Re: [PATCH 05/10] KVM: nVMX: Allocate shadow vmcs

[Gleb Natapov <gleb@redhat.com>]

On Thu, Apr 18, 2013 at 10:07:11AM +0300, Abel Gordon wrote:
> 
> 
> Gleb Natapov <gleb@redhat.com> wrote on 18/04/2013 09:38:43 AM:
> 
> > On Wed, Apr 17, 2013 at 08:07:40PM +0300, Abel Gordon wrote:
> > > Allocate a shadow vmcs used by the processor to shadow part of the
> fields
> > > stored in the software defined VMCS12 (let L1 access fields without
> causing
> > > exits). Note we keep a shadow vmcs only for the current vmcs12.
> > Once a vmcs12
> > > becomes non-current, its shadow vmcs is released.
> > >
> > >
> > > Signed-off-by: Abel Gordon <abelg@il.ibm.com>
> > > ---
> > >  arch/x86/kvm/vmx.c |   12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > >
> > > --- .before/arch/x86/kvm/vmx.c   2013-04-17 19:58:32.000000000 +0300
> > > +++ .after/arch/x86/kvm/vmx.c   2013-04-17 19:58:32.000000000 +0300
> > > @@ -355,6 +355,7 @@ struct nested_vmx {
> > >     /* The host-usable pointer to the above */
> > >     struct page *current_vmcs12_page;
> > >     struct vmcs12 *current_vmcs12;
> > > +   struct vmcs *current_shadow_vmcs;
> > >
> > >     /* vmcs02_list cache of VMCSs recently used to run L2 guests */
> > >     struct list_head vmcs02_pool;
> > > @@ -5517,6 +5518,7 @@ static int handle_vmon(struct kvm_vcpu *
> > >  {
> > >     struct kvm_segment cs;
> > >     struct vcpu_vmx *vmx = to_vmx(vcpu);
> > > +   struct vmcs *shadow_vmcs;
> > >
> > >     /* The Intel VMX Instruction Reference lists a bunch of bits that
> > >      * are prerequisite to running VMXON, most notably cr4.VMXE must be
> > > @@ -5540,6 +5542,16 @@ static int handle_vmon(struct kvm_vcpu *
> > >        kvm_inject_gp(vcpu, 0);
> > >        return 1;
> > >     }
> > > +   if (enable_shadow_vmcs) {
> > > +      shadow_vmcs = alloc_vmcs();
> > > +      if (!shadow_vmcs)
> > > +         return -ENOMEM;
> > > +      /* mark vmcs as shadow */
> > > +      shadow_vmcs->revision_id |= (1u << 31);
> > > +      /* init shadow vmcs */
> > > +      vmcs_clear(shadow_vmcs);
> > > +      vmx->nested.current_shadow_vmcs = shadow_vmcs;
> > > +   }
> > >
> > Guest can ddos host by calling vmxon repeatedly causing host to leak
> > memory. This point to a bug in vmxon implementation. vmxon should call
> > nested_vmx_failInvalid() if (vmx->nested.vmxon).
> 
> Good point. I just checked the spec (VMXON pseudo-code) to verify
> the right emulation:
> According to the pseudo-code we should:
>   ELSE VMfail(“VMXON executed in VMX root operation”) which means:
> 
> VMfail(ErrorNumber):
>  IF VMCS pointer is valid
>  THEN VMfailValid(ErrorNumber);
>  ELSE VMfailInvalid;
>  FI;
> 
> 
> So, I'll call nested_vmx_failValid if nested.current_vmptr != -1ull
> Otherwise, I'll call nested_vmx_failInvalid.
> 
Just call nested_vmx_failValid(). It does that internally.

--
			Gleb.