linux-kernel - Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM

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Date:   Thu, 13 Jan 2022 10:27:30 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Igor Mammedov <imammedo@...hat.com>
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 1/12/22 14:58, Vitaly Kuznetsov wrote:
>> -	best = kvm_find_cpuid_entry(vcpu, 0xD, 1);
>> +	best = cpuid_entry2_find(entries, nent, 0xD, 1);
>>   	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>>   		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
>>   		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
>>   
>> -	best = kvm_find_kvm_cpuid_features(vcpu);
>> +	best = __kvm_find_kvm_cpuid_features(vcpu, vcpu->arch.cpuid_entries,
>> +					     vcpu->arch.cpuid_nent);
>>   	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
>
> I think this should be __kvm_find_kvm_cpuid_features(vcpu, entries, nent).
>

Of course.

>> 
>> +		case 0x1:
>> +			/* Only initial LAPIC id is allowed to change */
>> +			if (e->eax ^ best->eax || ((e->ebx ^ best->ebx) >> 24) ||
>> +			    e->ecx ^ best->ecx || e->edx ^ best->edx)
>> +				return -EINVAL;
>> +			break;
>
> This XOR is a bit weird.  In addition the EBX test is checking the wrong 
> bits (it checks whether 31:24 change and ignores changes to 23:0).

Indeed, however, I've tested CPU hotplug with QEMU trying different
CPUs in random order and surprisingly othing blew up, feels like QEMU
was smart enough to re-use the right fd)

>
> You can write just "(e->ebx & ~0xff000000u) != (best->ebx ~0xff000000u)".
>
>> 
>> +		default:
>> +			if (e->eax ^ best->eax || e->ebx ^ best->ebx ||
>> +			    e->ecx ^ best->ecx || e->edx ^ best->edx)
>> +				return -EINVAL;
>
> This one even more so.

Thanks for the early review, I'm going to prepare a selftest and send
this out.

-- 
Vitaly