[CVE-2026-93186][LOW] cxl/mbox: Clamp mailbox output allocation to the payload size [ Upstream

[CVE-2026-93186][LOW] cxl/mbox: Clamp mailbox output allocation to the payload size [ Upstream

[AL-KERNEL]

CVE: CVE-2026-93186
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: cxl/mbox: Clamp mailbox output allocation to the payload size [ Upstream
Commit: f5d2bbf0300f46307948864fbb97bce5097f4fe2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5d2bbf0300f46307948864fbb97bce5097f4fe2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93186
Analysis date: Fri, 18 Sep 2026 08:29:42 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93186 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93186

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: LOW
Manual review required: NO

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

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

CVE-2026-93186	LOW	https://www.kernelcve.org/list/?q=CVE-2026-93186 CHECK WITH IMPACT FROM ORIG NN LOW	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-789;**CWE-400;CWE-770;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'CXL_MEM_SEND_COMMAND bounds the input size but did not bound the user controlled output size before cxl_mbox_cmd_ctor allocated the output buffer with kvzalloc(). A very large out.size can drive an oversized allocation and can trigger a WARN or a kernel panic when panic_on_warn 1 is configured. The response copy path is already clamped to the mailbox payload size, so the patch context does not show an out of bounds write or another memory corruption primitive. For the CVSS the PR:L is used in the paranoid score because some deployments may delegate access to the CXL device node to a local service account or reduced capability root context. The issue is not network reachable and requires local access to the CXL mailbox ioctl path. Impact is denial of service through memory allocation pressure or panic_on_warn behavior, with no supported confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) 	MAYBE	WARNONLY SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

2. Signal breakdown

Conservative positive signals:

* +1 Conditional kernel DoS. User controlled out.size can drive an oversized kvzalloc() in the CXL mailbox ioctl path. The supplied commit states that values above INT_MAX can WARN and that panic_on_warn=1 can turn this into a panic.

Paranoid positive signals:

* +1 Conditional kernel DoS. Same as conservative.
* +1 Resource pressure / stronger availability interpretation. A very large local allocation request can create practical memory pressure or fatal behavior on hardened systems using panic_on_warn=1, but this remains resource exhaustion behavior rather than memory corruption.

Negative or non-awarded signals:

* 0 UAF not awarded. No stale pointer, lifetime violation, double release, or post-free use is shown.
* 0 Memory corruption not awarded. The commit explicitly says the response copy is already clamped to min(out.size, payload_size, device len), so no OOB write is established.
* 0 Confidentiality and integrity not awarded. No disclosure, overwrite, authorization bypass, or protected object modification is established.
* 0 Concrete persistent resource leak not awarded. This is an oversized allocation attempt, not a missing put/release or permanent leaked reference.
* 0 Remote reachability not awarded. The issue is local to the CXL mailbox ioctl path.
* 0 Local unprivileged trigger not awarded. Below-host-admin reachability is not proven by the supplied patch.
* No privilege penalty applied. Host-admin-equivalent access is plausible for CXL device management, but not positively established from the supplied evidence, so no -2 was applied.

3. Reachability analysis

The vulnerable path is CXL_MEM_SEND_COMMAND through the CXL mailbox command constructor, where out.size from userspace influenced the size passed to kvzalloc(). Triggering requires local access to the relevant CXL device ioctl. The supplied evidence does not prove whether this ioctl is reachable by an ordinary user, a delegated service account, or only a host administrator.

HostAdminRequired is therefore UNKNOWN. NamespaceScopedPrivilege is NO because no namespace-scoped capability path is indicated. There is no network trigger. A prior CXL device and driver path must exist, but no special attacker-controlled firmware or external device input is required for the described oversized allocation behavior.

No race is involved. The trigger is an ordered local ioctl input validation failure, not a timing window.

4. Primitive and severity interpretation

The exact primitive is uncontrolled allocation size caused by failing to clamp the requested output buffer size to the mailbox payload size. This is a resource-consumption and WARN/panic_on_warn issue.

PrimitiveClass is NONE because the patch does not establish UAF, OOB access, double free, refcount corruption, type confusion, authorization bypass, or semantic ownership violation. The demonstrated consequence is DoS through oversized allocation behavior and conditional kernel panic when panic_on_warn=1 is configured.

There is no concrete leverage beyond crash/resource pressure. ImportantCandidate is NO because no meaningful memory corruption, boundary bypass, repeated lifetime mutation, or exploitation primitive remains unresolved. AutoDowngradeAllowed is YES because the issue is a local resource allocation bug with no supported C/I impact and no established corruption primitive.

Likely operational severity is Low to ordinary Moderate.

5. One-sentence report phrase

CXL_MEM_SEND_COMMAND failed to clamp the user controlled output size before kvzalloc(), allowing a local caller with access to the CXL mailbox ioctl to request an oversized allocation that may cause memory pressure, WARN, or panic_on_warn based denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch and commit explicitly bound the impact to oversized allocation and conditional DoS, while also stating that the response copy path was already bounded and does not write beyond the mailbox payload.

7. Call-site confidence

Call-site confidence: medium

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

Patch: cxl/mbox: Clamp mailbox output allocation to the payload size [ Upstream
Commit: f5d2bbf0300f46307948864fbb97bce5097f4fe2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5d2bbf0300f46307948864fbb97bce5097f4fe2

Changed files:
  drivers/cxl/core/mbox.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=f5d2bbf0300f46307948864fbb97bce5097f4fe2

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

The original Linux kernel CVE announcement for CVE-2026-93186 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93186

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:L
  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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).