ax25: Fix reference count leak issues of ax25_dev

The ax25_addr_ax25dev() and ax25_dev_device_down() exist a reference
count leak issue of the object "ax25_dev".

Memory leak issue in ax25_addr_ax25dev():

The reference count of the object "ax25_dev" can be increased multiple
times in ax25_addr_ax25dev(). This will cause a memory leak.

Memory leak issues in ax25_dev_device_down():

The reference count of ax25_dev is set to 1 in ax25_dev_device_up() and
then increase the reference count when ax25_dev is added to ax25_dev_list.
As a result, the reference count of ax25_dev is 2. But when the device is
shutting down. The ax25_dev_device_down() drops the reference count once
or twice depending on if we goto unlock_put or not, which will cause
memory leak.

As for the issue of ax25_addr_ax25dev(), it is impossible for one pointer
to be on a list twice. So add a break in ax25_addr_ax25dev(). As for the
issue of ax25_dev_device_down(), increase the reference count of ax25_dev
once in ax25_dev_device_up() and decrease the reference count of ax25_dev
after it is removed from the ax25_dev_list.

Fixes: d01ffb9e ("ax25: add refcount in ax25_dev to avoid UAF bugs")
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/361bbf2a4b091e120006279ec3b382d73c4a0c17.1715247018.git.duoming@zju.edu.cnSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ax25/ax25_dev.c

@@ -39,6 +39,7 @@ ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
 		if (ax25cmp(addr, (const ax25_address *)ax25_dev->dev->dev_addr) == 0) {
 			res = ax25_dev;
 			ax25_dev_hold(ax25_dev);
+			break;
 		}
 	spin_unlock_bh(&ax25_dev_lock);
 
@@ -88,7 +89,6 @@ void ax25_dev_device_up(struct net_device *dev)
 	list_add(&ax25_dev->list, &ax25_dev_list);
 	dev->ax25_ptr     = ax25_dev;
 	spin_unlock_bh(&ax25_dev_lock);
-	ax25_dev_hold(ax25_dev);
 
 	ax25_register_dev_sysctl(ax25_dev);
 }
@@ -129,7 +129,6 @@ void ax25_dev_device_down(struct net_device *dev)
 unlock_put:
 	dev->ax25_ptr = NULL;
 	spin_unlock_bh(&ax25_dev_lock);
-	ax25_dev_put(ax25_dev);
 	netdev_put(dev, &ax25_dev->dev_tracker);
 	ax25_dev_put(ax25_dev);
 }