Re: [PATCH RFC] KVM: x86: Drop WARN_ON_ONCE() for concurrent PIC interrupt deassertion

Let's try again with
https://github.com/google/syzkaller/commit/36faab20efd417e2f41d35311544bebb8e3af80e

#syz reject

On Mon, Jun 22, 2026 at 6:19 PM 'syzbot' via
syzkaller-upstream-moderation
<[email protected]> wrote:
>
> A false positive warning can be triggered in kvm_check_and_inject_events()
> and vmx_check_nested_events() due to a natural race condition when a
> level-triggered PIC interrupt is deasserted concurrently with vCPU entry.
>
> The WARN_ON_ONCE(irq == -1) in kvm_check_and_inject_events() was added to
> catch cases where kvm_cpu_has_injectable_intr() returns true, but
> kvm_cpu_get_interrupt() returns -1 (meaning no interrupt is pending). The
> assumption was that this could only happen due to a KVM bug. However, for
> level-triggered PIC interrupts, a concurrent deassertion by the device
> makes this a perfectly valid and expected scenario.
>
> For example, if a device asserts a level-triggered interrupt, the IRR bit
> is set and kvm_cpu_has_injectable_intr() returns true. If the device then
> deasserts the interrupt before kvm_cpu_get_interrupt() is called, the IRR
> bit is cleared, and kvm_cpu_get_interrupt() correctly returns -1.
>
> A similar issue exists in vmx_check_nested_events(), where
> kvm_cpu_has_interrupt() is checked first. If it returns true because a PIC
> interrupt is pending, but the interrupt is concurrently deasserted, both
> kvm_cpu_get_extint() and kvm_apic_has_interrupt() will return -1,
> triggering a WARN_ON_ONCE(irq < 0).
>
> Remove the WARN_ON_ONCE() macros in both functions, as WARN_ON must not be
> used for conditions that can legitimately happen, and pr_err() should be
> used instead if necessary. In this case, no error logging is needed because
> the code already handles irq == -1 gracefully by skipping the injection,
> which is the correct behavior when a level-triggered interrupt is
> deasserted before it can be injected.
>
> ------------[ 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>
>
> 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=8568ceee-8527-4f42-b840-1fd49498df2e
> 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..72d434f3f 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4446,7 +4446,12 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>                 }
>
>                 irq = kvm_apic_has_interrupt(vcpu);
> -               if (WARN_ON_ONCE(irq < 0))
> +               /*
> +                * A level-triggered PIC interrupt can be deasserted concurrently,
> +                * making it possible for irq to be -1 even if kvm_cpu_has_interrupt()
> +                * returned true.
> +                */
> +               if (irq < 0)
>                         goto no_vmexit;
>
>                 /*
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0550359ed..af36643b9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10857,7 +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)) {
> +                       /*
> +                        * A level-triggered PIC interrupt can be deasserted
> +                        * concurrently with vCPU entry, making it possible
> +                        * for irq to be -1 even if kvm_cpu_has_injectable_intr()
> +                        * returned true.
> +                        */
> +                       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);
>
>
> base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6
> --
> This is an AI-generated patch subject to moderation.
> Reply with '#syz upstream' to Sign-off the patch as a human author
> and send it to the upstream kernel mailing lists.
> Reply with '#syz reject' to reject it ('#syz unreject' to undo).
>
> See https://goo.gle/syzbot-ai-patches for information about AI-generated patches.
> You can comment on the patch as usual, syzbot will try to address
> the comments and send a new version of the patch if necessary.
> syzbot engineers can be reached at [email protected].
>
> --
> You received this message because you are subscribed to the Google Groups "syzkaller-upstream-moderation" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].
> To view this discussion visit https://groups.google.com/d/msgid/syzkaller-upstream-moderation/077c14b4-6f79-45a9-9b11-bd625c8ecff3%40mail.kernel.org.