[PATCH v3 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID

[Emanuele Giuseppe Esposito]

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 <eespo...@redhat.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;
+               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;
 }
 
-- 
2.30.2