[CVE-2026-98747][MODERATE REGULAR] serial: core: fix NULL pointer dereference in serial_core_unregister_port()

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

CVE: CVE-2026-98747
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: serial: core: fix NULL pointer dereference in serial_core_unregister_port()
Commit: 54abbb862366dcaf1f22f1e968358147b7d0547d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54abbb862366dcaf1f22f1e968358147b7d0547d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98747
Analysis date: Mon, 19 Oct 2026 03:29:16 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A host administrator can trigger a NULL pointer dereference during serial driver unbind when no port device is registered, causing an Oops or policy-dependent kernel panic without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-98747 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98747

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 REGULAR
Manual review required: NO

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-2026-98747	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98747 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE RESULT=DIFF V39_AEGIS_ONE_STEP_UP=LOW->MODERATE TEMP_DISTANCE_MERGE_SKIP=AEGIS_LOW_ELIGIBLE_NO_POSITIVE_SIGNAL (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in serial_core_unregister_port() occurs because port_dev can be NULL after port teardown or when port registration fails before serial_core_port_device_add(). The function passed this NULL pointer to serial_core_get_ctrl_dev(), which dereferenced it and could cause a kernel Oops or panic during serial driver unbind or device removal. A local actor able to initiate the affected unregister path can trigger denial of service. For the CVSS the PR:L value is used for the paranoid score because delegated device management, container root, or a restricted service account may have sufficient control without unrestricted host administrator privileges. The base score uses PR:H because sysfs driver unbind and host device management normally require administrative privileges. The issue is not network reachable. Impact is limited to availability because the demonstrated primitive is a near NULL invalid read with no supported UAF, OOB access, information leak, or controlled write. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to MODERATE because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.3 and ActionableScore 0 with primitive class CLASS_C and manual review NO. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	MAYBE	KASAN NULLPTR INIT PRINTF-TTY SIMPLEFIX ERRORPATH HARDWARE LINUS SYZBOT DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

## 1. ActionableScore

* Raw conservative score: -2
* Conservative score: 0
* Raw paranoid score: -1
* Paranoid score: 0
* Applied lower score bound: conservative -2 to 0 and paranoid -1 to 0
* Final recommended bucket: **Ordinary Moderate**::

## 2. Signal breakdown

Conservative signals:

* +0: The established primitive is a near-NULL invalid read in `serial_core_get_ctrl_dev()`. It is not a UAF, OOB access, write, or lifetime mutation.
* -2: The demonstrated trigger reaches `serial8250_remove()` through the host-global sysfs `unbind_store` device-management interface, which requires host-admin-equivalent control.
* Raw conservative total: 0 - 2 = -2. The final score is bounded at 0.

Paranoid signals:

* +1: The NULL dereference can produce a kernel Oops and may become a host panic when fatal-Oops policy is enabled.
* -2: The demonstrated trigger still requires host-level driver unbind control.
* Raw paranoid total: 1 - 2 = -1. The final score is bounded at 0.

Not awarded:

* No UAF or stale post-lifetime object use is established.
* No repeated release or refcount mutation exists.
* No attacker-controlled write, controlled reclaim, OOB access, disclosure, or authorization bypass exists.
* No additional availability point is awarded for the same NULL-dereference failure.
* Remote and namespace-scoped reachability are not established.
* B5-R does not apply because there is no heap OOB write or retry-accumulated offset.
* B3-PENDING does not apply because there is no shared-backing write.
* B4-PENDING does not apply because no DENY-to-ALLOW authorization decision changes.

## 3. Reachability analysis

The demonstrated path is `unbind_store()` to `serial8250_remove()` to `serial8250_unregister_port()` and then `serial_core_unregister_port()`. Writing the driver unbind interface is host-global device administration and is not an attacker-created user or network namespace operation.

`HostAdminRequired=YES` is therefore supported for the shown trigger. `RequiresHostUID0=UNKNOWN` is used because numeric UID 0 is not the only theoretical representation of host-admin-equivalent authority. No network trigger, special race timing, or ordinary unprivileged path is shown. Physical device removal may reach related cleanup code, but the supplied evidence does not establish it as an attacker-controlled security path.

## 4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference. `port->port_dev` can legitimately be NULL after teardown or when registration failed before `serial_core_port_device_add()`. The old code passed that value directly to `serial_core_get_ctrl_dev()`, which dereferenced a field near offset `0x40`.

This is Class C invalid-state behavior. It is neither active-object UAF nor fixed stale storage because no object is shown to have been freed and subsequently accessed through a dangling pointer. The registration error-path assignment prevents a removed device pointer from remaining published, while mutex-protected access and the NULL check prevent the demonstrated dereference.

The demonstrated consequence is an Oops and possible policy-dependent panic. No useful primitive remains after excluding the crash. `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES` are appropriate because no unresolved corruption, lifetime, authorization, or trust-boundary mechanism exists.

## 5. One-sentence report phrase

A host administrator can trigger a NULL pointer dereference during serial driver unbind when no port device is registered, causing an Oops or policy-dependent kernel panic without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is a host-admin-triggered near-NULL read with crash-only behavior and no UAF, write, authorization bypass, or cross-boundary primitive.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: serial: core: fix NULL pointer dereference in serial_core_unregister_port()
Commit: 54abbb862366dcaf1f22f1e968358147b7d0547d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54abbb862366dcaf1f22f1e968358147b7d0547d

Commit description:

port->port_dev is NULL when no port device is installed: it is cleared
on teardown, and never set if registration failed before
serial_core_port_device_add().  serial_core_unregister_port() passes it
straight to serial_core_get_ctrl_dev(), which dereferences it:

  KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047]
  RIP: serial_core_unregister_port
  Call Trace:
   serial8250_unregister_port
   serial8250_remove
   unbind_store

Return early when there is no port device, and read port->port_dev
under port_mutex.

Also clear port->port_dev on the serial_core_register_port() error
path, where the port device has already been removed.

Fixes: 84a9582 ("serial: core: Start managing serial controllers to enable runtime PM")
Reported-by: syzbot+9f57c1b2792029198fcf@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9f57c1b2792029198fcf
Cc: stable@kernel.org
Assisted-by: Claude:claude-opus-5
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Ruslan Valiyev <linuxoid@gmail.com>
Link: https://syzkaller.appspot.com/bug?extid=9f57c1b2792029198fcf
Link: https://lore.kernel.org/all/20260826073237.1377668-1-linuxoid@gmail.com/
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Link: https://patch.msgid.link/20260826084654.1392851-1-linuxoid@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/tty/serial/serial_core.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=54abbb862366dcaf1f22f1e968358147b7d0547d

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

The original Linux kernel CVE announcement for CVE-2026-98747 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98747

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  Paranoid ActionableScore: 0

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).