Re: [PATCH RFC] KVM: x86: Drop WARN_ON_ONCE() for concurrently disappearing interrupts

On Tue, Jun 23, 2026 at 1:47 PM 'syzbot' via
syzkaller-upstream-moderation
<[email protected]> wrote:
>
> A warning can be triggered in kvm_check_and_inject_events() when an
> interrupt disappears between the time it is checked via
> kvm_cpu_has_injectable_intr() and the time it is fetched via
> kvm_cpu_get_interrupt(). This occurs because the warning incorrectly
> assumes that if an interrupt is injectable, fetching it must always return
> a valid interrupt vector (i.e., not -1).
>
> However, this assumption is broken by level-triggered interrupts that are
> deasserted concurrently by another thread. For example, if a misconfigured
> PIT or a PCI device asserts and then immediately deasserts a
> level-triggered interrupt, the VCPU thread might see the pending interrupt
> during the check but find it gone during the fetch, resulting in
> kvm_cpu_get_interrupt() returning -1.
>
> The warning manifests as follows:
>
> ------------[ cut here ]------------
> irq == -1
> WARNING: arch/x86/kvm/x86.c:10860 at kvm_check_and_inject_events
> arch/x86/kvm/x86.c:10860 [inline]
> WARNING: arch/x86/kvm/x86.c:10860 at vcpu_enter_guest
> arch/x86/kvm/x86.c:11356 [inline]
> WARNING: arch/x86/kvm/x86.c:10860 at vcpu_run+0x57ec/0x7950
> arch/x86/kvm/x86.c:11770
> RIP: 0010:kvm_check_and_inject_events arch/x86/kvm/x86.c:10860 [inline]
> RIP: 0010:vcpu_enter_guest arch/x86/kvm/x86.c:11356 [inline]
> RIP: 0010:vcpu_run+0x57ec/0x7950 arch/x86/kvm/x86.c:11770
> Call Trace:
>  <TASK>
>  kvm_arch_vcpu_ioctl_run+0x1193/0x2070 arch/x86/kvm/x86.c:12125
>  kvm_vcpu_ioctl+0xa61/0xfd0 virt/kvm/kvm_main.c:4470
>  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>
>
> Since this is a legitimate Time-Of-Check to Time-Of-Use (TOCTOU) race
> condition that can occur during normal operation, WARN_ON_ONCE() must not
> be used for conditions that can legitimately happen. The patch removes the
> WARN_ON_ONCE() in kvm_check_and_inject_events() and replaces it with a
> pr_err_ratelimited() to log the event instead. The same flawed assumption
> exists in vmx_check_nested_events(), where kvm_apic_has_interrupt() can
> return -1 after kvm_cpu_has_interrupt() returns true. The patch applies the
> same fix there by removing the WARN_ON_ONCE() and gracefully handling the
> missing interrupt with a rate-limited error message.
>
> Fixes: bf672720e83c ("KVM: x86: check the kvm_cpu_get_interrupt result before using it")
> Assisted-by: Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=dd769db18693736eee89
> Link: https://syzkaller.appspot.com/ai_job?id=b37654f7-62ad-4e4a-a244-0949c0ca4246
> To: "Borislav Petkov" <[email protected]>
> To: "Dave Hansen" <[email protected]>
> To: <[email protected]>
> To: "Ingo Molnar" <[email protected]>
> To: "Paolo Bonzini" <[email protected]>
> To: "Sean Christopherson" <[email protected]>
> To: "Thomas Gleixner" <[email protected]>
> To: <[email protected]>
> Cc: "H. Peter Anvin" <[email protected]>
> Cc: <[email protected]>
>
> ---
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 3fe88f29b..c3e9c9d9b 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c

Let's not touch nested.c for now. Respin with just the arch/x86/kvm/x86.c part.

> @@ -4446,8 +4446,11 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>                 }
>
>                 irq = kvm_apic_has_interrupt(vcpu);
> -               if (WARN_ON_ONCE(irq < 0))
> +               if (irq < 0) {
> +                       pr_err_ratelimited(
> +                               "KVM: APIC interrupt disappeared between checking and fetching\n");
>                         goto no_vmexit;
> +               }
>
>                 /*
>                  * If the IRQ is L2's PI notification vector, process posted
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0550359ed..c5b4cddd9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10857,10 +10857,13 @@ static int kvm_check_and_inject_events(struct kvm_vcpu *vcpu,
>                 if (r) {
>                         int irq = kvm_cpu_get_interrupt(vcpu);
>
> -                       if (!WARN_ON_ONCE(irq == -1)) {
> +                       if (irq != -1) {
>                                 kvm_queue_interrupt(vcpu, irq, false);
>                                 kvm_x86_call(inject_irq)(vcpu, false);
>                                 WARN_ON(kvm_x86_call(interrupt_allowed)(vcpu, true) < 0);
> +                       } else {
> +                               pr_err_ratelimited(
> +                                       "KVM: interrupt disappeared between checking and fetching\n");
>                         }
>                 }
>                 if (kvm_cpu_has_injectable_intr(vcpu))
>
>
> base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6
> --
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