KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)

The bounds check was present in KVM_GET_SUPPORTED_CPUID but not
KVM_GET_EMULATED_CPUID.

Reported-by: syzbot+e3f4897236c4eeb8af4f@syzkaller.appspotmail.com
Fixes: 84cffe49 ("kvm: Emulate MOVBE", 2013-10-29)
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -504,7 +504,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 
 	r = -E2BIG;
 
-	if (*nent >= maxnent)
+	if (WARN_ON(*nent >= maxnent))
 		goto out;
 
 	do_host_cpuid(entry, function, 0);
@@ -815,6 +815,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 static int do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 func,
 			 int *nent, int maxnent, unsigned int type)
 {
+	if (*nent >= maxnent)
+		return -E2BIG;
+
 	if (type == KVM_GET_EMULATED_CPUID)
 		return __do_cpuid_func_emulated(entry, func, nent, maxnent);