Code: static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
{
	int r;
	struct kvm_vcpu *vcpu, *v;

	vcpu = kvm_arch_vcpu_create(kvm, id);
	if (IS_ERR(vcpu))
		return PTR_ERR(vcpu);

	preempt_notifier_init(&vcpu->preempt_notifier, &kvm_preempt_ops);

	r = kvm_arch_vcpu_setup(vcpu);
	if (r)
 		goto vcpu_destroy;
 
 	mutex_lock(&kvm->lock);
 	if (atomic_read(&kvm->online_vcpus) == KVM_MAX_VCPUS) {
 		r = -EINVAL;
 		goto unlock_vcpu_destroy;
	}

	kvm_for_each_vcpu(r, v, kvm)
		if (v->vcpu_id == id) {
			r = -EEXIST;
			goto unlock_vcpu_destroy;
		}

	BUG_ON(kvm->vcpus[atomic_read(&kvm->online_vcpus)]);

	/* Now it's all set up, let userspace reach it */
	kvm_get_kvm(kvm);
	r = create_vcpu_fd(vcpu);
	if (r < 0) {
		kvm_put_kvm(kvm);
		goto unlock_vcpu_destroy;
	}

	kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
	smp_wmb();
	atomic_inc(&kvm->online_vcpus);

	mutex_unlock(&kvm->lock);
	return r;

unlock_vcpu_destroy:
	mutex_unlock(&kvm->lock);
vcpu_destroy:
	kvm_arch_vcpu_destroy(vcpu);
	return r;
}

Vulnerability Type: DoS 
CWE ID: CWE-399
Summary: The KVM implementation in the Linux kernel before 3.3.6 allows host OS users to cause a denial of service (NULL pointer dereference and host OS crash) by making a KVM_CREATE_IRQCHIP ioctl call after a virtual CPU already exists.
Commit Message: KVM: Ensure all vcpus are consistent with in-kernel irqchip settings

(cherry picked from commit 3e515705a1f46beb1c942bb8043c16f8ac7b1e9e)

If some vcpus are created before KVM_CREATE_IRQCHIP, then
irqchip_in_kernel() and vcpu->arch.apic will be inconsistent, leading
to potential NULL pointer dereferences.

Fix by:
- ensuring that no vcpus are installed when KVM_CREATE_IRQCHIP is called
- ensuring that a vcpu has an apic if it is installed after KVM_CREATE_IRQCHIP

This is somewhat long winded because vcpu->arch.apic is created without
kvm->lock held.

Based on earlier patch by Michael Ellerman.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>