Re: [syzbot] [kvm?] WARNING in vcpu_run

[syzbot <syzbot+1522459a74d26b0ac33a@syzkaller.appspotmail.com>]

> #syz test git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

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> master
>
> From 2090baeb71f48c9ad62129bd457a2c3bb5f0ed55 Mon Sep 17 00:00:00 2001
> From: Alessandro Ratti <alessandro@0x65c.net>
> Date: Sat, 3 Jan 2026 22:01:32 +0100
> Subject: [PATCH] KVM: x86: Handle -EBUSY from nested event check in
>  vcpu_block()
>
> When a vCPU running in nested guest mode attempts to block (e.g. due to
> HLT), kvm_check_nested_events() may return -EBUSY to indicate that a
> nested event is pending but cannot be injected immediately, such as
> when event delivery is temporarily blocked in the guest.
>
> Currently, vcpu_block() treats this as a generic error and exits to
> userspace. This can cause the vCPU to repeatedly block without making
> forward progress, delaying nested event injection and potentially
> leading to guest hangs under rare timing conditions.
>
> Handle -EBUSY explicitly by returning to the vCPU run loop and retrying
> guest entry instead of blocking. This allows nested event delivery to
> complete once the temporary blocking condition clears.
>
> This issue was triggered by syzkaller during nested virtualization
> stress testing.
>
> Fixes: 45405155d876 ("KVM: x86: WARN if a vCPU gets a valid wakeup
> that KVM can't yet inject")
> Reported-by: syzbot+1522459a74d26b0ac33a@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?id=b646ef310afe5b51ae0372e1de8fdd68baad9eb5
> Signed-off-by: Alessandro Ratti <alessandro@0x65c.net>
> ---
>  arch/x86/kvm/x86.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ff8812f3a129..d5cf9a7ff8c5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11596,7 +11596,15 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
>         if (is_guest_mode(vcpu)) {
>                 int r = kvm_check_nested_events(vcpu);
>
> -               WARN_ON_ONCE(r == -EBUSY);
> +               /*
> +                * -EBUSY indicates a nested event is pending but cannot be
> +                * injected immediately (e.g., event delivery is temporarily
> +                * blocked). Return to the vCPU run loop to retry guest entry
> +                * instead of blocking, which would lose the pending event.
> +                */
> +               if (r == -EBUSY)
> +                       return 1;
> +
>                 if (r < 0)
>                         return 0;
>         }
> -- 
> 2.52.0