[CVE-2026-98754][MODERATE 7.0] tty: vcc: hold port lock when clearing tty pointer in vcc_cleanup

[CVE-2026-98754][MODERATE 7.0] tty: vcc: hold port lock when clearing tty pointer in vcc_cleanup

[AL-KERNEL]

CVE: CVE-2026-98754
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tty: vcc: hold port lock when clearing tty pointer in vcc_cleanup
Commit: 88f6ecb058801e9ce7b9aabc966530118d7cd53c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=88f6ecb058801e9ce7b9aabc966530118d7cd53c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98754
Analysis date: Sat, 24 Oct 2026 06:31:53 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A missing port lock lets an LDC callback dereference a freed VCC tty during concurrent cleanup, causing a race-dependent kernel UAF and likely crash.

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

The original Linux kernel CVE announcement for CVE-2026-98754 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98754

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.

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

CVE-2026-98754	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98754 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race condition in the VCC tty cleanup path allows vcc_cleanup to clear port tty lacking port lock while an LDC callback reads and uses the same pointer under that lock. The callback can consequently dereference tty port after cleanup has destroyed and freed the associated object, providing a concrete use after free dereference and a likely kernel crash. For the CVSS the PR:L value reflects that reliable triggering normally requires access to the VCC tty or control over a local process using it together with carefully timed LDC activity. The race requires a timing window and is therefore AC:H. This is not an ordinary IP network attack, although LDC events can originate from a virtual domain channel on affected SPARC virtualization systems. The base impact is denial of service. The paranoid score treats reuse of the freed tty object as potential kernel memory corruption with confidentiality and integrity impact, but an arbitrary write or reliable privilege escalation path is not established. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	PRINTF-TTY UAF LINUS KPANIC 	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1.
* Conservative score: 4. Real-UAF floor 4 applied: 1 -> 4.
* Raw paranoid score: 1.
* Paranoid score: 4. No additional exploit primitive is technically supported.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 — Real UAF: `vcc_cleanup()` can free/destroy the tty-associated state while `vcc_event()` retains and dereferences the stale `port->tty`.
* +1 — Plausible kernel crash: dereferencing freed `tty->port` can fault.
* -1 — Hard timing: triggering requires cleanup to overlap an LDC callback in the unlocked window.
* -1 — Rare, difficult-to-instantiate path: VCC is tied to specialized SPARC virtual-domain/LDC environments.
* Floor 4 — The stale reference is genuinely consumed and host-admin-only triggering is not proven.
* No controlled reclaim, attacker-shaped replacement, arbitrary write, callback hijack, disclosure, or repeated release/refcount mutation is shown.
* No extra availability point: it would duplicate the same crash.
* No C/I points: speculative reuse of the freed tty object is insufficient.
* B5-R, B3-PENDING, and B4-PENDING do not apply.

3. Reachability analysis

The actor needs access to an affected VCC tty and appropriately timed LDC activity. The supplied evidence does not establish whether a normal guest, local user, VMM component, or host administrator can complete that sequence. There is no namespace-capability evidence. The vulnerable platform and virtual-domain channel are prerequisites; remote IP reachability is not shown.

4. Primitive and severity reading

This is an active structured-object UAF, but the shown stale use is narrow: a cleanup/LDC-callback race followed by dereference of `tty->port`. The lock added around clearing `port->tty` closes the check/use synchronization gap.

The patch supports CLASS_B lifetime corruption and likely crash, but not useful post-reclamation field consumption or another non-crash primitive. It is therefore not presently an Important candidate. Auto-downgrade remains allowed because the demonstrated behavior is constrained, race-dependent, platform-specific, and crash-oriented.

5. One-sentence report phrase

A missing port lock lets an LDC callback dereference a freed VCC tty during concurrent cleanup, causing a race-dependent kernel UAF and likely crash.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm the VCC/LDC attacker role and whether an untrusted guest or delegated tty user can reliably drive both sides of the race.

7. Call-site confidence

Call-site confidence: medium

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

Patch: tty: vcc: hold port lock when clearing tty pointer in vcc_cleanup
Commit: 88f6ecb058801e9ce7b9aabc966530118d7cd53c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=88f6ecb058801e9ce7b9aabc966530118d7cd53c

Commit description:

vcc_cleanup() sets port->tty to NULL without holding port->lock, racing
with vcc_event() LDC callbacks that read port->tty under port->lock and
then use tty->port. This can cause a use-after-free when the callback
dereferences tty->port after cleanup has already destroyed and freed it.

Assisted-by: AISLE:Snapshot
Cc: stable <stable@kernel.org>
Signed-off-by: Joshua Rogers <linux@joshua.hu>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://patch.msgid.link/20260803-aisle-tty-vcc-v2-2-a50bc9b15193@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/tty/vcc.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=88f6ecb058801e9ce7b9aabc966530118d7cd53c

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

The original Linux kernel CVE announcement for CVE-2026-98754 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98754

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:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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).