[CVE-2026-98812][MODERATE REGULAR] net/mctp: publish the mctp_dev only after it is initialised [ Upstream

[CVE-2026-98812][MODERATE REGULAR] net/mctp: publish the mctp_dev only after it is initialised [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98812
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/mctp: publish the mctp_dev only after it is initialised [ Upstream
Commit: af6e4f931afc2d670d133aa2bedc8b6e3753c618
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af6e4f931afc2d670d133aa2bedc8b6e3753c618
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98812
Analysis date: Mon, 19 Oct 2026 05:52:21 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
An RCU publication race in `mctp_add_dev()` can expose a partially initialized device to a local MCTP `sendmsg` caller, causing a NULL-pointer dereference and possible kernel denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-98812 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98812

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-2026-98812	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98812 CHECK WITH IMPACT FROM ORIG NN LOW | 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:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-665;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'An RCU publication race in mctp_add_dev can expose a partially initialized mctp_dev to concurrent readers because dev.mctp_ptr becomes visible before mdev.dev is assigned. A local process using an MCTP socket can reach the vulnerable state through sendmsg while the device is being added, causing a NULL-pointer dereference in the kernel. For the CVSS the PR:L value is used because reliable triggering requires a local user or process context capable of using the MCTP socket interface, but it does not necessarily require full administrative control. AC:H reflects the narrow timing window between publication of the object and completion of its initialization. The demonstrated impact is a kernel Oops or crash with C:N and I:N because the available code shows a NULL dereference rather than UAF, OOB access, or a write primitive. The issue is not directly network reachable from a remote MCTP peer based on the supplied call path. The paranoid score raises availability from A:L to A:H for systems where an Oops causes a panic or leaves the networking subsystem or host unusable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold 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 INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.0 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. 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	KASAN NULLPTR HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Weak or indirect corruption candidate. RCU exposes a partially initialized active object whose `mdev->dev` field is NULL.
* +1 Reliable kernel fault. KASAN demonstrates a NULL-pointer dereference during `sendmsg`, with kernel Oops or panic possible.
* +1 Local unprivileged trigger. The reader path is reached through a userspace MCTP `sendmsg` operation and does not itself require host-admin privileges.
* -1 Hard timing requirement. The vulnerable interval is between `rcu_assign_pointer()` and the immediately following `mdev->dev` assignment during device addition, which is a narrow and potentially one-shot window.

Paranoid difference:

* The -1 timing penalty is withheld because repeated `sendmsg` calls can continuously exercise the reader while device registration occurs. This supports score 3 but does not establish reliable exploitation.

Not awarded:

* No UAF. The published `mctp_dev` remains allocated and only contains an uninitialized NULL field.
* No OOB write, arbitrary write, disclosure, controlled reclaim, refcount mutation, or authorization bypass.
* No extra availability point because it would double-count the same NULL-pointer fault.
* No remote-reachability point. The demonstrated path begins with local `sendmsg`.
* V33 does not apply because no attacker-influenced corruption of active structured state is established.
* B5-R does not apply because there is no retry-accumulated heap OOB write.
* B3-PENDING and B4-PENDING do not apply.

3. Reachability analysis

A local process can repeatedly call `sendmsg` on an available MCTP socket while `mctp_add_dev()` publishes a new device. The attacker does not need host UID 0 for the demonstrated reader operation, but an MCTP device and route must already exist or be undergoing registration.

Device creation or registration may involve a separate privileged actor or device event. This is an environmental prerequisite rather than a privilege exercised by the sending process. The supplied evidence does not establish that the complete setup can be created inside an attacker-controlled namespace, so `NamespaceScopedPrivilege` remains UNKNOWN.

No direct trigger from a remote MCTP peer is shown. No mandatory special physical hardware is established. Repeated sends are inexpensive, but opportunities are limited by device-add events and the narrow publication window.

4. Primitive and severity interpretation

The exact primitive is unsafe RCU publication of a partially initialized `mctp_dev`. Readers can legitimately obtain the object after `rcu_assign_pointer()`, but its `dev` member has not yet been assigned. The resulting use is a NULL-pointer dereference through an otherwise live active structured object.

This is not a stale post-lifetime reference, UAF, or memory overwrite. It is therefore CLASS_C rather than CLASS_B or CLASS_A. The demonstrated consequence is kernel Oops or DoS. Host-wide panic depends on configuration and fault handling.

There is no useful non-crash primitive after excluding the NULL dereference. `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES` follow from the absence of memory corruption, lifetime corruption, security-boundary bypass, or unresolved integrity primitive. Manual review remains recommended because the path is locally reachable and crosses an RCU publication boundary.

5. One-sentence report phrase

An RCU publication race in `mctp_add_dev()` can expose a partially initialized device to a local MCTP `sendmsg` caller, causing a NULL-pointer dereference and possible kernel denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is a locally reachable RCU initialization race with a demonstrated kernel fault, but no UAF, write primitive, authorization bypass, or Important-grade exploitation mechanism is established.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/mctp: publish the mctp_dev only after it is initialised [ Upstream
Commit: af6e4f931afc2d670d133aa2bedc8b6e3753c618
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af6e4f931afc2d670d133aa2bedc8b6e3753c618

Changed files:
  include/linux/sockptr.h
  net/mctp/route.c
  security/selinux/hooks.c
  net/socket.c
  kernel/sched/fair.c
  include/linux/rcupdate.h
  kernel/sched/core.c
  kernel/time/clockevents.c
  fs/file.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S
  net/mctp/device.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=af6e4f931afc2d670d133aa2bedc8b6e3753c618

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

The original Linux kernel CVE announcement for CVE-2026-98812 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98812

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 2
  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).