[PATCH v4 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.

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 */
+	if (array->nent >= array->maxnent)
+		return -E2BIG;
+
+	entry.function = func;
+	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
+
+out:
 	return 0;
 }
 
-- 
2.30.2