[CVE-2026-90094][MODERATE REGULAR] arm64: process: Fix context switching MTE store-only tag check [ Upstream

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

CVE: CVE-2026-90094
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: arm64: process: Fix context switching MTE store-only tag check [ Upstream
Commit: 00a73ce437e27fdc636a5ede445a4a491e01788d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00a73ce437e27fdc636a5ede445a4a491e01788d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90094
Analysis date: Fri, 18 Sep 2026 00:17:43 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90094 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90094

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.

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

CVE-2026-90094	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90094 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-693;CWE-665;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '5.8';DESCR 'An arm64 context switch can leave SCTLR_EL1.TCSO0 enabled after a task opts into MTE store only tag checking because that bit was omitted from SCTLR_USER_MASK. A later task scheduled on the same CPU can therefore inherit store only behavior and unexpectedly lose MTE tag checks for loads, weakening a memory safety boundary and allowing invalid tagged loads that the task expected to fault. For the CVSS the PR:L is used because an unprivileged local process can normally configure its own MTE mode and influence the stale CPU state, although reliable impact depends on scheduling and on a following MTE enabled task using the same CPU. The issue is not network reachable. The direct defect is a cross task protection state leak rather than a demonstrated UAF or arbitrary write. The lost load checks can plausibly create limited confidentiality and integrity impact and may contribute to a crash in a protected application. The paranoid score uses C:L I:L A:H as the highest still defensible interpretation because downstream impact depends on another memory safety defect and does not justify C:H or I:H.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)  SKIP CVE-2026-90094 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX LEAK HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

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

1. ActionableScore

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

2. Signal breakdown

* +1 Local unprivileged trigger. A user task can opt into MTE store-only checking without host-admin-equivalent privilege.
* +1 Cross-task security-policy contamination. SCTLR_EL1.TCSO0 can remain active across a context switch and weaken MTE load-tag checking for a different task.
* +1 Weak but concrete exploitation concern. Attacker-influenced architectural state changes whether another task accepts invalid tagged loads, although no independent memory-corruption primitive is established.
* No race penalty. The defect is a deterministic failure to restore per-task state, while scheduling determines which subsequent task is affected.
* No hardware penalty. MTE-capable arm64 hardware is required, but the supplied evidence does not establish that it is sufficiently rare to justify a reduction.
* No crash point. The patch does not establish a kernel crash or host-wide DoS.
* No confidentiality or integrity points. No disclosure, unauthorized write, or protected-object access is demonstrated.
* No UAF, controlled reclaim, arbitrary write, authorization bypass, privilege escalation, or remote reachability is established.
* No paranoid increase. Stronger impact would require a separate memory-safety defect or an attacker-influenced victim data path not demonstrated here.

3. Reachability analysis

An ordinary local process can select the MTE store-only mode and leave TCSO0 set on the CPU. Meaningful impact additionally requires another MTE-using task to run on that CPU before the architectural state is corrected and to rely on load-tag checks.

No capability or namespace-scoped privilege is involved, so host administration is not required and namespace delegation is irrelevant. MTE-capable arm64 hardware and a victim using MTE are concrete prerequisites. The attacker can repeatedly yield or generate scheduling activity, but exact victim placement is not fully controlled. This is not a network-triggerable issue and is not a conventional timing race.

4. Primitive and severity interpretation

The exact primitive is cross-task leakage of a security-relevant CPU control bit. It causes a later task to inherit store-only MTE semantics and lose expected tag checks for loads.

This is invalid active security state rather than a UAF, stale heap object, ownership violation, or direct kernel-memory corruption. No repeated release or refcount mutation exists. The demonstrated consequence is weakening of a memory-safety mitigation. Use of that weakening for disclosure, modification, process compromise, or privilege escalation requires an additional vulnerability and is speculative.

PrimitiveClass is CLASS_C because the patch establishes invalid security state with no independently useful corruption primitive. ImportantCandidate is NO because no direct C/I impact, LPE, or strong exploitation leverage is shown. AutoDowngradeAllowed is NO because a cross-task protection-mechanism failure should remain visible for architecture-specific review even though current evidence does not support Important severity.

5. One-sentence report phrase

An unprivileged arm64 task can leave MTE store-only checking enabled across a context switch, causing a later task on the same CPU to lose expected load-tag checks without establishing a direct memory-corruption or privilege-escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review should confirm the exact user-facing MTE mode-selection interface, per-thread versus process semantics, scheduler persistence, and whether realistic shared-memory or sandboxed workloads provide attacker influence over a victim that relies on load-tag checking.

7. Call-site confidence

Call-site confidence: medium

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

Patch: arm64: process: Fix context switching MTE store-only tag check [ Upstream
Commit: 00a73ce437e27fdc636a5ede445a4a491e01788d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00a73ce437e27fdc636a5ede445a4a491e01788d

Changed files:
  arch/arm64/include/asm/processor.h

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=00a73ce437e27fdc636a5ede445a4a491e01788d

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

The original Linux kernel CVE announcement for CVE-2026-90094 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90094

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:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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).