Re: [PATCH] KVM: x86/xen: Fix bug in kvm_xen_vcpu_set_attr()

[David Woodhouse]

On Thu, 2022-07-28 at 14:47 -0500, Coleman Dietsch wrote:
> This crash appears to be happening when vcpu->arch.xen.timer is
> already set and kvm_xen_init_timer(vcpu) is called.
> 
> During testing with the syzbot reproducer code it seemed apparent
> that the else if statement in the KVM_XEN_VCPU_ATTR_TYPE_TIMER switch
> case was not being reached, which is where the
> kvm_xen_stop_timer(vcpu) call is located.
> 
> Link: https://syzkaller.appspot.com/bug?id=8234a9dfd3aafbf092cc5a7cd9842e3ebc45fc42
> 
> Reported-and-tested-by: syzbot+e54f930ed78eb0f85281@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> Signed-off-by: Coleman Dietsch <dietschc@xxxxxxx>


Modulo the cosmetic issues discussed,

Acked-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

Thanks.

> ---
>  arch/x86/kvm/xen.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 610beba35907..4b4b985813c5 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -707,6 +707,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  		break;
>  
>  	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> +		/* Stop current timer if it is enabled */
> +		if (kvm_xen_timer_enabled(vcpu)) {
> +			kvm_xen_stop_timer(vcpu);
> +			vcpu->arch.xen.timer_virq = 0;
> +		}
> +
>  		if (data->u.timer.port) {
>  			if (data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
>  				r = -EINVAL;
> @@ -720,9 +726,6 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  				kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
>  						    data->u.timer.expires_ns -
>  						    get_kvmclock_ns(vcpu->kvm));
> -		} else if (kvm_xen_timer_enabled(vcpu)) {
> -			kvm_xen_stop_timer(vcpu);
> -			vcpu->arch.xen.timer_virq = 0;
>  		}
>  
>  		r = 0;
>

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