Re: [PATCH v2 1/4] kvm: cpuid: adjust the returned nent field of kvm_cpuid2 for KVM_GET_SUPPORTED_CPUID and KVM_GET_EMULATED_CPUID

[Emanuele Giuseppe Esposito]

On 31/03/2021 20:31, Sean Christopherson wrote:
> On Wed, Mar 31, 2021, Emanuele Giuseppe Esposito wrote:
> > Calling the kvm KVM_GET_[SUPPORTED/EMULATED]_CPUID ioctl requires
> > a nent field inside the kvm_cpuid2 struct to be big enough to contain
> > all entries that will be set by kvm.
> > Therefore if the nent field is too high, kvm will adjust it to the
> > right value. If too low, -E2BIG is returned.
> >
> > However, when filling the entries do_cpuid_func() requires an
> > additional entry, so if the right nent is known in advance,
> > giving the exact number of entries won't work because it has to be
> > increased by one.
>
> I'd strong prefer to reword the shortlog and changelog.  It's not immediately
> obvious what this is changing without the context from the v1 thread.  E.g.
>
>    KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID
>
>    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.

I will update it with v3, thanks.

> > Signed-off-by: Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/cpuid.c | 35 ++++++++++++++++++-----------------
> >   1 file changed, 18 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 6bd2f8b830e4..02a51f921548 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -567,34 +567,34 @@ 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;
> > +	entry.function = func;
> > +	entry.index = 0;
> > +	entry.flags = 0;
>
> Depending on the leaf, eax/ebx/ecx/edx will be left uninitialized.  This wasn't
> a bug before since @array is zeroed on allocation.
>
> What about pre-checking @func?  I don't particular like the duplicate checks,
> but none of the solutions are particularly elegant.  E.g.
You're right, I should have zeroed it. I agree that memsetting and 
memcopying is not elegant either, but unless I am missing something and 
it changes the intended behavior, IMHO this avoids duplicate checks and 
makes it simpler to add a new 'func'.

Emanuele
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6bd2f8b830e4..9824947bd5ad 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -565,14 +565,18 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
>          return entry;
>   }
>
> -static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> +static noinline int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>   {
>          struct kvm_cpuid_entry2 *entry;
>
> +       if (func != 0 && func != 1 && func != 7)
> +               return 0;
> +
>          if (array->nent >= array->maxnent)
>                  return -E2BIG;
>
> -       entry = &array->entries[array->nent];
> +       entry = &array->entries[array->nent++];
> +
>          entry->function = func;
>          entry->index = 0;
>          entry->flags = 0;
> @@ -580,19 +584,17 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>          switch (func) {
>          case 0:
>                  entry->eax = 7;
> -               ++array->nent;
>                  break;
>          case 1:
>                  entry->ecx = F(MOVBE);
> -               ++array->nent;
>                  break;
>          case 7:
>                  entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>                  entry->eax = 0;
>                  entry->ecx = F(RDPID);
> -               ++array->nent;
> -       default:
>                  break;
> +       default:
> +               BUG();
>          }
>
>          return 0
>
>
> >   
> >   	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;
> > +		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;
> >   }
> >   
> > @@ -975,6 +975,7 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
> >   
> >   	if (cpuid->nent < 1)
> >   		return -E2BIG;
> > +
> >   	if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
> >   		cpuid->nent = KVM_MAX_CPUID_ENTRIES;
> >   
> > --
> > 2.30.2