Re: [PATCH v4] KVM: x86: Fix KVM_GET_CPUID2 ioctl to return cpuid entries count

[Paolo Bonzini]

On 29/04/21 03:00, Sean Christopherson wrote:
> On Wed, Apr 28, 2021, Valeriy Vdovin wrote:
> > It's very explicit by the code that it was designed to receive some
> > small number of entries to return E2BIG along with the corrected number.
>
> LOL, saying KVM_GET_CPUID2 was "designed" is definitely giving the KVM
> forefathers the benefit of the doubt.

I was going to make a different joke, i.e. that KVM_GET_CPUID2 was 
indeed designed the way Valeriy described, but that design was forgotten 
soon after.

Really, this ioctl has been such a trainwreck that I think the only good 
solution here is to drop it.

Paolo

> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index efc7a82ab140..3f941b1f4e78 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -4773,14 +4773,17 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> >   		r = -EFAULT;
> >   		if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
> >   			goto out;
> > +
> >   		r = kvm_vcpu_ioctl_get_cpuid2(vcpu, &cpuid,
> >   					      cpuid_arg->entries);
> > -		if (r)
> > +
> > +		if (r && r != -E2BIG)
> >   			goto out;
> > -		r = -EFAULT;
> > -		if (copy_to_user(cpuid_arg, &cpuid, sizeof(cpuid)))
> > +
> > +		if (copy_to_user(cpuid_arg, &cpuid, sizeof(cpuid))) {
> > +			r = -EFAULT;
> >   			goto out;
>
> As I pointed out[*], copying the number of entries but not the entries themselves
> is wrong.  All of my feedback on v1 still stands.
>
> [*] https://lkml.kernel.org/r/YIl4M/GgaYvwNuXv@xxxxxxxxxx
>
> > -		r = 0;
> > +		}
> >   		break;
> >   	}
> >   	case KVM_GET_MSRS: {