[CVE-2026-89981][MODERATE REGULAR] arm64: Don't read GMID_EL1 when MTE is disabled

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

CVE: CVE-2026-89981
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: arm64: Don't read GMID_EL1 when MTE is disabled
Commit: d2d22ff377cd43f2e9660426ff22e3cd4c534637
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2d22ff377cd43f2e9660426ff22e3cd4c534637
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89981
Analysis date: Wed, 16 Sep 2026 08:34:57 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
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-2026-89981 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89981

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: NO

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

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

CVE-2026-89981	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89981 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-703;CWE-754;CWE-248;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'An arm64 pKVM host can panic because __cpuinfo_store_cpu may read GMID_EL1 when MTE is disabled by the kernel configuration or by arm64.nomte. KVM sets HCR_EL2.TID5 in this state, so the GMID_EL1 access is trapped and pKVM can inject an UNDEF that the host cannot handle after a CPU is offlined and brought back online. For the CVSS the PR:L is used in the paranoid score because some deployments may delegate CPU hotplug or host power management actions to a reduced capability service account or orchestration component rather than full host administration. The issue is not network reachable and requires local control over the affected host environment. Impact is denial of service via host kernel panic. No memory corruption, information disclosure, or privilege escalation primitive is indicated by the code path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)  SKIP CVE-2026-89981 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	INIT HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1. No floor or cap applied.
* Raw paranoid score: 1
* Paranoid score: 1. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Reliable kernel crash / strong DoS. The commit shows an unhandled UNDEF in the host and a kernel panic after `__cpuinfo_store_cpu()` reads `GMID_EL1` while MTE access is trapped by pKVM.

Negative signals:

* -2 Host-admin-equivalent trigger requirement would normally apply because CPU offline / online control is a host management operation. However the raw score is floored at practical minimum 1 for an actual host panic already counted above.

Important non-awarded signals:

* RealUAF not awarded. No freed object, stale pointer, double release, refcount misuse, or post-lifetime object use is established.
* Memory corruption not awarded. The primitive is an invalid system-register access that raises UNDEF, not heap/list/page/object corruption.
* C/I impact not awarded. No information disclosure, integrity corruption, privilege escalation, arbitrary write, or protected-object access is shown.
* Remote reachability not awarded. The trigger is CPU hotplug on the affected host, not packet or network input.
* KVM guest-to-host leverage not awarded. Although pKVM is involved, supplied evidence does not show that malicious guest code can trigger the complete sequence or obtain a host corruption primitive.
* Extra Availability not awarded because the same kernel panic is already counted once as reliable DoS.
* Manual-review preserving UAF / repeated-release / RCU lifetime rules do not apply.

3. Reachability analysis

The affected path is arm64 CPU feature discovery during CPU bring-up. The issue occurs only in pKVM when MTE is disabled by `CONFIG_ARM64_MTE=n` or `arm64.nomte`, because KVM sets `HCR_EL2.TID5` and `GMID_EL1` access traps.

The complete trigger described by the commit requires a CPU to be offlined and brought back online. That is normally a host-admin-equivalent operation through CPU hotplug control. No ordinary local user path is established. No namespace-scoped capability path is established. No network path is established.

Prior configuration requirements exist: arm64 pKVM, MTE disabled, and CPU hotplug activity. These are deployment conditions, but the decisive reachability constraint is host CPU management control. The issue is not a race in the exploitability sense. It is an ordered state/configuration mismatch during CPU online.

4. Primitive and severity interpretation

The exact primitive is an invalid read of `GMID_EL1` when effective MTE availability is disabled and the register is trapped by pKVM. This causes an unhandled host UNDEF and kernel panic.

This is not active structured UAF, not fixed stale storage, and not a repeated release/refcount mutation. The demonstrated consequence is host DoS. The speculative consequence beyond crash is weak because the patch does not establish memory corruption, object lifetime corruption, guest-controlled host state corruption, or authorization bypass.

PrimitiveClass is CLASS_C because the behavior is crash/DoS from invalid-state handling. ImportantCandidate is NO because no meaningful exploitation primitive beyond panic is established and the trigger is host-admin-equivalent. AutoDowngradeAllowed is YES because no unresolved corruption, ownership, authorization, or boundary-bypass primitive remains.

Likely operational severity is ordinary Moderate or lower for security triage, while still important as a stability fix for affected pKVM hosts.

5. One-sentence report phrase

An arm64 pKVM host can panic when `__cpuinfo_store_cpu()` reads `GMID_EL1` despite MTE being disabled, but triggering requires host CPU hotplug control and no memory corruption or privilege escalation primitive is established.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch supports host panic only, with host-admin-equivalent CPU hotplug required and no UAF, corruption, C/I impact, guest-only trigger, or authorization bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: arm64: Don't read GMID_EL1 when MTE is disabled
Commit: d2d22ff377cd43f2e9660426ff22e3cd4c534637
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2d22ff377cd43f2e9660426ff22e3cd4c534637

Commit description:

__cpuinfo_store_cpu() gates the GMID_EL1 read on the raw
ID_AA64PFR1_EL1, so it reads the register even when the kernel has
disabled MTE (CONFIG_ARM64_MTE=n or arm64.nomte). KVM sets HCR_EL2.TID5
in that case, and pKVM injects an UNDEF the host cannot handle:

  Internal error: Oops - Undefined instruction: 0000000002000000 [#1]  SMP
  pc : __cpuinfo_store_cpu+0xf4/0x264
  Kernel panic - not syncing: Attempted to kill the idle task!

Only pKVM reaches it, and only after a CPU is offlined and brought back
online: its CPU_ON relay sets the host HCR before the CPU enters EL1,
while plain nVHE sets it at CPUHP_AP_KVM_ONLINE.

Gate the read on the CPU's own ID_AA64PFR1_EL1 with the command-line
override applied, and on CONFIG_ARM64_MTE, which no register reflects.
The boot CPU stores its registers before init_cpu_features() strips an
unsafe override, so clamp against the hardware value here too.

Fixes: f35abcb ("KVM: arm64: Trap MTE access and discovery when MTE is disabled")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/include/asm/cpu.h
  arch/arm64/include/asm/cpufeature.h
  arch/arm64/kernel/cpufeature.c
  arch/arm64/kernel/cpuinfo.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=d2d22ff377cd43f2e9660426ff22e3cd4c534637

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

The original Linux kernel CVE announcement for CVE-2026-89981 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89981

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:H
  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: unknown
  Paranoid ActionableScore: 1

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).