linux-kernel - Re: [PATCH 14/25] KVM: TDX: initialize VM with TDX specific parameters

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Message-ID: <ZtlWzUO+VGzt7Z89@yzhao56-desk.sh.intel.com>
Date: Thu, 5 Sep 2024 14:59:25 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Tony Lindgren <tony.lindgren@...ux.intel.com>
CC: Rick Edgecombe <rick.p.edgecombe@...el.com>, <seanjc@...gle.com>,
	<pbonzini@...hat.com>, <kvm@...r.kernel.org>, <kai.huang@...el.com>,
	<isaku.yamahata@...il.com>, <xiaoyao.li@...el.com>,
	<linux-kernel@...r.kernel.org>, Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH 14/25] KVM: TDX: initialize VM with TDX specific
 parameters

On Mon, Sep 02, 2024 at 01:31:29PM +0300, Tony Lindgren wrote:
> On Thu, Aug 29, 2024 at 02:27:56PM +0800, Yan Zhao wrote:
> > On Mon, Aug 12, 2024 at 03:48:09PM -0700, Rick Edgecombe wrote:
> > > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > > 
> > ...
> > > +static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
> > > +{
> ...
> 
> > > +	kvm_tdx->tsc_offset = td_tdcs_exec_read64(kvm_tdx, TD_TDCS_EXEC_TSC_OFFSET);
> > > +	kvm_tdx->attributes = td_params->attributes;
> > > +	kvm_tdx->xfam = td_params->xfam;
> > > +
> > > +	if (td_params->exec_controls & TDX_EXEC_CONTROL_MAX_GPAW)
> > > +		kvm->arch.gfn_direct_bits = gpa_to_gfn(BIT_ULL(51));
> > > +	else
> > > +		kvm->arch.gfn_direct_bits = gpa_to_gfn(BIT_ULL(47));
> > > +
> > Could we introduce a initialized field in struct kvm_tdx and set it true
> > here? e.g
> > +       kvm_tdx->initialized = true;
> > 
> > Then reject vCPU creation in tdx_vcpu_create() before KVM_TDX_INIT_VM is
> > executed successfully? e.g.
> > 
> > @@ -584,6 +589,9 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu)
> >         struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> >         struct vcpu_tdx *tdx = to_tdx(vcpu);
> > 
> > +       if (!kvm_tdx->initialized)
> > +               return -EIO;
> > +
> >         /* TDX only supports x2APIC, which requires an in-kernel local APIC. */
> >         if (!vcpu->arch.apic)
> >                 return -EINVAL;
> > 
> > Allowing vCPU creation only after TD is initialized can prevent unexpected
> > userspace access to uninitialized TD primitives.
> 
> Makes sense to check for initialized TD before allowing other calls. Maybe
> the check is needed in other places too in additoin to the tdx_vcpu_create().
Do you mean in places checking is_hkid_assigned()?
> 
> How about just a function to check for one or more of the already existing
> initialized struct kvm_tdx values?
Instead of checking multiple individual fields in kvm_tdx or vcpu_tdx, could we
introduce a single state field in the two strutures and utilize a state machine
for check (as Chao Gao pointed out at [1]) ?

e.g.
Now TD can have 5 states: (1)created, (2)initialized, (3)finalized,
                          (4)destroyed, (5)freed.
Each vCPU has 3 states: (1) created, (2) initialized, (3)freed

All the states are updated by a user operation (e.g. KVM_TDX_INIT_VM,
KVM_TDX_FINALIZE_VM, KVM_TDX_INIT_VCPU) or a x86 op (e.g. vm_init, vm_destroy,
vm_free, vcpu_create, vcpu_free).


     TD                                   vCPU
(1) created(set in op vm_init)
(2) initialized
(indicate tdr_pa != 0 && HKID assigned)

                                          (1) created (set in op vcpu_create)

                                          (2) initialized

                                    (can call INIT_MEM_REGION, GET_CPUID here)


(3) finalized

                                 (tdx_vcpu_run(), tdx_handle_exit() can be here)


(4) destroyed (indicate HKID released)

                                         (3) freed

(5) freed


[1] https://lore.kernel.org/kvm/ZfvI8t7SlfIsxbmT@chao-email/#t