Re: [BUG] arch/x86/kvm/vmx/vmx_onhyperv.h:109:36: error: dereference of NULL ‘0’

Sean Christopherson
2024-08-14T15:08:08Z

On Wed, Aug 14, 2024, Vitaly Kuznetsov wrote:
> What I meant is something along these lines (untested):
>
> diff --git a/arch/x86/kvm/vmx/vmx_onhyperv.h b/arch/x86/kvm/vmx/vmx_onhyperv.h
> index eb48153bfd73..e2d8c67d0cad 100644
> --- a/arch/x86/kvm/vmx/vmx_onhyperv.h
> +++ b/arch/x86/kvm/vmx/vmx_onhyperv.h
> @@ -104,6 +104,14 @@ static inline void evmcs_load(u64 phys_addr)
> struct hv_vp_assist_page *vp_ap =
> hv_get_vp_assist_page(smp_processor_id());
>
> + /*
> + * When enabling eVMCS, KVM verifies that every CPU has a valid hv_vp_assist_page()
> + * and aborts enabling the feature otherwise. CPU onlining path is also checked in
> + * vmx_hardware_enable(). With this, it is impossible to reach here with vp_ap == NULL
> + * but compilers may still complain.
> + */
> + BUG_ON(!vp_ap);

A full BUG_ON() is overkill, and easily avoided. If we want to add a sanity
check here and do more than just WARN, then it's easy enough to plumb in @vcpu
and make this a KVM_BUG_ON() so that the VM dies, i.e. so that KVM doesn't risk
corrupting the guest somehow.

> +
> if (current_evmcs->hv_enlightenments_control.nested_flush_hypercall)
> vp_ap->nested_control.features.directhypercall = 1;
> vp_ap->current_nested_vmcs = phys_addr;
>
> the BUG_ON() will silence compiler warning as well as become a sentinel
> for future code changes.
>
> --
> Vitaly
>