infiniband: fix a possible use-after-free bug

ucma_process_join() will free the new allocated "mc" struct,
if there is any error after that, especially the copy_to_user().

But in parallel, ucma_leave_multicast() could find this "mc"
through idr_find() before ucma_process_join() frees it, since it
is already published.

So "mc" could be used in ucma_leave_multicast() after it is been
allocated and freed in ucma_process_join(), since we don't refcnt
it.

Fix this by separating "publish" from ID allocation, so that we
can get an ID first and publish it later after copy_to_user().

Fixes: c8f6a362bf3e ("RDMA/cma: Add multicast communication support")
Reported-by: Noam Rathaus <noamr@beyondsecurity.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
static int ucma_open(struct inode *inode, struct file *filp)
{
	struct ucma_file *file;

	file = kmalloc(sizeof *file, GFP_KERNEL);
	if (!file)
		return -ENOMEM;

	file->close_wq = alloc_ordered_workqueue("ucma_close_id",
						 WQ_MEM_RECLAIM);
	if (!file->close_wq) {
		kfree(file);
		return -ENOMEM;
	}

	INIT_LIST_HEAD(&file->event_list);
	INIT_LIST_HEAD(&file->ctx_list);
	init_waitqueue_head(&file->poll_wait);
	mutex_init(&file->mut);

	filp->private_data = file;
	file->filp = filp;

	return nonseekable_open(inode, filp);
}

static int ucma_open(struct inode *inode, struct file *filp)
{
	struct ucma_file *file;

	file = kmalloc(sizeof *file, GFP_KERNEL);
	if (!file)
		return -ENOMEM;

	file->close_wq = alloc_ordered_workqueue("ucma_close_id",
						 WQ_MEM_RECLAIM);
	if (!file->close_wq) {
		kfree(file);
		return -ENOMEM;
	}

	INIT_LIST_HEAD(&file->event_list);
	INIT_LIST_HEAD(&file->ctx_list);
	init_waitqueue_head(&file->poll_wait);
	mutex_init(&file->mut);

	filp->private_data = file;
	file->filp = filp;

	return nonseekable_open(inode, filp);
}
