linux-kernel - Re: [PATCH v3 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED

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Date:   Tue, 6 Apr 2021 16:09:00 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Jim Mattson <jmattson@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
        Alexander Graf <graf@...zon.com>,
        Andrew Jones <drjones@...hat.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 1/4] KVM: x86: Fix a spurious -E2BIG in
 KVM_GET_EMULATED_CPUID

On Tue, Apr 06, 2021, Vitaly Kuznetsov wrote:
> Emanuele Giuseppe Esposito <eesposit@...hat.com> writes:
> 
> > When retrieving emulated CPUID entries, check for an insufficient array
> > size if and only if KVM is actually inserting an entry.
> > If userspace has a priori knowledge of the exact array size,
> > KVM_GET_EMULATED_CPUID will incorrectly fail due to effectively requiring
> > an extra, unused entry.
> >
> > Signed-off-by: Emanuele Giuseppe Esposito <eesposit@...hat.com>
> > ---
> >  arch/x86/kvm/cpuid.c | 33 ++++++++++++++++-----------------
> >  1 file changed, 16 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 6bd2f8b830e4..27059ddf9f0a 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -567,34 +567,33 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
> >  
> >  static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> >  {
> > -	struct kvm_cpuid_entry2 *entry;
> > -
> > -	if (array->nent >= array->maxnent)
> > -		return -E2BIG;
> > +	struct kvm_cpuid_entry2 entry;
> >  
> > -	entry = &array->entries[array->nent];
> > -	entry->function = func;
> > -	entry->index = 0;
> > -	entry->flags = 0;
> > +	memset(&entry, 0, sizeof(entry));
> > +	entry.function = func;
> >  
> >  	switch (func) {
> >  	case 0:
> > -		entry->eax = 7;
> > -		++array->nent;
> > +		entry.eax = 7;
> >  		break;
> >  	case 1:
> > -		entry->ecx = F(MOVBE);
> > -		++array->nent;
> > +		entry.ecx = F(MOVBE);
> >  		break;
> >  	case 7:
> > -		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > -		entry->eax = 0;
> > -		entry->ecx = F(RDPID);
> > -		++array->nent;
> > -	default:
> > +		entry.flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > +		entry.eax = 0;
> 
> Nitpick: there's no need to set entry.eax = 0 as the whole structure was
> zeroed. Also, '|=' for flags could be just '='.

Agreed on dropping "entry.eax = 0".  I could go either way on flags; I do like
that the "|=" is consistent with do_host_cpuid().

> > +		entry.ecx = F(RDPID);
> >  		break;
> > +	default:
> > +		goto out;
> >  	}
> >  
> > +	if (array->nent >= array->maxnent)
> > +		return -E2BIG;
> > +
> > +	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
> > +
> > +out:
> >  	return 0;
> >  }
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> 
> -- 
> Vitaly
>