Re: [PATCH v2 04/13] KVM: x86: dynamic kvm_apic_map

[Yang Zhang]

On 2016/7/11 15:43, Paolo Bonzini wrote:
> On 11/07/2016 08:07, Yang Zhang wrote:
> > >      mutex_lock(&kvm->arch.apic_map_lock);
> > >
> > > +    kvm_for_each_vcpu(i, vcpu, kvm)
> > > +        if (kvm_apic_present(vcpu))
> > > +            max_id = max(max_id, kvm_apic_id(vcpu->arch.apic));
> > > +
> > > +    new = kzalloc(sizeof(struct kvm_apic_map) +
> > > +                  sizeof(struct kvm_lapic *) * (max_id + 1),
> > > GFP_KERNEL);
> > > +
> >
> > I think this may cause the host runs out of memory if a malicious guest
> > did follow thing:
> > 1. vcpu a is doing apic map recalculation.
> > 2. vcpu b write the apic id with 0xff
> > 3. then vcpu b enable the x2apic: in kvm_lapic_set_base(), we will set
> > apic_base to new value before reset the apic id.
> > 4. vcpu a may see the x2apic enabled in vcpu b plus an old apic
> > id(0xff), and max_id will become (0xff >> 24).
>
> The bug is not really here but in patch 6---but you're right nevertheless!
>
> I guess the easiest solution is to replace kvm_apic_id with a field in
> struct kvm_lapic, which is already shifted right by 24 in xAPIC mode.

Or we can just simply put the assignment of apic_base to the end.

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index fdc05ae..9c69059 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1745,7 +1745,6 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 
value)
                return;
        }

-       vcpu->arch.apic_base = value;

        /* update jump label if enable bit changes */
        if ((old_value ^ value) & MSR_IA32_APICBASE_ENABLE) {
@@ -1753,7 +1752,6 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 
value)
                        static_key_slow_dec_deferred(&apic_hw_disabled);
                else
                        static_key_slow_inc(&apic_hw_disabled.key);
-               recalculate_apic_map(vcpu->kvm);
        }

        if ((old_value ^ value) & X2APIC_ENABLE) {
@@ -1764,6 +1762,8 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 
value)
                        kvm_x86_ops->set_virtual_x2apic_mode(vcpu, false);
        }

+       vcpu->arch.apic_base = value;
+       recalculate_apic_map(vcpu->kvm);
        apic->base_address = apic->vcpu->arch.apic_base &
                             MSR_IA32_APICBASE_BASE;


btw, i noticed that there is no apic map recalculation after turn off 
the x2apic mode.Is it correct?

--
best regards
yang