On Tue, Apr 06, 2021, Vitaly Kuznetsov wrote:
> Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx> 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@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..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@xxxxxxxxxx>
>
> --
> Vitaly
>
