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Date: Wed, 1 Mar 2023 22:14:19 +0100
From: Alexander Graf <graf@...zon.com>
To: Zhi Wang <zhi.wang.linux@...il.com>
CC: Michael Roth <michael.roth@....com>, <kvm@...r.kernel.org>,
<linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
<linux-crypto@...r.kernel.org>, <x86@...nel.org>,
<linux-kernel@...r.kernel.org>, <tglx@...utronix.de>,
<mingo@...hat.com>, <jroedel@...e.de>, <thomas.lendacky@....com>,
<hpa@...or.com>, <ardb@...nel.org>, <pbonzini@...hat.com>,
<seanjc@...gle.com>, <vkuznets@...hat.com>, <jmattson@...gle.com>,
<luto@...nel.org>, <dave.hansen@...ux.intel.com>, <slp@...hat.com>,
<pgonda@...gle.com>, <peterz@...radead.org>,
<srinivas.pandruvada@...ux.intel.com>, <rientjes@...gle.com>,
<dovmurik@...ux.ibm.com>, <tobin@....com>, <bp@...en8.de>,
<vbabka@...e.cz>, <kirill@...temov.name>, <ak@...ux.intel.com>,
<tony.luck@...el.com>, <marcorr@...gle.com>,
<sathyanarayanan.kuppuswamy@...ux.intel.com>,
<alpergun@...gle.com>, <dgilbert@...hat.com>, <jarkko@...nel.org>,
<ashish.kalra@....com>, <nikunj.dadhania@....com>,
Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH RFC v8 47/56] KVM: SVM: Support SEV-SNP AP Creation NAE
event
On 28.02.23 21:47, Zhi Wang wrote:
> On Fri, 24 Feb 2023 13:37:48 +0100
> Alexander Graf <graf@...zon.com> wrote:
>
>> On 20.02.23 19:38, Michael Roth wrote:
>>> From: Tom Lendacky <thomas.lendacky@....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@....com>
>>> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
>>> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
>>> [mdr: add handling for restrictedmem]
>>> Signed-off-by: Michael Roth <michael.roth@....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
Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879
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