[CVE-2026-74517][MODERATE 7.0] KVM: x86: Cancel delayed I/O APIC EOI handling before destroying vCPUs

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

CVE: CVE-2026-74517
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: x86: Cancel delayed I/O APIC EOI handling before destroying vCPUs
Commit: ed56a6b58222f9c1f4115a0bd2788dd6ed6022e2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed56a6b58222f9c1f4115a0bd2788dd6ed6022e2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74517
Analysis date: Sat, 15 Aug 2026 17:11:26 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in KVM x86 VM destruction can allow delayed I/O APIC EOI work to run after vCPUs are freed, causing a host kernel use-after-free and likely local denial of service for users able to access `/dev/kvm`.

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

The original Linux kernel CVE announcement for CVE-2026-74517 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74517

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 7.0
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-2026-74517	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A host kernel use after free can occur in KVM x86 because delayed IO APIC EOI work may run after vCPUs have already been destroyed. The delayed worker can re enter interrupt delivery and read freed vCPU or LAPIC related state in __kvm_irq_delivery_to_apic_fast, as shown by the KASAN slab use after free report. For the CVSS the PR:L is used because reliable triggering normally requires a local user or virtualization service account that can create, run, and destroy KVM VMs through /dev/kvm, rather than full host administrator privileges. The issue is not directly network reachable and depends on VM lifecycle control plus a race window during destruction. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, confidentiality and integrity impact are also considered because the primitive is a host kernel UAF in KVM interrupt delivery and may require manual review for privilege escalation risk.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	READ DANGER NULLPTR VIRT INIT UAF DMAorINTERRUPT HARDWARE KPANIC 	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**. A local user or virtualization service account with access to `/dev/kvm` can create, run, and destroy VMs.
* **Memory corruption, weak race-UAF primitive: +1**. The patch and KASAN trace show a real slab-use-after-free, but only a stale read is demonstrated.
* **Real lifetime corruption: +1**. Delayed I/O APIC EOI work can outlive vCPU destruction and access freed vCPU/LAPIC-related state.
* **Reliable kernel crash / strong DoS: +1**. KASAN reports UAF in host kernel workqueue context, and the realistic impact is host kernel crash.
* **Broad/common virtualization exposure: +1**. KVM is widely deployed on virtualization hosts and developer systems.
* **Hard or unreliable race / special timing required: -1**. Triggering depends on a VM teardown window and delayed work timing.

Conservative total: **4**

Paranoid additional signal:

* **Availability impact realistic: +1**. In the paranoid view, the UAF occurs in host kernel KVM interrupt delivery and can plausibly cause a system-wide host crash, not just a contained VM failure.

Paranoid total: **5**

Not awarded:

* **No remote reachable signal**. The bug is not network-triggerable by itself.
* **No strong LPE signal**. The patch shows UAF, but not attacker-controlled reclaim, overwrite, type confusion, callback control, or refcount takeover.
* **No confidentiality/integrity signal in the conservative score**. The demonstrated primitive is stale read/use-after-free leading to crash, not an info leak or write primitive.

## 3. Reachability analysis

The bug is reachable by a local actor that can use KVM, typically a user in the `kvm` group, a VM management service account, or a reduced-privilege virtualization component. Full host root is not required in many deployments, so treating this as local PR:L is more practical than PR:H.

Namespaces do not automatically make this reachable, because `/dev/kvm` access is still required. However, containerized or delegated virtualization environments can expose KVM to non-root service accounts.

The path is not directly network reachable. A remote attacker would need a separate path to control VM lifecycle operations through a management plane.

Realistic exploitation conditions require creating/running a VM, arranging delayed I/O APIC EOI work, and destroying the VM in the vulnerable lifetime window. That makes this timing-sensitive, but the KASAN trace confirms the lifetime bug is real.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a confirmed host-kernel UAF in KVM, not merely a warning or validation bug. The realistic demonstrated impact is local DoS via host kernel crash.

It is not a clear Important-class issue from the patch alone because there is no demonstrated reclaim, overwrite, type confusion, or controlled object replacement. The paranoid score is therefore kept at **5**, making it an **Actionable Moderate** that should not be auto-closed without review.

