Re: [PATCH 4/4] KVM: X86: Potential 'index out of range' bug

[Sean Christopherson <seanjc@google.com>]

On Fri, Sep 03, 2021, Vitaly Kuznetsov wrote:
> Jiang Jiasheng <jiasheng@iscas.ac.cn> writes:
> 
> > The kvm_get_vcpu() will call for the array_index_nospec()
> > with the value of atomic_read(&(v->kvm)->online_vcpus) as size,
> > and the value of constant '0' as index.
> > If the size is also '0', it will be unreasonabe
> > that the index is no less than the size.
> >
> 
> Can this really happen?
> 
> 'online_vcpus' is never decreased, it is increased with every
> kvm_vm_ioctl_create_vcpu() call when a new vCPU is created and is set to
> 0 when all vCPUs are destroyed (kvm_free_vcpus()).
> 
> kvm_guest_time_update() takes a vcpu as a parameter, this means that at
> least 1 vCPU is currently present so 'online_vcpus' just can't be zero.

Agreed, but doing kvm_get_vcpu() is ugly and overkill.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 86539c1686fa..cc1cb9a401cd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2969,7 +2969,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
                                       offsetof(struct compat_vcpu_info, time));
        if (vcpu->xen.vcpu_time_info_set)
                kvm_setup_pvclock_page(v, &vcpu->xen.vcpu_time_info_cache, 0);
-       if (v == kvm_get_vcpu(v->kvm, 0))
+       if (!kvm_vcpu_get_idx(v))
                kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
        return 0;
 }