ipmi: Remove smi_msg from waiting_rcv_msgs list before handle_one_recv_msg()

Commit 7ea0ed2b5be8 ("ipmi: Make the message handler easier to use for
SMI interfaces") changed handle_new_recv_msgs() to call handle_one_recv_msg()
for a smi_msg while the smi_msg is still connected to waiting_rcv_msgs list.
That could lead to following list corruption problems:

1) low-level function treats smi_msg as not connected to list

handle_one_recv_msg() could end up calling smi_send(), which
assumes the msg is not connected to list.

For example, the following sequence could corrupt list by
doing list_add_tail() for the entry still connected to other list.

handle_new_recv_msgs()
msg = list_entry(waiting_rcv_msgs)
handle_one_recv_msg(msg)
handle_ipmb_get_msg_cmd(msg)
smi_send(msg)
spin_lock(xmit_msgs_lock)
list_add_tail(msg)
spin_unlock(xmit_msgs_lock)

2) race between multiple handle_new_recv_msgs() instances

handle_new_recv_msgs() once releases waiting_rcv_msgs_lock before calling
handle_one_recv_msg() then retakes the lock and list_del() it.

If others call handle_new_recv_msgs() during the window shown below
list_del() will be done twice for the same smi_msg.

handle_new_recv_msgs()
spin_lock(waiting_rcv_msgs_lock)
msg = list_entry(waiting_rcv_msgs)
spin_unlock(waiting_rcv_msgs_lock)
|
| handle_one_recv_msg(msg)
|
spin_lock(waiting_rcv_msgs_lock)
list_del(msg)
spin_unlock(waiting_rcv_msgs_lock)

Fixes: 7ea0ed2b5be8 ("ipmi: Make the message handler easier to use for SMI interfaces")
Signed-off-by: Jun'ichi Nomura <[e-mail]>
[Added a comment to describe why this works.]
Signed-off-by: Corey Minyard <[e-mail]>
Cc: [e-mail] # 3.19
Tested-by: Ye Feng <[e-mail]>

drivers/char/ipmi/ipmi_msghandler.c

···
 	while (!list_empty(&intf->waiting_rcv_msgs)) {
 		smi_msg = list_entry(intf->waiting_rcv_msgs.next,
 				     struct ipmi_smi_msg, link);
+		list_del(&smi_msg->link);
 		if (!run_to_completion)
 			spin_unlock_irqrestore(&intf->waiting_rcv_msgs_lock,
 					       flags);
···
 		if (rv > 0) {
 			/*
 			 * To preserve message order, quit if we
-			 * can't handle a message.
+			 * can't handle a message.  Add the message
+			 * back at the head, this is safe because this
+			 * tasklet is the only thing that pulls the
+			 * messages.
 			 */
+			list_add(&smi_msg->link, &intf->waiting_rcv_msgs);
 			break;
 		} else {
-			list_del(&smi_msg->link);
 			if (rv == 0)
 				/* Message handled */
 				ipmi_free_smi_msg(smi_msg);