[CVE-2026-98755][MODERATE REGULAR] tty: vcc: zero-initialize control packet in vcc_send_ctl()

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

CVE: CVE-2026-98755
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tty: vcc: zero-initialize control packet in vcc_send_ctl()
Commit: 6f1a08ef02825b440126c406ea63ef6128888e15
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f1a08ef02825b440126c406ea63ef6128888e15
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98755
Analysis date: Fri, 23 Oct 2026 20:51:41 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A partially initialized VCC control packet can disclose residual kernel stack data from the control domain to a connected LDC peer, with the practical impact depending on packet repeatability and the sensitivity of the leaked byte.

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

The original Linux kernel CVE announcement for CVE-2026-98755 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98755

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: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-98755	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98755 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N';*CWE-457;*CWE-200;CWE-665;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N';BEST CVSS score: '7.4';DESCR 'An information disclosure occurs in vcc_send_ctl() because the stack allocated vio_vcc control packet is only partially initialized before it is passed to ldc_write(). The uninitialized tag.stype_env field can contain residual kernel stack data which is then disclosed from the control domain to the connected LDC peer. For the CVSS the PR:N value is used because the peer does not need privileges inside the vulnerable control domain, although the attacker must control or access a logical domain connected through the LDC channel. AV:A is used because LDC is an adjacent inter domain transport rather than an arbitrary remote IP network. The demonstrated impact is confidentiality loss without memory corruption, kernel modification, or denial of service. The base score uses C:L because each packet appears to expose only a small uninitialized field. The paranoid score uses C:H to preserve manual review sensitivity if repeated control exchanges can disclose varying sensitive stack contents across the domain boundary. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7.4 and the production ActionableScore returned 3 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 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 V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	WRITE OOB PRINTF-TTY SIMPLEFIX LEAK HARDWARE LINUS PACKET  V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +1 Confidentiality plausible. The commit explicitly establishes that uninitialized kernel stack data from `tag.stype_env` is copied into a packet and sent through `ldc_write()`.
* +1 Cross-boundary isolation impact. Data moves from the kernel of the control domain to a connected LDC peer, crossing a logical-domain trust boundary.
* +1 Practical disclosure chain. The complete low-level chain is established as partially initialized stack object, packet transmission, and peer-visible data.

No negative points were applied. The small apparent size of `tag.stype_env` limits demonstrated impact, but the supplied source does not establish that its value is harmless or constant.

Important non-awarded signals:

* No OOB write, UAF, active-state corruption, authorization bypass, or memory-lifetime violation is present.
* No integrity or availability consequence is established.
* Repeated disclosure of varying stack contents is not established by the visible call sites.
* High-volume or substantial kernel disclosure is not established, so CLASS_A was not assigned.
* Direct remote IP reachability and ordinary local-user triggering are not established.
* B5-R does not apply because there is no retry-accumulated heap OOB write.
* 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 recipient is a logical domain connected to the control domain through LDC. The peer does not need execution privileges inside the vulnerable kernel merely to receive a transmitted packet. However, the supplied context does not show whether the peer can directly and repeatedly cause `vcc_send_ctl()` to run, or whether a control-domain administrator must first initiate the relevant console operation.

The complete trigger privilege is therefore unresolved. `HostAdminRequired=UNKNOWN` and `RequiresHostUID0=UNKNOWN` avoid treating prior logical-domain provisioning as proof that every disclosure requires host administration. Linux user or network namespaces are not involved. Special SPARC logical-domain and virtual-console infrastructure is required, but no rarity penalty was applied solely because the subsystem is specialized. There is no race.

4. Primitive and severity interpretation

The exact primitive is a peer-visible disclosure of residual kernel stack data caused by transmitting a partially initialized `struct vio_vcc`. This is a constrained information-disclosure primitive, not memory corruption and not an internal invalid read with no output path.

The demonstrated consequence is disclosure of the uninitialized `tag.stype_env` field across the control-domain boundary. The amount and sensitivity of leaked data appear limited, but the value can originate from prior kernel stack contents. Repeated leakage of different sensitive bytes is a plausible question for call-site review, not an established scored consequence.

The issue is CLASS_B because a real cross-boundary disclosure exists, but substantial disclosure or broader exploitation leverage is not established. `ImportantCandidate=NO` because there is no evidence of credentials, pointers, cryptographic material, repeated arbitrary stack sampling, privilege escalation, or host compromise. `AutoDowngradeAllowed=NO` preserves review because the recipient trust boundary, packet frequency, attacker influence over triggering, and actual leaked values remain unresolved.

Likely operational severity is Low to Moderate confidentiality impact in systems using the affected VCC and LDC configuration.

5. One-sentence report phrase

A partially initialized VCC control packet can disclose residual kernel stack data from the control domain to a connected LDC peer, with the practical impact depending on packet repeatability and the sensitivity of the leaked byte.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Confirm the width of `tag.stype_env`, all callers of `vcc_send_ctl()`, whether an untrusted peer can induce repeated control packets, and whether successive packets can expose varying control-domain stack contents.

7. Call-site confidence

Call-site confidence: medium

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

Patch: tty: vcc: zero-initialize control packet in vcc_send_ctl()
Commit: 6f1a08ef02825b440126c406ea63ef6128888e15
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f1a08ef02825b440126c406ea63ef6128888e15

Commit description:

The stack-allocated struct vio_vcc pkt was partially initialized,
leaving the tag.stype_env field uninitialized before being sent via
ldc_write(), potentially leaking kernel stack data to the LDC peer.

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-v2-1-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=6f1a08ef02825b440126c406ea63ef6128888e15

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

The original Linux kernel CVE announcement for CVE-2026-98755 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98755

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:A/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
  The Best / paranoid CVSS score: 7.4

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: 3

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).