KVM: Forbid /dev/kvm being opened by a compat task when CONFIG_KVM_COMPAT=n

On a system without KVM_COMPAT, we prevent IOCTLs from being issued
by a compat task. Although this prevents most silly things from
happening, it can still confuse a 32bit userspace that is able
to open the kvm device (the qemu test suite seems to be pretty
mad with this behaviour).

Take a more radical approach and return a -ENODEV to the compat
task.
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -124,7 +124,13 @@ static long kvm_vcpu_compat_ioctl(struct file *file, unsigned int ioctl,
 #else
 static long kvm_no_compat_ioctl(struct file *file, unsigned int ioctl,
 				unsigned long arg) { return -EINVAL; }
-#define KVM_COMPAT(c)	.compat_ioctl	= kvm_no_compat_ioctl
+
+static int kvm_no_compat_open(struct inode *inode, struct file *file)
+{
+	return is_compat_task() ? -ENODEV : 0;
+}
+#define KVM_COMPAT(c)	.compat_ioctl	= kvm_no_compat_ioctl,	\
+			.open		= kvm_no_compat_open
 #endif
 static int hardware_enable_all(void);
 static void hardware_disable_all(void);