linux-kernel - Re: [PATCH v11 019/113] KVM: TDX: initialize VM with TDX specific parameters

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Date:   Mon, 27 Feb 2023 13:44:30 -0800
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Li, Xiaoyao" <xiaoyao.li@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH v11 019/113] KVM: TDX: initialize VM with TDX specific
 parameters

On Tue, Jan 17, 2023 at 12:19:15PM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> > +/*
> > + * cpuid entry lookup in TDX cpuid config way.
> > + * The difference is how to specify index(subleaves).
> 
> AFAICT you only have one caller here.  If this is the only difference, will it
> be simpler to ask caller to simply convert TDX_CPUID_NO_SUBLEAF to 0, so this
> function can perhaps be removed?

I removed the function as it turned out only one caller needs it after revise.


> > +static int setup_tdparams(struct kvm *kvm, struct td_params *td_params,
> > +			struct kvm_tdx_init_vm *init_vm)
> > +{
> > +	const struct kvm_cpuid2 *cpuid = &init_vm->cpuid;
> > +	const struct kvm_cpuid_entry2 *entry;
> > +	u64 guest_supported_xcr0;
> > +	u64 guest_supported_xss;
> > +	int max_pa;
> > +	int i;
> > +
> > +	if (kvm->created_vcpus)
> > +		return -EBUSY;
> > +	td_params->max_vcpus = kvm->max_vcpus;
> > +	td_params->attributes = init_vm->attributes;
> > +	if (td_params->attributes & TDX_TD_ATTRIBUTE_PERFMON) {
> > +		/*
> > +		 * TODO: save/restore PMU related registers around TDENTER.
> > +		 * Once it's done, remove this guard.
> > +		 */
> > +		pr_warn("TD doesn't support perfmon yet. KVM needs to save/restore "
> > +			"host perf registers properly.\n");
> > +		return -EOPNOTSUPP;
> > +	}
> > +
> > +	for (i = 0; i < tdx_caps.nr_cpuid_configs; i++) {
> > +		const struct tdx_cpuid_config *config = &tdx_caps.cpuid_configs[i];
> > +		const struct kvm_cpuid_entry2 *entry =
> > +			tdx_find_cpuid_entry(cpuid, config->leaf, config->sub_leaf);
> > +		struct tdx_cpuid_value *value = &td_params->cpuid_values[i];
> > +
> > +		if (!entry)
> > +			continue;
> > +
> > +		value->eax = entry->eax & config->eax;
> > +		value->ebx = entry->ebx & config->ebx;
> > +		value->ecx = entry->ecx & config->ecx;
> > +		value->edx = entry->edx & config->edx;
> > +	}
> 
> A comment to explain above would be helpful, i.e TDX requires the number and the
> order of those entries in TD_PARAMS's cpuid_values[] must be in the same number
> and order with TDSYSINFO's CPUID_CONFIG.
> 
> Also, this code depends on @td_params already being zeroed.  Perhaps also point
> it out.

Ok, I'll add a comment.


> [snip]
> 
> > +static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
> > +{
> > 
> [snip]
> 
> > +
> > +	ret = setup_tdparams(kvm, td_params, init_vm);
> > +	if (ret)
> > +		goto out;
> > +
> > +	ret = __tdx_td_init(kvm, td_params);
> > +	if (ret)
> > +		goto out;
> > +
> > +	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;
> > +
> > +out:
> > +	/* kfree() accepts NULL. */
> > +	kfree(init_vm);
> > +	kfree(td_params);
> 
> So looks KVM doesn't CPUID configurations that are passed to the TDX module.  
> 
> IIUC, KVM still depends on userspace to later use KVM_SET_CPUID2 to fill the
> _same_ CPUID entries for each vcpu?  If so, what if userspace didn't provide
> consistent CPUIDs in KVM_SET_CPUID2?  Should we verify in KVM_SET_CPUID2 that
> CPUIDs are consistent?

Yes. guest might be confused, but KVM is fine with it. I don't think check is
necessary.

Because already user space is required to configure consistent CPUIDs (and MSRs)
and KVM doesn't do such consistency check for it except minimum check for KVM to
work correctly. e.g. the highest leaf number. This patch doesn't make it worse.


> I am thinking if some #VE handling requires CPUID to make some decision, then
> inconsistent CPUIDs will cause trouble, but I don't have an example now.

Do you mean TDG.VP.VMCALL<CPUID>?  Because guest TD doesn't VMM, #VE handler in
guest should sanitize the result and should be robust against attack by 
returning broken value for TDG.VP.VMCALL<CPUID>.(or other TDG.VP.VMCALL)
-- 
Isaku Yamahata <isaku.yamahata@...il.com>