Re: [PATCH v4 3/3] KVM: nVMX: Emulate EPTP switching for the L1 hypervisor

[Bandan Das]

Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> writes:

> [David did a great review, so I'll just point out things I noticed.]
>
> 2017-07-11 09:51+0200, David Hildenbrand:
>> On 10.07.2017 22:49, Bandan Das wrote:
>> > When L2 uses vmfunc, L0 utilizes the associated vmexit to
>> > emulate a switching of the ept pointer by reloading the
>> > guest MMU.
>> > 
>> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> > Signed-off-by: Bandan Das <bsd@xxxxxxxxxx>
>> > ---
>> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> > @@ -7784,11 +7801,46 @@ static int handle_vmfunc(struct kvm_vcpu *vcpu)
>> >  	}
>> >  
>> >  	vmcs12 = get_vmcs12(vcpu);
>> > -	if ((vmcs12->vm_function_control & (1 << function)) == 0)
>> > +	if (((vmcs12->vm_function_control & (1 << function)) == 0) ||
>> > +	    WARN_ON_ONCE(function))
>> 
>> "... instruction causes a VM exit if the bit at position EAX is 0 in the
>> VM-function controls (the selected VM function is
>> not enabled)."
>> 
>> So g2 can trigger this WARN_ON_ONCE, no? I think we should drop it then
>> completely.
>
> It assumes that vm_function_control is not > 1, which is (should be)
> guaranteed by VM entry check, because the nested_vmx_vmfunc_controls MSR
> is 1.
>
>> > +		goto fail;
>
> The rest of the code assumes that the function is
> VMX_VMFUNC_EPTP_SWITCHING, so some WARN_ON_ONCE is reasonable.
>
> Writing it as
>
>   WARN_ON_ONCE(function != VMX_VMFUNC_EPTP_SWITCHING)
>
> would be cleared and I'd prefer to move the part that handles
> VMX_VMFUNC_EPTP_SWITCHING into a new function. (Imagine that Intel is
> going to add more than one VM FUNC. :])

IMO, for now, this should be fine because we are not even passing through the
hardware's eptp switching. Even if there are other vm functions, they
won't be available for the nested case and cause any conflict.

>> > +	if (!nested_cpu_has_ept(vmcs12) ||
>> > +	    !nested_cpu_has_eptp_switching(vmcs12))
>> > +		goto fail;
>
> This brings me to a missing vm-entry check:
>
>  If âEPTP switchingâ VM-function control is 1, the âenable EPTâ
>  VM-execution control must also be 1. In addition, the EPTP-list address
>  must satisfy the following checks:
>  â Bits 11:0 of the address must be 0.
>  â The address must not set any bits beyond the processorâs
>    physical-address width.
>
> so this one could be
>
>   if (!nested_cpu_has_eptp_switching(vmcs12) ||
>       WARN_ON_ONCE(!nested_cpu_has_ept(vmcs12)))

I will reverse the order here but the vm entry check is unnecessary because
the check on the list address is already being done in this function.

> after adding the check.