KVM: x86: Check userspace CPUID array size after validating sub-leaf

Verify that the next sub-leaf of CPUID 0x4 (or 0x8000001d) is valid
before rejecting the entire KVM_GET_SUPPORTED_CPUID due to insufficent
space in the userspace array.

Note, although this is technically a bug, it's not visible to userspace
as KVM_GET_SUPPORTED_CPUID is guaranteed to fail on KVM_CPUID_SIGNATURE,
which is hardcoded to be added after the affected leafs.  The real
motivation for the change is to tightly couple the nent/maxnent and
do_host_cpuid() sequences in preparation for future cleanup.
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -552,12 +552,12 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 
 		/* read more entries until cache_type is zero */
 		for (i = 1; ; ++i) {
-			if (*nent >= maxnent)
-				goto out;
-
 			cache_type = entry[i - 1].eax & 0x1f;
 			if (!cache_type)
 				break;
+
+			if (*nent >= maxnent)
+				goto out;
 			do_host_cpuid(&entry[i], function, i);
 			++*nent;
 		}