[CVE-2026-45987][LOW] KVM: nSVM: Sync interrupt shadow to cached vmcb12 after VMRUN of L2

[CVE-2026-45987][LOW] KVM: nSVM: Sync interrupt shadow to cached vmcb12 after VMRUN of L2

[AL-KERNEL]

CVE: CVE-2026-45987
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: nSVM: Sync interrupt shadow to cached vmcb12 after VMRUN of L2
Commit: 1709418535a8df95532999d61b03d59975280258
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1709418535a8df95532999d61b03d59975280258
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45987
Analysis date: Wed, 27 May 2026 11:54:48 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
KVM nSVM could lose L2 interrupt shadow state during nested state restore, causing an L2 vCPU hang under specific restore ordering without memory corruption or host privilege impact.

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

The original Linux kernel CVE announcement for CVE-2026-45987 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45987

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-2026-45987	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-664;CWE-755;CWE-670;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'KVM nSVM did not copy int_state from vmcb02 into the cached vmcb12 after running L2, so nested state save and restore could lose the L2 interrupt shadow state. If KVM_SET_VCPU_EVENTS is applied before KVM_SET_NESTED_STATE, the interrupt shadow may be restored to vmcb01 instead of vmcb02 and the L2 vCPU can hang, for example around HLT and wakeup interrupt ordering. For the CVSS the PR:L is used because triggering requires local access to KVM or control of a VMM that can run nested SVM and restore nested state. AC:H is used because the issue depends on nested virtualization, migration or state restore ordering, and a specific interrupt shadow condition. The issue is not network reachable and does not involve UAF, out of bounds access, arbitrary write, information disclosure, or host privilege escalation. Impact is denial of service only, mainly a stuck L2 vCPU or nested guest.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	DANGER VIRT HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Local trigger through KVM/VMM control: +1
  Triggering requires local access to KVM userspace or control of a VMM managing nested SVM state.

* Availability impact realistic: +1
  Incorrect interrupt shadow restore can hang the L2 vCPU or nested guest.

* Reliable guest DoS plausible: +1
  The failure mode is a stuck L2 around interrupt delivery and HLT ordering after nested state restore.

Subtracting signals:

* Special configuration/state ordering: -1
  Requires nested SVM and specific ordering of `KVM_SET_VCPU_EVENTS` before `KVM_SET_NESTED_STATE`.

* Impact mostly confined to nested guest: -1
  The described effect is L2 vCPU hang, not host crash, host memory corruption, or host privilege bypass.

No signals apply for remote reachability, memory corruption, UAF, arbitrary write, information disclosure, host privilege escalation, or a demonstrated security-boundary escape.

## 3. Reachability analysis

The issue is local to a KVM host/VMM workflow. A user or process controlling a VMM can run nested SVM and restore nested state in the problematic order. A normal remote attacker cannot trigger it directly.

Namespaces and containers matter only if `/dev/kvm` and nested virtualization control are delegated. The issue is not default-reachable to arbitrary local users unless KVM access is granted.

## 4. Severity interpretation

This behaves like a Low-like nested virtualization state-restore DoS. It is a correctness bug in saving/restoring L2 interrupt shadow state, not memory corruption or a guest-to-host escape.

Theoretical security impact is limited because the described consequence is a stuck L2 vCPU. Realistic evidence does not support C/I impact or host compromise.

## 5. One-sentence report phrase

KVM nSVM could lose L2 interrupt shadow state during nested state restore, causing an L2 vCPU hang under specific restore ordering without memory corruption or host privilege impact.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a nested guest availability/correctness issue with no supported memory corruption, remote exposure, or host security-boundary escape.

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

Patch: KVM: nSVM: Sync interrupt shadow to cached vmcb12 after VMRUN of L2
Commit: 1709418535a8df95532999d61b03d59975280258
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1709418535a8df95532999d61b03d59975280258

Commit description:

After VMRUN in guest mode, nested_sync_control_from_vmcb02() syncs
fields written by the CPU from vmcb02 to the cached vmcb12. This is
because the cached vmcb12 is used as the authoritative copy of some of
the controls, and is the payload when saving/restoring nested state.

int_state is also written by the CPU, specifically bit 0 (i.e.
SVM_INTERRUPT_SHADOW_MASK) for nested VMs, but it is not sync'd to
cached vmcb12. This does not cause a problem if KVM_SET_NESTED_STATE
preceeds KVM_SET_VCPU_EVENTS in the restore path, as an interrupt shadow
would be correctly restored to vmcb02 (KVM_SET_VCPU_EVENTS overwrites
what KVM_SET_NESTED_STATE restored in int_state).

However, if KVM_SET_VCPU_EVENTS preceeds KVM_SET_NESTED_STATE, an
interrupt shadow would be restored into vmcb01 instead of vmcb02. This
would mostly be benign for L1 (delays an interrupt), but not for L2. For
L2, the vCPU could hang (e.g. if a wakeup interrupt is delivered before
a HLT that should have been in an interrupt shadow).

Sync int_state to the cached vmcb12 in nested_sync_control_from_vmcb02()
to avoid this problem. With that, KVM_SET_NESTED_STATE restores the
correct interrupt shadow state, and if KVM_SET_VCPU_EVENTS follows it
would overwrite it with the same value.

Fixes: cc440cd ("KVM: nSVM: implement KVM_GET_NESTED_STATE and KVM_SET_NESTED_STATE")
CC: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260225005950.3739782-3-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/svm/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=1709418535a8df95532999d61b03d59975280258

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

The original Linux kernel CVE announcement for CVE-2026-45987 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45987

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:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 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).