## 5. One-sentence report phrase

A race in KVM x86 VM destruction can allow delayed I/O APIC EOI work to run after vCPUs are freed, causing a host kernel use-after-free and likely local denial of service for users able to access `/dev/kvm`.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is confirmed KVM host-kernel memory corruption with a real UAF trace. Even though the demonstrated primitive is mainly DoS, KVM UAFs historically deserve manual review for possible privilege escalation or host escape relevance.

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

Patch: KVM: x86: Cancel delayed I/O APIC EOI handling before destroying vCPUs
Commit: ed56a6b58222f9c1f4115a0bd2788dd6ed6022e2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed56a6b58222f9c1f4115a0bd2788dd6ed6022e2

Commit description:

Cancel (and flush) the I/O APIC's delayed EOI handling work during the
"pre VM destroy" phase, before vCPUs are destroyed, as processing the EOI
broadcast will inject another IRQ if the line is asserted, i.e. will try
to deliver an IRQ to the target vCPU(s).  Canceling the work after vCPUs
are destroyed leads to UAF if the delayed work is processed after vCPUs are
destroyed.

  BUG: KASAN: slab-use-after-free in __kvm_irq_delivery_to_apic_fast+0x9bf/0xa20 arch/x86/kvm/lapic.c:1250
  Read of size 8 at addr ffff8880499abea0 by task kworker/1:2/1218

  CPU: 1 UID: 0 PID: 1218 Comm: kworker/1:2 Not tainted 7.1.0-rc7 #5 PREEMPT(lazy)
  Hardware name: QEMU Ubuntu 25.10 PC v2 (i440FX + PIIX, + 10.1 machine, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
  Workqueue: events kvm_ioapic_eoi_inject_work
  Call Trace:
   <TASK>
   __dump_stack lib/dump_stack.c:94
   dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
   print_address_description mm/kasan/report.c:378
   print_report+0x139/0x4ad mm/kasan/report.c:482
   kasan_report+0xe4/0x1d0 mm/kasan/report.c:595
   __kvm_irq_delivery_to_apic_fast+0x9bf/0xa20 arch/x86/kvm/lapic.c:1250
   __kvm_irq_delivery_to_apic+0xd8/0xbf0 arch/x86/kvm/lapic.c:1345
   kvm_irq_delivery_to_apic arch/x86/kvm/lapic.h:129
   ioapic_service+0x308/0x590 arch/x86/kvm/ioapic.c:492
   kvm_ioapic_eoi_inject_work+0x13c/0x190 arch/x86/kvm/ioapic.c:532
   process_one_work+0xa59/0x19a0 kernel/workqueue.c:3314
   process_scheduled_works kernel/workqueue.c:3397
   worker_thread+0x5eb/0xe50 kernel/workqueue.c:3478
   kthread+0x370/0x450 kernel/kthread.c:436
   ret_from_fork+0x72b/0xd30 arch/x86/kernel/process.c:158
   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
   </TASK>

Note, the VM is unreachable once kvm_destroy_vm() starts, and scheduling
new work via kvm_ioapic_send_eoi() can only be done via KVM_RUN, i.e.
requires a live vCPU.

Alternatively, KVM could simply destroy the I/O APIC during the "pre" phase
of VM destruction, but that gets more than a bit sketchy as KVM expects the
I/O APIC to exist if ioapic_in_kernel() is true, and nested virtualization
in particular has a bad habit of touching VM-scope state during vCPU
destruction.  E.g. attempting to free the PIC during the pre phase would
lead to a NULL pointer dereference in kvm_cpu_has_extint(), and it's not
hard to imagine the I/O APIC having a similar flaw.

Fixes: 17bcd71 ("KVM: x86: Free vCPUs before freeing VM state")
Reported-by: <zdi-disclosures@trendmicro.com>
Reported-by: Zhong Wang <wangzhong.c0ss4ck@bytedance.com>
Reported-by: Xuanqing Shi <shixuanqing.11@bytedance.com>
Cc: stable@vger.kernel.org
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260727171718.543491-1-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/lapic.c
  lib/dump_stack.c
  mm/kasan/report.c
  arch/x86/kvm/lapic.h
  arch/x86/kvm/ioapic.c
  kernel/workqueue.c
  kernel/kthread.c
  arch/x86/kernel/process.c
  arch/x86/entry/entry_64.S
  arch/x86/kvm/x86.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=ed56a6b58222f9c1f4115a0bd2788dd6ed6022e2

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

The original Linux kernel CVE announcement for CVE-2026-74517 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74517

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).