Re: [PATCH RFC v8 47/56] KVM: SVM: Support SEV-SNP AP Creation NAE event

[Alexander Graf <graf@amazon.com>]

On 28.02.23 21:47, Zhi Wang wrote:
> On Fri, 24 Feb 2023 13:37:48 +0100
> Alexander Graf <graf@amazon.com> wrote:
>
>> On 20.02.23 19:38, Michael Roth wrote:
>>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>>
>>> Add support for the SEV-SNP AP Creation NAE event. This allows SEV-SNP
>>> guests to alter the register state of the APs on their own. This allows
>>> the guest a way of simulating INIT-SIPI.
>>>
>>> A new event, KVM_REQ_UPDATE_PROTECTED_GUEST_STATE, is created and used
>>> so as to avoid updating the VMSA pointer while the vCPU is running.
>>>
>>> For CREATE
>>>     The guest supplies the GPA of the VMSA to be used for the vCPU with
>>>     the specified APIC ID. The GPA is saved in the svm struct of the
>>>     target vCPU, the KVM_REQ_UPDATE_PROTECTED_GUEST_STATE event is added
>>>     to the vCPU and then the vCPU is kicked.
>>>
>>> For CREATE_ON_INIT:
>>>     The guest supplies the GPA of the VMSA to be used for the vCPU with
>>>     the specified APIC ID the next time an INIT is performed. The GPA is
>>>     saved in the svm struct of the target vCPU.
>>>
>>> For DESTROY:
>>>     The guest indicates it wishes to stop the vCPU. The GPA is cleared
>>>     from the svm struct, the KVM_REQ_UPDATE_PROTECTED_GUEST_STATE event is
>>>     added to vCPU and then the vCPU is kicked.
>>>
>>> The KVM_REQ_UPDATE_PROTECTED_GUEST_STATE event handler will be invoked
>>> as a result of the event or as a result of an INIT. The handler sets the
>>> vCPU to the KVM_MP_STATE_UNINITIALIZED state, so that any errors will
>>> leave the vCPU as not runnable. Any previous VMSA pages that were
>>> installed as part of an SEV-SNP AP Creation NAE event are un-pinned. If
>>> a new VMSA is to be installed, the VMSA guest page is pinned and set as
>>> the VMSA in the vCPU VMCB and the vCPU state is set to
>>> KVM_MP_STATE_RUNNABLE. If a new VMSA is not to be installed, the VMSA is
>>> cleared in the vCPU VMCB and the vCPU state is left as
>>> KVM_MP_STATE_UNINITIALIZED to prevent it from being run.
>>>
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>>> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
>>> [mdr: add handling for restrictedmem]
>>> Signed-off-by: Michael Roth <michael.roth@amd.com>
>>
>> What is the intended boot sequence for SEV-SNP guests? FWIW with this
>> interface in place, guests will typically use in-guest VMSA pages to
>> hold secondary vcpu state. But that means we're now allocating 4kb of
>> memory for every vcpu that we create that will be for most of the
>> guest's lifetime superfluous.
>>
>> Wouldn't it make more sense to have a model where we only allocate the
>> VMSA for the boot CPU and leave secondary allocation to the guest? We
>> already need firmware changes for SEV-SNP - may as well make this one more.
>>
>> [...]
>>
>>> +
>>> +static int sev_snp_ap_creation(struct vcpu_svm *svm)
>>> +{
>>> +       struct kvm_sev_info *sev = &to_kvm_svm(svm->vcpu.kvm)->sev_info;
>>> +       struct kvm_vcpu *vcpu = &svm->vcpu;
>>> +       struct kvm_vcpu *target_vcpu;
>>> +       struct vcpu_svm *target_svm;
>>> +       unsigned int request;
>>> +       unsigned int apic_id;
>>> +       bool kick;
>>> +       int ret;
>>> +
>>> +       request = lower_32_bits(svm->vmcb->control.exit_info_1);
>>> +       apic_id = upper_32_bits(svm->vmcb->control.exit_info_1);
>>> +
>>> +       /* Validate the APIC ID */
>>> +       target_vcpu = kvm_get_vcpu_by_id(vcpu->kvm, apic_id);
>>
>> Out of curiosity: The target CPU can be my own vCPU, right?
>>
>>
>>> +       if (!target_vcpu) {
>>> +               vcpu_unimpl(vcpu, "vmgexit: invalid AP APIC ID [%#x] from guest\n",
>>> +                           apic_id);
>>> +               return -EINVAL;
>>> +       }
>>> +
>>> +       ret = 0;
>>> +
>>> +       target_svm = to_svm(target_vcpu);
>>> +
>>> +       /*
>>> +        * The target vCPU is valid, so the vCPU will be kicked unless the
>>> +        * request is for CREATE_ON_INIT. For any errors at this stage, the
>>> +        * kick will place the vCPU in an non-runnable state.
>>> +        */
>>> +       kick = true;
>>> +
>>> +       mutex_lock(&target_svm->sev_es.snp_vmsa_mutex);
>>> +
>>> +       target_svm->sev_es.snp_vmsa_gpa = INVALID_PAGE;
>>> +       target_svm->sev_es.snp_ap_create = true;
>>> +
>>> +       /* Interrupt injection mode shouldn't change for AP creation */
>>> +       if (request < SVM_VMGEXIT_AP_DESTROY) {
>>> +               u64 sev_features;
