Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

[Maxim Levitsky]

On Thu, 2022-01-13 at 10:27 +0100, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@xxxxxxxxxx> 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.
> 
I also looked at this recently (due to other reasons) and I found out that
qemu picks a parked vcpu by its vcpu_id which is its initial apic id,
thus apic id related features should not change.

Take a look at 'kvm_get_vcpu' in qemu source.
Maybe old qemu versions didn't do this?

Best regards,
Thanks,
	Maxim Levitsky