[CVE-2026-64604][MODERATE REGULAR] KVM: VMX: Grab vmcs12 on CR8 interception update iff vCPU is in guest mode

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

CVE: CVE-2026-64604
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: VMX: Grab vmcs12 on CR8 interception update iff vCPU is in guest mode
Commit: c7cd3605244c924249dea32632e1bc3e89bda543
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c7cd3605244c924249dea32632e1bc3e89bda543
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64604
Analysis date: Thu, 06 Aug 2026 04:03:41 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended but not Important-class
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-64604 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64604

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.

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

CVE-2026-64604	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-667;CWE-703;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'KVM VMX CR8 intercept update could call get_vmcs12 even when the vCPU is not in guest mode, triggering a lockdep warning during vCPU creation. The observed path is local through KVM ioctls such as KVM_CREATE_VCPU and not through network traffic. For the CVSS the PR:L is used because a local process needs access to /dev/kvm, which is often delegated to non-root users through the kvm group. Impact is primarily a warning or local denial of service on systems configured to panic on warnings. No memory corruption, rmap stale pointer, SPTE overwrite, information leak, or privilege escalation primitive is indicated by this patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended but not Important-class (with actual score 3)	YES	DANGER VIRT HARDWARE SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended but not Important-class**::

2. Signal breakdown
* Local unprivileged trigger: +1. A local process with access to `/dev/kvm` can reach the path via KVM ioctls such as `KVM_CREATE_VCPU`. This is often delegated to non-root users through the `kvm` group.
* Broad/common subsystem exposure: +1. KVM is a common virtualization subsystem on affected hosts, although not universally enabled or network-exposed.
* Paranoid availability concern: +1. The observed failure is a WARN/lockdep assertion, not a demonstrated crash, but hardened systems with `panic_on_warn` may convert this into host DoS.
* No memory corruption signal. The patch only moves `get_vmcs12()` behind `is_guest_mode(vcpu)`. It does not show UAF, stale rmap, SPTE overwrite, double free, OOB access, refcount abuse, or type confusion.
* No LPE signal. There is no plausible privilege-escalation primitive supported by the patch.
* No remote reachability. Triggering requires local KVM ioctl access.

3. Reachability analysis
A local process that can open and use `/dev/kvm` can plausibly trigger the warning during vCPU creation. This is not reachable through network traffic or guest packet/data input. Namespace impact is limited: access depends on whether `/dev/kvm` is exposed to a container or delegated service account. The affected path is relevant mainly on systems running KVM/VMX virtualization.

Call-site confidence: high. The commit message includes the exact stack path from `KVM_CREATE_VCPU` through `kvm_arch_vcpu_create()` and `kvm_lapic_reset()` into `vmx_update_cr8_intercept()`.

4. Severity interpretation
This behaves like a low-end Moderate or borderline triage item, not an Important-class kernel vulnerability. The realistic impact is a local warning and possible DoS only on systems where warnings are fatal. Theoretical KVM concern is worth noting, but the patch context does not support memory corruption, confidentiality impact, integrity impact, guest-to-host escape, or privilege escalation.

5. One-sentence report phrase
KVM VMX could call `get_vmcs12()` while the vCPU was not in guest mode during CR8 intercept updates, allowing a local `/dev/kvm` user to trigger a lockdep warning during vCPU creation, with practical impact limited to warning-only behavior or DoS on `panic_on_warn` systems.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED.
Reason: the score is borderline only because KVM is security-sensitive and `/dev/kvm` may be available to non-root users, but the patch itself supports warning-only impact and no concrete corruption or privilege-escalation primitive.

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

Patch: KVM: VMX: Grab vmcs12 on CR8 interception update iff vCPU is in guest mode
Commit: c7cd3605244c924249dea32632e1bc3e89bda543
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c7cd3605244c924249dea32632e1bc3e89bda543

Commit description:

When updating CR8 intercepts, get vmcs12 if and only if the vCPU is in
guest mode so that a future change can have update CR8 intercepts during
vCPU creation, without running afoul of get_vmcs12()'s lockdep assertion.

  ------------[ cut here ]------------
  debug_locks && !(lock_is_held(&(&vcpu->mutex)->dep_map) || !refcount_read(&vcpu->kvm->users_count))
  WARNING: arch/x86/kvm/vmx/nested.h:61 at get_vmcs12 arch/x86/kvm/vmx/nested.h:60 [inline], CPU#0: syz.2.19/5879
  WARNING: arch/x86/kvm/vmx/nested.h:61 at vmx_update_cr8_intercept+0x3de/0x4e0 arch/x86/kvm/vmx/vmx.c:6879, CPU#0: syz.2.19/5879
  Modules linked in:
  CPU: 0 UID: 0 PID: 5879 Comm: syz.2.19 Not tainted syzkaller #0 PREEMPT(full)
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
  RIP: 0010:get_vmcs12 arch/x86/kvm/vmx/nested.h:60 [inline]
  RIP: 0010:vmx_update_cr8_intercept+0x3de/0x4e0 arch/x86/kvm/vmx/vmx.c:6879
  Call Trace:
   <TASK>
   apic_update_ppr arch/x86/kvm/lapic.c:984 [inline]
   kvm_lapic_reset+0x1c24/0x2980 arch/x86/kvm/lapic.c:3023
   kvm_vcpu_reset+0x44c/0x1bf0 arch/x86/kvm/x86.c:12986
   kvm_arch_vcpu_create+0x746/0x8b0 arch/x86/kvm/x86.c:12847
   kvm_vm_ioctl_create_vcpu+0x428/0x930 virt/kvm/kvm_main.c:4201
   kvm_vm_ioctl+0x893/0xd50 virt/kvm/kvm_main.c:5159
   vfs_ioctl fs/ioctl.c:51 [inline]
   __do_sys_ioctl fs/ioctl.c:597 [inline]
   __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>

No functional change intended.

Reported-by: syzbot ci <syzbot+ci493c6d734b63e050@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/6a2adf3b.3b0a2d4e.8c8d1.0012.GAE@google.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260618174347.1981064-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/vmx/nested.h
  arch/x86/kvm/vmx/vmx.c
  arch/x86/kvm/lapic.c
  arch/x86/kvm/x86.c
  virt/kvm/kvm_main.c
  fs/ioctl.c
  arch/x86/entry/syscall_64.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=c7cd3605244c924249dea32632e1bc3e89bda543

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

The original Linux kernel CVE announcement for CVE-2026-64604 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64604

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: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: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: 2
  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).