ipmi: Add a limit on the number of users that may use IPMI

Each user uses memory, we need limits to avoid a rogue program from
running the system out of memory.

Based on work by Chen Guanqiao <chen.chenchacha@foxmail.com>

Cc: Chen Guanqiao <chen.chenchacha@foxmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

drivers/char/ipmi/ipmi_msghandler.c

@@ -145,6 +145,12 @@ module_param(default_max_retries, uint, 0644);
 MODULE_PARM_DESC(default_max_retries,
 		 "The time (milliseconds) between retry sends in maintenance mode");
 
+/* The default maximum number of users that may register. */
+static unsigned int max_users = 30;
+module_param(max_users, uint, 0644);
+MODULE_PARM_DESC(max_users,
+		 "The most users that may use the IPMI stack at one time.");
+
 /* Call every ~1000 ms. */
 #define IPMI_TIMEOUT_TIME	1000
 
@@ -442,6 +448,7 @@ struct ipmi_smi {
 	 */
 	struct list_head users;
 	struct srcu_struct users_srcu;
+	atomic_t nr_users;
 
 	/* Used for wake ups at startup. */
 	wait_queue_head_t waitq;
@@ -1230,6 +1237,11 @@ int ipmi_create_user(unsigned int          if_num,
 	goto out_kfree;
 
  found:
+	if (atomic_add_return(1, &intf->nr_users) > max_users) {
+		rv = -EBUSY;
+		goto out_kfree;
+	}
+
 	INIT_WORK(&new_user->remove_work, free_user_work);
 
 	rv = init_srcu_struct(&new_user->release_barrier);
@@ -1262,6 +1274,7 @@ int ipmi_create_user(unsigned int          if_num,
 	return 0;
 
 out_kfree:
+	atomic_dec(&intf->nr_users);
 	srcu_read_unlock(&ipmi_interfaces_srcu, index);
 	vfree(new_user);
 	return rv;
@@ -1336,6 +1349,7 @@ static void _ipmi_destroy_user(struct ipmi_user *user)
 	/* Remove the user from the interface's sequence table. */
 	spin_lock_irqsave(&intf->seq_lock, flags);
 	list_del_rcu(&user->link);
+	atomic_dec(&intf->nr_users);
 
 	for (i = 0; i < IPMI_IPMB_NUM_SEQ; i++) {
 		if (intf->seq_table[i].inuse
@@ -3529,6 +3543,7 @@ int ipmi_add_smi(struct module         *owner,
 	if (slave_addr != 0)
 		intf->addrinfo[0].address = slave_addr;
 	INIT_LIST_HEAD(&intf->users);
+	atomic_set(&intf->nr_users, 0);
 	intf->handlers = handlers;
 	intf->send_info = send_info;
 	spin_lock_init(&intf->seq_lock);