Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check_equal()

[Paolo Bonzini]

On 1/24/22 17:52, Sean Christopherson wrote:
> On Mon, Jan 24, 2022, Vitaly Kuznetsov wrote:
> > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> > > > +	if (memcmp(e2, vcpu->arch.cpuid_entries, nent * sizeof(*e2)))
> > > > +		return -EINVAL;
> > >
> > > Hmm, not sure about that due to the padding in struct kvm_cpuid_entry2.
> > >    It might break userspace that isn't too careful about zeroing it.
>
> Given that we already are fully committed to potentially breaking userspace by
> disallowing KVM_SET_CPUID{2} after KVM_RUN, we might as well get greedy.

Hmm, I thought this series was because we were _not_ fully committed. :)

> > FWIW, QEMU zeroes the whole thing before setting individual CPUID
> > entries. Legacy KVM_SET_CPUID call is also not afffected as it copies
> > entries to a newly allocated "struct kvm_cpuid_entry2[]" and explicitly
> > zeroes padding.
> >
> > Do we need to at least add a check for ".flags"?
>
> Yes.

Yes, we do.  Alternatively, we can replace memdup with a copy in the 
style of KVM_CPUID.

Paolo