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

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Message-ID: <50136685-706e-fc6a-0a77-97e584e74f93@redhat.com>
Date:   Wed, 12 Jan 2022 19:39:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

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).

> 
> +		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).

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.

Paolo