>>> +
>>> +               sev_features = vcpu->arch.regs[VCPU_REGS_RAX];
>>> +               sev_features ^= sev->sev_features;
>>> +               if (sev_features & SVM_SEV_FEAT_INT_INJ_MODES) {
>>> +                       vcpu_unimpl(vcpu, "vmgexit: invalid AP injection mode [%#lx] from guest\n",
>>> +                                   vcpu->arch.regs[VCPU_REGS_RAX]);
>>> +                       ret = -EINVAL;
>>> +                       goto out;
>>> +               }
>>> +       }
>>> +
>>> +       switch (request) {
>>> +       case SVM_VMGEXIT_AP_CREATE_ON_INIT:
>>> +               kick = false;
>>> +               fallthrough;
>>> +       case SVM_VMGEXIT_AP_CREATE:
>>> +               if (!page_address_valid(vcpu, svm->vmcb->control.exit_info_2)) {
>>> +                       vcpu_unimpl(vcpu, "vmgexit: invalid AP VMSA address [%#llx] from guest\n",
>>> +                                   svm->vmcb->control.exit_info_2);
>>> +                       ret = -EINVAL;
>>> +                       goto out;
>>> +               }
>>> +
>>> +               /*
>>> +                * Malicious guest can RMPADJUST a large page into VMSA which
>>> +                * will hit the SNP erratum where the CPU will incorrectly signal
>>> +                * an RMP violation #PF if a hugepage collides with the RMP entry
>>> +                * of VMSA page, reject the AP CREATE request if VMSA address from
>>> +                * guest is 2M aligned.
>>
>> This will break genuine current Linux kernels that just happen to
>> allocate a guest page, no? In fact, given enough vCPUs you're almost
>> guaranteed to hit an aligned structure somewhere. What is the guest
>> supposed to do in that situation?
>>
>>
>>> +                */
>>> +               if (IS_ALIGNED(svm->vmcb->control.exit_info_2, PMD_SIZE)) {
>>> +                       vcpu_unimpl(vcpu,
>>> +                                   "vmgexit: AP VMSA address [%llx] from guest is unsafe as it is 2M aligned\n",
>>> +                                   svm->vmcb->control.exit_info_2);
>>> +                       ret = -EINVAL;
>>> +                       goto out;
>>> +               }
>>> +
>>> +               target_svm->sev_es.snp_vmsa_gpa = svm->vmcb->control.exit_info_2;
>>> +               break;
>>> +       case SVM_VMGEXIT_AP_DESTROY:
>>
>> I don't understand the destroy path. Why does this case destroy anything?
>>
>>
>>> +               break;
>>> +       default:
>>> +               vcpu_unimpl(vcpu, "vmgexit: invalid AP creation request [%#x] from guest\n",
>>> +                           request);
>>> +               ret = -EINVAL;
>>> +               break;
>>> +       }
>>> +
>>> +out:
>>> +       if (kick) {
>>> +               if (target_vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)
>>> +                       target_vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
>>
>> What if the guest AP goes through a create -> destroy -> create cycle?
>> Will it stay runnable while destroyed?
> The code is not very straightforward.
>
> 1) target_svm->sev_es.snp_vmsa_gpa is set as INVALID_PAGE in the beginning of this function.
>
> 2) If a DESTROY is hit in this function, target_svm->sev_es.snp_vmsa_gpa will be
> left as INVALID_PAGE.
>
> 3) At the end of this function, it calls kvm_make_request(KVM_REQ_UPDATE_PROTECTED_GUEST_STATE).
>
> 4) In the vcpu_enter_guest(), the kvm_vcpu_reset()->sev_snp_init_protected_guest_state()
> ->__sev_snp_init_protected_guest_state() is called.
>
> 5) The mp_state is set to KVM_MP_STATE_STOPPED by default and the runtime VMSA is
> cleared. Then the it will be initialized according to the guest's
> configuration.
>
> 6) As the snp_vmsa_gpa is set as INVALID_PAGE in 1, the mp_state will be left as
> KVM_MP_STATE_STOPPED.
>
> 7) With this code piece:
>
> +                       kvm_vcpu_reset(vcpu, true);
> +                       if (vcpu->arch.mp_state != KVM_MP_STATE_RUNNABLE)
> +                               goto out;
>
> vcpu_enter_guest() bails out.


Thanks a lot Zhi for the detailed explanation! I think this code flow 
wants to become slightly more obvious. For example, if we just said

   case SVM_VMGEXIT_AP_DESTROY:
     /* This will tell __sev_snp_update_protected_guest_state to unmap 
the VMSA */
     target_svm->sev_es.snp_vmsa_gpa = INVALID_PAGE;
     break;

We'd get a big win in readability with little effort. It makes it 
immediately obvious where to look for the destroy operation.


Alex





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