgts()
Commit: 7507bd1885643d0461a6017767492450af1ce2a3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7507bd1885643d0461a6017767492450af1ce2a3

Commit description:

When free_tgts() frees all structures in ce->tlist, ce->tgthint
is left pointing to one of the freed cache_dfs_tgt structures.

If ce->tgthint is not reset before it is used later, it results
in a use-after-free.

Set ce->tgthint to NULL in free_tgts() after the elements are
freed to reflect that no elements remain.

Fixes: 54be1f6 ("cifs: Add DFS cache routines")
Cc: stable@vger.kernel.org # depends on: smb: client: harden DFS cache against invalid target hints
Signed-off-by: Fredric Cover <fredric.cover.lkernel@gmail.com>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Signed-off-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/dfs_cache.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=7507bd1885643d0461a6017767492450af1ce2a3

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

The original Linux kernel CVE announcement for CVE-2026-89636 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89636

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:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 5
  Paranoid ActionableScore: 6

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).