KVM: arm/arm64: vgic: Avoid injecting reserved IRQ numbers

commit 4839ddc2 upstream.

Commit fd1d0ddf (KVM: arm/arm64: check IRQ number on userland
injection) rightly limited the range of interrupts userspace can
inject in a guest, but failed to consider the (unlikely) case where
a guest is configured with 1024 interrupts.

In this case, interrupts ranging from 1020 to 1023 are unuseable,
as they have a special meaning for the GIC CPU interface.

Make sure that these number cannot be used as an IRQ. Also delete
a redundant (and similarily buggy) check in kvm_set_irq.
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
[ kamal: backport to 3.19-stable: no change to kvm_set_irq ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

virt/kvm/arm/vgic.c

@@ -1722,7 +1722,7 @@ int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int irq_num,
 			goto out;
 	}
 
-	if (irq_num >= kvm->arch.vgic.nr_irqs)
+	if (irq_num >= min(kvm->arch.vgic.nr_irqs, 1020))
 		return -EINVAL;
 
 	vcpu_id = vgic_update_irq_pending(kvm, cpuid, irq_num, level);