[CVE-2026-98173][MODERATE 7.0] smb: client: fix use-after-free of iface in cifs_try_adding_channels() [ Upstream

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

CVE: CVE-2026-98173
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: fix use-after-free of iface in cifs_try_adding_channels() [ Upstream
Commit: c941f1ebfd26f683de81091473f3596a218abff0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c941f1ebfd26f683de81091473f3596a218abff0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98173
Analysis date: Thu, 08 Oct 2026 04:08:51 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
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
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The original Linux kernel CVE announcement for CVE-2026-98173 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98173

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-2026-98173	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98173 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	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-362;CWE-825;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A use after free in the SMB client multichannel path occurs because cifs_try_adding_channels can retain an unreferenced iface list entry while dropping iface_lock to open another channel. A concurrent SMB interface refresh can remove and release that entry, after which list iteration and channel selection dereference freed iface memory. A remote SMB server that can cause interface advertisements to change during SMB multichannel setup may trigger the race. For the CVSS the PR:N rating is used because the attacker needs no local account or privileges on the client beyond the ability to act as the connected SMB server or influence its server traffic. Network reachability is conditional on the client connecting to an attacker controlled or compromised SMB server. The base impact is kernel crash and denial of service. The UAF can involve stale heap object accesses and reference count operations, so the paranoid score retains possible confidentiality integrity and privilege escalation impact pending manual review. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	LOCK DISK UAF SERVERTOCLIENT LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF or active lifetime violation. `niface` can be freed by concurrent interface refresh after its last list-held reference is dropped, while `cifs_try_adding_channels()` retains and later dereferences that stale pointer.
* +2 Remote reachable conditionally. A malicious or compromised SMB server can influence advertised interface state during an existing SMB3 multichannel client session. The vulnerable processing is driven by SMB protocol activity, although the victim must already connect to that server.
* +1 Reliable kernel crash or strong DoS concern. Traversal of `niface->iface_head.next` and reads of fields such as `rdma_capable` and `is_active` occur after possible `kfree()`, plausibly causing an Oops or kernel crash.

No negative race penalty was applied. The patch establishes a concurrent window, but does not prove that the timing is uniquely one-shot, externally uncontrollable, or materially difficult to repeat. The added attempts cap also indicates that repeated interface refresh activity was considered.

Important non-awarded signals:

* Controlled reclaim or attacker-controlled replacement of the freed `cifs_server_iface` object is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Structured stale-object exploitation leverage was not separately awarded. The stale object is active and structured, but supplied evidence does not establish attacker-controlled post-free contents or a useful replacement-object interpretation.
* Weak LPE concern was not awarded solely because this is a UAF.
* No confidentiality or integrity point was awarded. There is no established information disclosure, protected-state modification, or privilege-escalation chain.
* No extra Availability point was awarded because the possible crash is already counted as the DoS signal.
* Concrete repeated release/refcount mutation was not awarded. The interface-refresh path can perform `kref_put()` and free the object while the stale list pointer survives, but the supplied control flow does not establish an exact later second `put` or `release` through that same dangling reference after the first release. A possible stale `kref_get()` is concerning but is not the concrete repeated-release pattern required by the dedicated rule.
* FLOOR 5 was not mechanically applied. The raw score already reaches 5 through the concrete UAF, conditional remote reachability, and realistic crash consequence.

3. Reachability analysis

The relevant attacker model is a malicious SMB server, or an attacker able to alter SMB server traffic sufficiently to affect interface advertisements. The target is an SMB client using SMB3 multichannel functionality.

The client must have an active connection to the attacker-controlled or compromised server and must enter the additional-channel path while an interface refresh removes an interface that is no longer advertised. This is not an inbound SMB server vulnerability on the victim. It is a server-to-client attack path over an existing SMB client connection.

HostAdminRequired is NO because the remote attacker does not require host administrator privileges on the affected client to send or influence the relevant SMB server responses. This does not establish that an ordinary local user can independently configure the full SMB multichannel client setup. Therefore NamespaceScopedPrivilege is UNKNOWN and no local-unprivileged trigger bonus is awarded.

No special hardware, boot option, firmware condition, or privileged device-management operation is indicated. SMB multichannel is a feature-dependent prerequisite, but the supplied evidence does not establish it as rare or attacker-unreachable enough to justify a niche-subsystem penalty.

4. Primitive and severity interpretation

The exact primitive is a real active-object UAF. A list iterator saves `niface` while holding `iface_lock`, then drops the lock during `cifs_ses_add_channel()`. A concurrent interface refresh can unlink the saved entry and execute `kref_put()`, with `release_iface()` performing `kfree()`. The iterator subsequently advances through the freed list node and consumes freed `cifs_server_iface` fields.

This is an active structured kernel object rather than fixed stale storage. The stale path performs linked-list traversal and accesses interface state used for channel selection. The supplied patch also shows that normal operation may later take a reference on selected interfaces, but it does not prove attacker-controlled replacement, refcount takeover, callback dispatch, arbitrary read/write, or controlled post-free object contents.

This is not merely an ordinary cleanup dereference. The stale pointer is used during ongoing multichannel channel-selection processing. Nevertheless, there is no patch-supported concrete second `kref_put()` or other repeated lifetime-destruction operation on the exact same dangling reference that would justify the dedicated repeated-release/refcount signal.

PrimitiveClass is CLASS_B because a concrete UAF and stale active-object traversal are established, but no controlled reclaim, directed corruption, security-boundary bypass, or demonstrated privilege escalation is established. The demonstrated practical consequence is kernel crash or denial of service. Confidentiality, integrity compromise, and privilege escalation remain speculative possibilities rather than established consequences.

ImportantCandidate is YES because this is a conditionally network-reachable active-object UAF with unresolved post-free behavior in SMB client kernel code. AutoDowngradeAllowed is NO because automatically treating the issue as an ordinary crash-only Moderate issue would discard a concrete remote-reachable lifetime-corruption primitive that merits manual exploitation-leverage review.

Likely operational severity is Actionable Moderate at minimum. It may become Important if further analysis establishes reliable attacker-controlled interface-refresh timing together with allocator reuse, stale refcount mutation, replacement-object control, disclosure, or privilege-impacting corruption.

5. One-sentence report phrase

A remote SMB server may trigger a use-after-free in the Linux SMB client multichannel interface-selection path by refreshing advertised interfaces while the client retains an unreferenced list entry, potentially causing a kernel crash and requiring manual review for post-free exploitation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

A real active-object UAF is established in a conditionally network-reachable SMB client path, but the available patch does not resolve whether an attacker can obtain controlled reuse, stale refcount manipulation, information disclosure, or privilege escalation beyond kernel DoS.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: fix use-after-free of iface in cifs_try_adding_channels() [ Upstream
Commit: c941f1ebfd26f683de81091473f3596a218abff0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c941f1ebfd26f683de81091473f3596a218abff0

Changed files:
  fs/smb/client/sess.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=c941f1ebfd26f683de81091473f3596a218abff0

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

The original Linux kernel CVE announcement for CVE-2026-98173 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98173

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: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: unknown
  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).