KVM: Reject device ioctls from processes other than the VM's creator

KVM's API requires thats ioctls must be issued from the same process
that created the VM.  In other words, userspace can play games with a
VM's file descriptors, e.g. fork(), SCM_RIGHTS, etc..., but only the
creator can do anything useful.  Explicitly reject device ioctls that
are issued by a process other than the VM's creator, and update KVM's
API documentation to extend its requirements to device ioctls.

Fixes: 852b6d57dc ("kvm: add device control API")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -2905,6 +2905,9 @@ static long kvm_device_ioctl(struct file *filp, unsigned int ioctl,
 {
 	struct kvm_device *dev = filp->private_data;
 
+	if (dev->kvm->mm != current->mm)
+		return -EIO;
+
 	switch (ioctl) {
 	case KVM_SET_DEVICE_ATTR:
 		return kvm_device_ioctl_attr(dev, dev->ops->set_attr, arg);