KVM: x86: avoid vmalloc(0) in the KVM_SET_CPUID

This causes an ugly dmesg splat.  Beautified syzkaller testcase:

    #include <unistd.h>
    #include <sys/syscall.h>
    #include <sys/ioctl.h>
    #include <fcntl.h>
    #include <linux/kvm.h>

    long r[8];

    int main()
    {
        struct kvm_cpuid2 c = { 0 };
        r[2] = open("/dev/kvm", O_RDWR);
        r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
        r[4] = ioctl(r[3], KVM_CREATE_VCPU, 0x8);
        r[7] = ioctl(r[4], KVM_SET_CPUID, &c);
        return 0;
    }

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/cpuid.c

@@ -181,19 +181,22 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
 			     struct kvm_cpuid_entry __user *entries)
 {
 	int r, i;
-	struct kvm_cpuid_entry *cpuid_entries;
+	struct kvm_cpuid_entry *cpuid_entries = NULL;
 
 	r = -E2BIG;
 	if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
 		goto out;
 	r = -ENOMEM;
-	cpuid_entries = vmalloc(sizeof(struct kvm_cpuid_entry) * cpuid->nent);
-	if (!cpuid_entries)
-		goto out;
-	r = -EFAULT;
-	if (copy_from_user(cpuid_entries, entries,
-			   cpuid->nent * sizeof(struct kvm_cpuid_entry)))
-		goto out_free;
+	if (cpuid->nent) {
+		cpuid_entries = vmalloc(sizeof(struct kvm_cpuid_entry) *
+					cpuid->nent);
+		if (!cpuid_entries)
+			goto out;
+		r = -EFAULT;
+		if (copy_from_user(cpuid_entries, entries,
+				   cpuid->nent * sizeof(struct kvm_cpuid_entry)))
+			goto out;
+	}
 	for (i = 0; i < cpuid->nent; i++) {
 		vcpu->arch.cpuid_entries[i].function = cpuid_entries[i].function;
 		vcpu->arch.cpuid_entries[i].eax = cpuid_entries[i].eax;
@@ -212,9 +215,8 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
 	kvm_x86_ops->cpuid_update(vcpu);
 	r = kvm_update_cpuid(vcpu);
 
-out_free:
-	vfree(cpuid_entries);
 out:
+	vfree(cpuid_entries);
 	return r;
 }