[CVE-2026-46295][MODERATE REGULAR] KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty

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

CVE: CVE-2026-46295
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty
Commit: bb1703949dcaa9a49c338dee075f659f4634214d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb1703949dcaa9a49c338dee075f659f4634214d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46295
Analysis date: Mon, 08 Jun 2026 15:12:08 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended but not Actionable Moderate
Manual review required: YES

Summary:
A race in KVM x86 posted interrupt synchronization can cause incorrect `max_irr` reporting and trigger a host WARNING during nested VM execution, leading mainly to DoS risk in configurations where warnings are fatal.

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

The original Linux kernel CVE announcement for CVE-2026-46295 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46295

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.

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

CVE-2026-46295	MODERATE	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-362;CWE-703;CWE-754;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 x86 posted interrupt handling can report an incorrect max_irr when a race occurs between PIR harvesting on the target vCPU and setting PID.ON on a sender vCPU. The interrupt is not lost because it already resides in IRR, but the second sync path can see PID.ON with an empty PIR and return -1 without scanning existing IRR. This can trigger a WARNING in vmx_check_nested_events and waste an L2 VM Enter Exit cycle. For the CVSS the PR:L is used because an attacker needs the ability to run or control local KVM guest execution, likely with nested virtualization or similar stress conditions. The issue is not network reachable and does not show a UAF, OOB access, arbitrary write, or information leak primitive. Impact is mainly denial of service through warning spam or degraded host responsiveness, with A:H only in the paranoid score if panic_on_warn or similar policy turns the warning into a host crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended but not Actionable Moderate (with actual score 3) 	MAYBE	VIRT WARNONLY RACE HARDWARE TEST  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended but not Actionable Moderate**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or low privilege trigger: +1
  A local user or process with access to `/dev/kvm` can potentially exercise the affected KVM vCPU path. This is PR:L rather than PR:H in many virtualization deployments.

* Broad/common subsystem exposure: +1
  KVM x86 is a common virtualization subsystem on hosts running VMs.

* Hard or timing dependent race: -1
  The issue requires a specific interleaving between posted interrupt delivery and PIR to IRR synchronization.

* Availability impact realistic: +1
  The demonstrated effect is a kernel WARNING and wasted nested VM entry/exit work. This is availability relevant, but weak in the conservative case because the interrupt is not lost and no corruption is shown.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1
  Only for configurations where `panic_on_warn` or similar policy turns the WARNING into a host crash.

Not counted:

* Generic memory corruption: +0
  The patch does not show UAF, double free, OOB write, arbitrary write, SPTE/rmap lifetime corruption, or page-cache corruption.

* Privilege escalation plausible: +0
  No realistic LPE primitive is supported by the patch context.

* Remote reachable: +0
  This is not network-triggerable directly.

## 3. Reachability analysis

The bug is reachable through local KVM execution paths, especially nested VM or posted interrupt stress scenarios. A normal remote network attacker cannot directly trigger it unless they already control workload execution inside a VM management environment.

Privileges depend on deployment. On many hosts, access to `/dev/kvm` is granted to users in a `kvm` group or to VM management services, so PR:L is the practical interpretation. Containers only matter if `/dev/kvm` is passed through or if a containerized workload can control KVM vCPU execution.

The path is common on KVM virtualization hosts, but the demonstrated trigger depends on nested VM stress and a narrow race. The interrupt is explicitly not lost, and the main consequence is inconsistent interrupt reporting that causes a WARNING and wasted VM entry/exit work.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like KVM correctness issue in the conservative case. It deserves manual review because it is a KVM race in interrupt handling, but the evidence supports DoS only.

Theoretical risk is limited because there is no demonstrated memory lifetime bug, no KVM MMU rmap/SPTE stale pointer pattern, and no attacker-controlled overwrite or disclosure primitive. The paranoid score only raises availability for `panic_on_warn` style deployments and does not treat this as privilege escalation or memory corruption.

## 5. One-sentence report phrase

A race in KVM x86 posted interrupt synchronization can cause incorrect `max_irr` reporting and trigger a host WARNING during nested VM execution, leading mainly to DoS risk in configurations where warnings are fatal.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This is a KVM race in nested interrupt handling, so it should not be blindly auto-closed, but current evidence supports warning or DoS only and does not justify Important-class handling.

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

Patch: KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty
Commit: bb1703949dcaa9a49c338dee075f659f4634214d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb1703949dcaa9a49c338dee075f659f4634214d

Commit description:

Fall back to apic_find_highest_vector() when PID.ON is set but PIR
turns out to be empty, to correctly report the highest pending interrupt
from the existing IRR.

In a nested VM stress test, the following WARNING fires in
vmx_check_nested_events() when kvm_cpu_has_interrupt() reports a pending
interrupt but the subsequent kvm_apic_has_interrupt() (which invokes
vmx_sync_pir_to_irr() again) returns -1:

  WARNING: CPU: 99 PID: 57767 at arch/x86/kvm/vmx/nested.c:4449 vmx_check_nested_events+0x6bf/0x6e0 [kvm_intel]
  Call Trace:
   kvm_check_and_inject_events
   vcpu_enter_guest.constprop.0
   vcpu_run
   kvm_arch_vcpu_ioctl_run
   kvm_vcpu_ioctl
   __x64_sys_ioctl
   do_syscall_64
   entry_SYSCALL_64_after_hwframe

The root cause is a race between vmx_sync_pir_to_irr() on the target vCPU
and __vmx_deliver_posted_interrupt() on a sender vCPU.  The sender
performs two individually-atomic operations that are not a single
transaction:

  1. pi_test_and_set_pir(vector)  -- sets the PIR bit
  2. pi_test_and_set_on()         -- sets PID.ON

The following interleaving triggers the bug:

  Sender vCPU (IPI):              Target vCPU (1st sync_pir_to_irr):
  B1: set PIR[vector]
                                  A1: pi_clear_on()
                                  A2: pi_harvest_pir() -> sees B1 bit
                                  A3: xchg() -> consumes bit, PIR=0
                                      (1st sync returns correct max_irr)
  B2: set PID.ON = 1

                                  Target vCPU (2nd sync_pir_to_irr):
                                  C1: pi_test_on() -> TRUE (from B2)
                                  C2: pi_clear_on() -> ON=0
                                  C3: pi_harvest_pir() -> PIR empty
                                  C4: *max_irr = -1, early return
                                      IRR NOT SCANNED

The interrupt is not lost (it resides in the IRR from the first sync and
is recovered on the next vcpu_enter_guest() iteration), but the incorrect
max_irr causes a spurious WARNING and a wasted L2 VM-Enter/VM-Exit cycle.

Fixes: b41f863 ("KVM: VMX: Isolate pure loads from atomic XCHG when processing PIR")
Reported-by: Farrah Chen <farrah.chen@intel.com>
Analyzed-by: Chenyi Qiang <chenyi.qiang@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/kvm/20260428070349.1633238-1-chenyi.qiang@intel.com/T/
Link: https://patch.msgid.link/20260503201703.108231-2-pbonzini@redhat.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.c
  arch/x86/kvm/lapic.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=bb1703949dcaa9a49c338dee075f659f4634214d

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

The original Linux kernel CVE announcement for CVE-2026-46295 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46295

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: 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).