[CVE-2026-90067][MODERATE REGULAR] libceph: validate banner payload length [ Upstream

[CVE-2026-90067][MODERATE REGULAR] libceph: validate banner payload length [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90067
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: libceph: validate banner payload length [ Upstream
Commit: 279c0852999fd2384f4a88155091a99e81f96873
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=279c0852999fd2384f4a88155091a99e81f96873
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90067
Analysis date: Fri, 18 Sep 2026 04:45:47 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A malformed Ceph msgr2 banner with payload_len below 16 bytes can trigger a WARN in populate_in_iter during socket read setup, and the practical impact is denial of service only when the Ceph endpoint is reachable and WARN handling is fatal or disruptive.

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

The original Linux kernel CVE announcement for CVE-2026-90067 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90067

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-2026-90067	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90067 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-20;CWE-754;CWE-617;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '7.5';DESCR 'A malformed Ceph messenger v2 banner can violate the input state machine because process_banner_prefix accepts a payload_len smaller than the 16 byte minimum required for server_feat and server_req_feat. A remote Ceph peer or an attacker able to tamper with Ceph msgr2 traffic can send a zero length or otherwise too small banner payload and trigger a WARN in populate_in_iter during socket read setup. For the CVSS the PR:N is used because no privileges are required on the victim once the attacker can reach or impersonate the relevant Ceph peer endpoint. The issue is network reachable in Ceph deployments where msgr2 traffic is exposed to the attacker, but Ceph networks are usually isolated storage or data center networks rather than public Internet services. Impact is primarily denial of service. Base impact is a warning and failed connection handling, while the paranoid score treats the WARN as high availability impact when panic_on_warn or equivalent policy can turn it into a kernel crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE DANGER NETWORK HARDWARE LINUS SYZBOT  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative positive signals:

* +2 Remote reachable / directly network-triggerable. A malformed Ceph msgr2 banner with payload_len below the protocol minimum can be sent by a remote Ceph peer or by an attacker able to reach or impersonate the relevant Ceph endpoint.
* +0 WARN / invalid state. The demonstrated effect is a WARN in populate_in_iter after a 0 length socket read setup. WARN alone is not counted as reliable kernel crash for conservative scoring.

Conservative negative signals:

* No negative privilege penalty. Host-admin-equivalent privilege is not required for the network peer once the Ceph msgr2 endpoint is reachable.
* No niche-subsystem penalty. Ceph is deployment-specific, but the vulnerable protocol endpoint can be directly reached in deployments where Ceph traffic is exposed to the attacker.

Paranoid additional signal:

* +1 Reliable kernel crash / strong DoS under hardened policy. The paranoid interpretation treats the WARN as high availability impact only for configurations such as panic_on_warn or equivalent policy where WARN becomes fatal.

Important non-awarded signals:

* RealUAF not awarded. The patch shows no use-after-free, stale object lifetime bug, double release, or refcount misuse.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* OOB read or OOB write not demonstrated.
* Confidentiality and integrity impact not awarded. The issue is malformed input causing state-machine invariant violation and WARN, not disclosure or unauthorized modification.
* Extra availability point not awarded. The same WARN or crash consequence must not be counted twice.
* Security-boundary bypass not demonstrated.
* Privilege escalation not demonstrated.
* Concrete repeated release/refcount signal not applicable. There is no first release that can end object lifetime, no surviving exact dangling reference, and no second lifetime/refcount mutation.

## 3. Reachability analysis

The evaluated attacker is a remote Ceph protocol peer or an attacker able to tamper with Ceph msgr2 traffic. The trigger is sending a malformed banner prefix with payload_len smaller than 16 bytes, including payload_len 0.

HostAdminRequired is NO because the vulnerable operation is reached through network protocol parsing after the endpoint is reachable. No local host administrator action is required by the attacker at trigger time.

NamespaceScopedPrivilege is NO because this is not a local namespace capability path. No CAP_NET_ADMIN or user namespace condition is needed for the remote protocol trigger.

Prior privileged setup may exist operationally because Ceph services or clients must be configured and reachable, but that is normal deployment exposure rather than an attacker privilege requirement. Ceph networks are commonly isolated storage or data-center networks and are usually not public Internet services. This limits practical exposure but does not remove network reachability when the attacker is on that network.

No special hardware is required. No race or timing condition is involved. The trigger is a deterministic malformed-protocol input.

## 4. Primitive and severity interpretation

The exact primitive is invalid protocol input validation leading to a violated read-state invariant and WARN in populate_in_iter. The bug class is validation/state-machine DoS behavior.

PrimitiveClass is CLASS_C because the demonstrated behavior is primarily WARN or DoS-like invalid state handling. There is no concrete memory corruption, no UAF, no active structured stale object, no repeated lifetime mutation, and no authorization bypass.

The demonstrated consequence is warning and failed connection handling. The paranoid consequence is kernel crash only when WARN is fatal due to system policy such as panic_on_warn.

Leverage beyond crash is not established. There is no useful attacker-controlled memory primitive after excluding the WARN/crash behavior.

ImportantCandidate is NO because no Class A primitive and no meaningful Class B exploitation leverage is present. ManualReview is RECOMMENDED because the issue is network reachable and availability depends on target WARN handling policy. AutoDowngradeAllowed is YES because this is a Class C validation/DoS issue without unresolved corruption or boundary-bypass leverage.

Likely operational severity is ordinary to borderline Moderate for isolated Ceph deployments, with higher urgency only where untrusted hosts can reach Ceph msgr2 traffic or panic_on_warn makes WARN fatal.

## 5. One-sentence report phrase

A malformed Ceph msgr2 banner with payload_len below 16 bytes can trigger a WARN in populate_in_iter during socket read setup, and the practical impact is denial of service only when the Ceph endpoint is reachable and WARN handling is fatal or disruptive.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network reachable in exposed Ceph msgr2 deployments, but the patch establishes only malformed input leading to WARN or DoS behavior and does not show memory corruption, privilege escalation, or a security-boundary bypass.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: libceph: validate banner payload length [ Upstream
Commit: 279c0852999fd2384f4a88155091a99e81f96873
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=279c0852999fd2384f4a88155091a99e81f96873

Changed files:
  net/ceph/messenger_v2.c
  net/ceph/messenger.c
  kernel/workqueue.c
  kernel/kthread.c
  arch/x86/kernel/process.c
  arch/x86/entry/entry_64.S

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=279c0852999fd2384f4a88155091a99e81f96873

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

The original Linux kernel CVE announcement for CVE-2026-90067 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90067

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:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: 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).