[PATCH net] ax25: Fix refcount leak issues of ax25_dev

[Duoming Zhou]

There are two scenarios that might cause refcount leak
issues of ax25_dev.

Scenario one:

The refcount of ax25_dev potentially increase more
than once in ax25_addr_ax25dev(), which will cause
memory leak.

In order to fix the above issue, only increase the
refcount of ax25_dev once, when the res is not null.

Scenario two:

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

In order to mitigate the above issues, only increase
the refcount of ax25_dev when the ax25_dev is added
to the ax25_dev_list and decrease the refcount of
ax25_dev after it is removed from the ax25_dev_list.

What's more, the ax25_dev should not be deallocated
directly by kfree() in ax25_dev_free(), replace it
with ax25_dev_put() instead.

Fixes: d01ffb9eee4a ("ax25: add refcount in ax25_dev to avoid UAF bugs")
Reported by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
 net/ax25/ax25_dev.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
index 282ec581c07..0e6dd98d3fa 100644
--- a/net/ax25/ax25_dev.c
+++ b/net/ax25/ax25_dev.c
@@ -37,8 +37,9 @@ ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
 	for (ax25_dev = ax25_dev_list; ax25_dev != NULL; ax25_dev = ax25_dev->next)
 		if (ax25cmp(addr, (const ax25_address *)ax25_dev->dev->dev_addr) == 0) {
 			res = ax25_dev;
-			ax25_dev_hold(ax25_dev);
 		}
+	if (res)
+		ax25_dev_hold(res);
 	spin_unlock_bh(&ax25_dev_lock);
 
 	return res;
@@ -58,7 +59,6 @@ void ax25_dev_device_up(struct net_device *dev)
 		return;
 	}
 
-	refcount_set(&ax25_dev->refcount, 1);
 	dev->ax25_ptr     = ax25_dev;
 	ax25_dev->dev     = dev;
 	netdev_hold(dev, &ax25_dev->dev_tracker, GFP_KERNEL);
@@ -88,7 +88,7 @@ void ax25_dev_device_up(struct net_device *dev)
 	ax25_dev->next = ax25_dev_list;
 	ax25_dev_list  = ax25_dev;
 	spin_unlock_bh(&ax25_dev_lock);
-	ax25_dev_hold(ax25_dev);
+	refcount_set(&ax25_dev->refcount, 1);
 
 	ax25_register_dev_sysctl(ax25_dev);
 }
@@ -135,7 +135,6 @@ void ax25_dev_device_down(struct net_device *dev)
 
 unlock_put:
 	spin_unlock_bh(&ax25_dev_lock);
-	ax25_dev_put(ax25_dev);
 	dev->ax25_ptr = NULL;
 	netdev_put(dev, &ax25_dev->dev_tracker);
 	ax25_dev_put(ax25_dev);
@@ -208,7 +207,7 @@ void __exit ax25_dev_free(void)
 		s        = ax25_dev;
 		netdev_put(ax25_dev->dev, &ax25_dev->dev_tracker);
 		ax25_dev = ax25_dev->next;
-		kfree(s);
+		ax25_dev_put(s);
 	}
 	ax25_dev_list = NULL;
 	spin_unlock_bh(&ax25_dev_lock);
-- 
2.17.1