[CVE-2026-72278][LOW] KVM: arm64: nv: Re-translate VNCR before injecting abort

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

CVE: CVE-2026-72278
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: arm64: nv: Re-translate VNCR before injecting abort
Commit: ea7a76d7d614b5f82b4d0785f9af3550e860a71a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea7a76d7d614b5f82b4d0785f9af3550e860a71a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72278
Analysis date: Sat, 15 Aug 2026 20:38:32 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A VNCR abort handling bug in KVM arm64 nested virtualization can cause stale read-only VNCR translation state to be reused for a later write abort, leading to incorrect nested abort injection and nested guest denial of service without evidence of host memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-72278 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72278

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.

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

CVE-2026-72278	LOW	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:C/C:N/I:L/A:L';CWE-754;CWE-755;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.2';DESCR 'A VNCR abort handling bug in KVM arm64 nested virtualization can occur when the guest reads the VNCR page before writing to it, leaving the cached host translation backed by a read-only PFN. A later write abort may then be handled without re-fetching the mapping for write access, causing KVM to inject an incorrect abort into the nested guest and disrupting nested virtualization. For the CVSS the PR:L is used because reliable triggering requires local KVM access or control of a guest capable of exercising nested virtualization, not full host administrator privileges. The issue is not network reachable and is driven by local virtualization control paths rather than packet traffic. Impact is mainly denial of service or correctness loss for the nested guest. No concrete UAF, stale rmap traversal, OOB write, arbitrary write, host crash, or host privilege escalation primitive is visible from the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-72278 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 2a359e072596fcb2e9e85017a865e3618a2fe5b5 YES  NO NO unknown 	MAYBE	READ LOCK VIRT WARNONLY NOMEMCHK HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or low-privileged trigger: +1. Triggering requires local KVM access or control of a guest path capable of exercising arm64 nested virtualization.
* Availability impact realistic: +1. The bug can disrupt nested virtualization by injecting an incorrect abort into the nested guest.
* Rare AND difficult-to-reach subsystem/configuration: -1. The affected path is arm64 KVM nested virtualization VNCR handling, not a broadly exposed default host path.

Paranoid adjustment:

* Do not apply the rare-path penalty: +1 relative to conservative. If the evaluated environment uses arm64 nested virtualization as a supported workload feature, the path should not be treated as rare for that deployment.

Not awarded:

* Remote reachable: +0. No network-triggerable path is shown.
* Firmware-mediated external influence: +0. No firmware-controlled input is involved.
* Generic memory corruption: +0. The patch does not show UAF, OOB write, stale rmap traversal, double free, type confusion, or arbitrary write.
* Privilege escalation plausible: +0. No host memory corruption or guest-to-host escalation primitive is visible.
* Reliable kernel crash / strong host DoS: +0. The described consequence is incorrect VNCR abort handling and nested guest disruption, not a demonstrated host panic.

## 3. Reachability analysis

The bug is reachable through KVM arm64 nested virtualization when a guest reads the VNCR page before writing to it, causing the cached translation to be read-only. A later write abort may then be handled without re-fetching the mapping with write permissions. The realistic attacker is a local user with KVM access or an actor controlling a guest that can exercise nested virtualization. This is not directly network reachable and is not triggered by packet traffic. Namespace or container impact depends on whether `/dev/kvm` is delegated, but the issue still requires KVM access and nested virtualization capability. Call-site confidence: high, because the affected handler and changed logic are present in the patch.

## 4. Severity interpretation

This behaves more like an ordinary Moderate or Low-like virtualization correctness/DoS issue than an Important-class kernel vulnerability. The theoretical boundary impact is limited to incorrect nested guest behavior. The patch does not support a memory corruption primitive, host crash primitive, stale rmap issue, or privilege escalation path. A paranoid score of 2 is defensible for environments where arm64 nested virtualization is actively used, but it still remains below Actionable Moderate.

## 5. One-sentence report phrase

A VNCR abort handling bug in KVM arm64 nested virtualization can cause stale read-only VNCR translation state to be reused for a later write abort, leading to incorrect nested abort injection and nested guest denial of service without evidence of host memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch indicates a VNCR translation refresh and permission-handling correctness issue, with no concrete UAF, OOB write, stale rmap traversal, arbitrary write, host crash, or privilege escalation primitive.

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

Patch: KVM: arm64: nv: Re-translate VNCR before injecting abort
Commit: ea7a76d7d614b5f82b4d0785f9af3550e860a71a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea7a76d7d614b5f82b4d0785f9af3550e860a71a

Commit description:

KVM faults in the VNCR page with FOLL_WRITE whenever the guest aborts
for a write, similar to how a regular stage-2 mapping is handled. It is
entirely possible that the guest reads from the VNCR before writing to
it, in which case the PFN could only be read-only.

Invalidate the VNCR TLB and re-fetch the translation upon taking a VNCR
abort, allowing the host mapping to be faulted in for write the second
time around. Interestingly enough, this also satisfies the ordering
requirements of FEAT_ETS2/3 between descriptor updates and MMU faults.

Cc: stable@vger.kernel.org
Fixes: 2a359e0 ("KVM: arm64: nv: Handle mapping of VNCR_EL2 at EL2")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Oliver Upton <oupton@kernel.org>
Link: https://patch.msgid.link/20260618234207.1063941-4-oupton@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/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=ea7a76d7d614b5f82b4d0785f9af3550e860a71a

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

The original Linux kernel CVE announcement for CVE-2026-72278 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72278

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:L
  The Best / paranoid CVSS score: 5.2

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).