Re: [PATCH 1/2] KVM: nVMX: fix msr bitmaps to prevent L2 from accessing L0 x2APIC

[Yang Zhang]

On 2016/8/9 20:23, Radim KrÄmÃÅ wrote:
> 2016-08-09 17:32+0800, Yang Zhang:
> > On 2016/8/9 2:16, Radim KrÄmÃÅ wrote:
> > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > > @@ -6995,16 +6982,21 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
> > >  		return 1;
> > >  	}
> > >
> > > +	if (cpu_has_vmx_msr_bitmap()) {
> > > +		vmx->nested.msr_bitmap =
> > > +				(unsigned long *)__get_free_page(GFP_KERNEL);
> > > +		if (!vmx->nested.msr_bitmap)
> > > +			goto out_msr_bitmap;
> > > +	}
> > > +
> >
> > We export msr_bitmap to guest even it is not supported by hardware. So we
> > need to allocate msr_bitmap for L1 unconditionally.
>
> We do emulate the feature, but the vmx->nested.msr_bitmap is used only
> if VMX supports it to avoid some VM exits:
>
> > > @@ -9957,10 +9938,10 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
> > >  	}
> > >
> > >  	if (cpu_has_vmx_msr_bitmap() &&
> > > -	    exec_control & CPU_BASED_USE_MSR_BITMAPS) {
> > > -		nested_vmx_merge_msr_bitmap(vcpu, vmcs12);
> > > -		/* MSR_BITMAP will be set by following vmx_set_efer. */
> > > -	} else
> > > +	    exec_control & CPU_BASED_USE_MSR_BITMAPS &&
> > > +	    nested_vmx_merge_msr_bitmap(vcpu, vmcs12))
> > > +		; /* MSR_BITMAP will be set by following vmx_set_efer. */
> > > +	else
> > >  		exec_control &= ~CPU_BASED_USE_MSR_BITMAPS;
>
> The else branch is taken if !cpu_has_vmx_msr_bitmap() and disables msr
> bitmaps.  Similar check for vmx_set_msr_bitmap(), so the NULL doesn't
> even get written to VMCS.
>
> KVM always uses L1's msr bitmaps when emulating the feature.

Yes, you are right. I forget the bitmap only used by hardware.:(

--
Yang
Alibaba Cloud Computing