Re: [PATCH v4 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID

[Emanuele Giuseppe Esposito]

On 08/04/2021 22:29, Sean Christopherson wrote:
> On Thu, Apr 08, 2021, Emanuele Giuseppe Esposito wrote:
> > 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.
> >
> > Fixes: 433f4ba19041 ("KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)")
> > Signed-off-by: Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx>
> > ---
> >   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..d30194081892 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));
> >   
> >   	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.ecx = F(RDPID);
> >   		break;
> > +	default:
> > +		goto out;
> >   	}
> >   
> > +	/* This check is performed only when func is valid */
>
> Sorry to keep nitpicking and bikeshedding.  

No problem at all. Any comment is very welcome :)

Funcs aren't really "invalid", KVM
> just doesn't have any features it emulates in other leafs.  Maybe be more literal
> in describing what triggers the check?
>
> 	/* Check the array capacity iff the entry is being copied over. */

What I mean here is that a func is "valid" if it matches one of the 
cases of the switch statement. If it is not valid, it ends up in the 
default case. But I agree, will change the comment your suggestion and 
resend.

Thank you,
Emanuele

> Not a sticking point, so either way:
>
> Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
>
> > +	if (array->nent >= array->maxnent)
> > +		return -E2BIG;
> > +
> > +	entry.function = func;
> > +	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
> > +
> > +out:
> >   	return 0;
> >   }
> >   
> > --
> > 2.30.2