[CVE-2026-80873][LOW] KVM: arm64: nv: Write ESR_EL2 for injected nested SError exceptions

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

CVE: CVE-2026-80873
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: arm64: nv: Write ESR_EL2 for injected nested SError exceptions
Commit: 09f35145f3a4aacea4d9b914ad895bf5af8fc4ae
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09f35145f3a4aacea4d9b914ad895bf5af8fc4ae
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80873
Analysis date: Fri, 04 Sep 2026 18:29:24 -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-2026-80873 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80873

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-2026-80873	LOW	https://www.kernelcve.org/list/?q=CVE-2026-80873 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-665;CWE-703;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.8';DESCR 'KVM arm64 nested virtualization can expose stale ESR_EL2 state to an L2 hypervisor because injected nested SError exceptions did not write the constructed syndrome to ESR_EL2. A guest workload using nested virtualization may observe an incorrect exception syndrome and handle the event incorrectly, which can lead to guest side malfunction or guest denial of service. For the CVSS the PR:L is used because triggering requires local access to KVM or control of a guest context with nested virtualization enabled, but does not require full host administrator privileges. The issue is not network reachable and is not triggered by packet traffic. Impact is mainly guest side integrity and availability degradation. The patch context does not indicate host memory corruption, host privilege escalation, or host information disclosure.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-80873 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	VIRT WARNONLY SIMPLEFIX HARDWARE LINUS  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=CLASS_C
RealUAF=NO
HostAdminRequired=NO
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: 2
* Paranoid score: 2. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 conservative: KVM arm64 nested virtualization exposes incorrect guest-visible exception state. Injected nested SError does not update ESR_EL2, so an L2 hypervisor may observe a stale syndrome.
* +1 paranoid: limited guest-side availability or integrity impact is plausible if the L2 hypervisor makes decisions based on the stale ESR_EL2 value.
* No Real UAF awarded. The word stale refers to stale architectural register state, not post-free object use.
* No memory corruption awarded. The patch only adds a missing vcpu_write_sys_reg() for ESR_EL2.
* No controlled reclaim, arbitrary write, OOB access, refcount misuse, rmap issue, or host object lifetime violation is established.
* No reliable host kernel crash awarded. The described consequence is incorrect nested guest exception state.
* No KVM guest-to-host leverage awarded. There is no concrete host-memory corruption primitive or guest-to-host boundary escape.
* No remote reachability awarded. This is not triggered by network packets.
* No Important-grade security-boundary bypass awarded. The affected state is guest-visible nested virtualization state, not a demonstrated host privilege or data boundary bypass.

3. Reachability analysis

The evaluated attacker is a local KVM or guest-context attacker, not a remote network attacker. Triggering requires a KVM arm64 nested virtualization scenario where nested SError or nested SEA EASE handling is exercised. A malicious or buggy L2 hypervisor can observe stale ESR_EL2 after injected nested SError, but the supplied evidence does not show host memory corruption or host compromise.

Host administrator privilege is not positively required because the relevant operation is within KVM guest execution or a local VMM context with KVM access. Namespace-scoped privilege is not relevant. No prior special host setup beyond using nested virtualization is established from the patch. No special rare hardware penalty is applied beyond the normal arm64 KVM nested virtualization context.

4. Primitive and severity interpretation

The exact primitive is a missing architectural state update: ESR_EL2 is written for synchronous EL2 exception injection but was not written for SError injection. This is a logic and state-consistency bug, not a lifetime bug.

PrimitiveClass is CLASS_C because the demonstrated behavior is invalid guest-visible state. RealUAF is NO. There is no active structured stale host object and no fixed stale storage UAF. There is no concrete repeated release or refcount mutation.

The demonstrated consequence is stale ESR_EL2 observed by an L2 hypervisor, potentially causing incorrect guest-side exception handling or guest-side denial of service. The speculative consequence would be stronger guest-side malfunction, but there is no patch-supported path to host privilege escalation, host information disclosure, or guest-to-host memory corruption.

ImportantCandidate is NO because no Class A primitive and no meaningful unresolved Class B corruption primitive exists. AutoDowngradeAllowed is YES because the issue is a constrained nested-guest correctness problem without established host security impact.

5. One-sentence report phrase

KVM arm64 nested SError injection can leave ESR_EL2 stale for an L2 hypervisor, causing incorrect guest-side exception handling or guest denial of service, but the patch does not establish host memory corruption, host information disclosure, or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch shows a narrow nested virtualization state-consistency fix with no UAF, no refcount mutation, no OOB access, no host crash primitive, and no demonstrated guest-to-host security boundary bypass.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: arm64: nv: Write ESR_EL2 for injected nested SError exceptions
Commit: 09f35145f3a4aacea4d9b914ad895bf5af8fc4ae
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09f35145f3a4aacea4d9b914ad895bf5af8fc4ae

Commit description:

kvm_inject_el2_exception() writes ESR_EL2 for synchronous exceptions
but not for SError. enter_exception64() does not write ESR_ELx for any
exception type, so the constructed syndrome is dropped. A guest L2
hypervisor taking a nested SError observes stale ESR_EL2.

This affects both kvm_inject_nested_serror() and the EASE path in
kvm_inject_nested_sea().

Write ESR_EL2 for except_type_serror, matching except_type_sync.

Fixes: 77ee70a ("KVM: arm64: nv: Honor SError exception routing / masking")
Reported-by: sashiko <sashiko@sashiko.dev>
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260615131116.390977-1-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/emulate-nested.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=09f35145f3a4aacea4d9b914ad895bf5af8fc4ae

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

The original Linux kernel CVE announcement for CVE-2026-80873 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80873

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: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: 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).