Re: [PATCH v3 29/32] KVM: arm64: GICv4.1: Allow SGIs to switch between HW and SW interrupts

[Zenghui Yu]

Hi,

On 2020/1/15 10:49, Shaokun Zhang wrote:
> Hi Marc, [This is from Nianyao]
>
> On 2019/12/24 19:10, Marc Zyngier wrote:
> > In order to let a guest buy in the new, active-less SGIs, we
> > need to be able to switch between the two modes.
> >
> > Handle this by stopping all guest activity, transfer the state
> > from one mode to the other, and resume the guest.
> >
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > ---

[...]

> > diff --git a/virt/kvm/arm/vgic/vgic-v4.c b/virt/kvm/arm/vgic/vgic-v4.c
> > index c2fcde104ea2..063785fd2dc7 100644
> > --- a/virt/kvm/arm/vgic/vgic-v4.c
> > +++ b/virt/kvm/arm/vgic/vgic-v4.c
> > @@ -97,6 +97,102 @@ static irqreturn_t vgic_v4_doorbell_handler(int irq, void *info)
> >   	return IRQ_HANDLED;
> >   }
> >   
> > +static void vgic_v4_sync_sgi_config(struct its_vpe *vpe, struct vgic_irq *irq)
> > +{
> > +	vpe->sgi_config[irq->intid].enabled	= irq->enabled;
> > +	vpe->sgi_config[irq->intid].group 	= irq->group;
> > +	vpe->sgi_config[irq->intid].priority	= irq->priority;
> > +}
> > +
> > +static void vgic_v4_enable_vsgis(struct kvm_vcpu *vcpu)
> > +{
> > +	struct its_vpe *vpe = &vcpu->arch.vgic_cpu.vgic_v3.its_vpe;
> > +	int i;
> > +
> > +	/*
> > +	 * With GICv4.1, every virtual SGI can be directly injected. So
> > +	 * let's pretend that they are HW interrupts, tied to a host
> > +	 * IRQ. The SGI code will do its magic.
> > +	 */
> > +	for (i = 0; i < VGIC_NR_SGIS; i++) {
> > +		struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, i);
> > +		struct irq_desc *desc;
> > +		int ret;
> > +
> > +		if (irq->hw) {
> > +			vgic_put_irq(vcpu->kvm, irq);
> > +			continue;
> > +		}
> > +
> > +		irq->hw = true;
> > +		irq->host_irq = irq_find_mapping(vpe->sgi_domain, i);
>
> I think we need to check whether irq_find_mapping returns 0.
>
> > +		vgic_v4_sync_sgi_config(vpe, irq);
> > +		/*
> > +		 * SGIs are initialised as disabled. Enable them if
> > +		 * required by the rest of the VGIC init code.
> > +		 */
> > +		desc = irq_to_desc(irq->host_irq);
> > +		ret = irq_domain_activate_irq(irq_desc_get_irq_data(desc),
> > +					      false);
>
> If irq->host_irq is not valid , in irq_domain_activate_irq, it will trigger NULL pointer
> dereference in host kernel.
> I meet a problem here. When hw support GIC4.1, and host kernel is started with
> kvm-arm.vgic_v4_enable=0, starting a virtual machine will trigger NULL pointer
> dereference in host.

I think the thing is that we should _not_ try to configure vSGIs at all
if kvm-arm.vgic_v4_enable=0 (which indicates we don't allow use of the
GICv4 of direct injection).

We currently set kvm_vgic_global_state.has_gicv4_1 to true if HW support
GICv4.1, regardless whatever the gicv4_enable is (see patch#23 -
vgic_v3_probe).  I think this is what actually needs fixing.


Thanks,
Zenghui