The ax25_addr_ax25dev() exists a reference count leak issue of the
object "ax25_dev" and the ax25_dev_device_down() exists reference
count leak issues of the objects "ax25_dev" and "net_device".

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 so far.

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.

There is also a reference count leak issue of the object "net_device",
when the ax25 device is shutting down. The ax25_dev_device_down() drops
the reference count of net_device one or zero times depending on if we
goto unlock_put or not, which will cause memory leak.

In order to solve the above issues, use kernel circular doubly linked
list to implementate ax25_dev_list. As for ax25_addr_ax25dev() issue,
it is impossible for one pointer to be on a list twice. So add a break
in ax25_addr_ax25dev(). As for ax25_dev_device_down() issues, increase
the reference count of ax25_dev once in ax25_dev_device_up() and decrease
the reference count of ax25_dev and net_device after ax25_dev is removed
from the ax25_dev_list.

Fixes: d01ffb9eee4a ("ax25: add refcount in ax25_dev to avoid UAF bugs")
Suggested-by: Dan Carpenter <redacted>
Signed-off-by: Duoming Zhou <redacted>
---
Changes in v3:
  - Use kernel list to implementate ax25_dev_list.
  - Solve reference count leak issues in ax25_dev_device_down().

 include/net/ax25.h  |  4 ++--
 net/ax25/ax25_dev.c | 36 ++++++++++++------------------------
 2 files changed, 14 insertions(+), 26 deletions(-)

diff --git a/include/net/ax25.h b/include/net/ax25.h
index 0d939e5aee4..92c6aa4f9a6 100644
--- a/include/net/ax25.h
+++ b/include/net/ax25.h

@@ -216,7 +216,7 @@ typedef struct {
 struct ctl_table;
 
 typedef struct ax25_dev {
-	struct ax25_dev		*next;
+	struct list_head	list;
 
 	struct net_device	*dev;
 	netdevice_tracker	dev_tracker;

@@ -330,7 +330,7 @@ int ax25_addr_size(const ax25_digi *);
 void ax25_digi_invert(const ax25_digi *, ax25_digi *);
 
 /* ax25_dev.c */
-extern ax25_dev *ax25_dev_list;
+static struct list_head ax25_dev_list;
 extern spinlock_t ax25_dev_lock;
 
 #if IS_ENABLED(CONFIG_AX25)

diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
index 282ec581c07..fbaaba0351e 100644
--- a/net/ax25/ax25_dev.c
+++ b/net/ax25/ax25_dev.c

@@ -22,11 +22,11 @@
 #include <net/sock.h>
 #include <linux/uaccess.h>
 #include <linux/fcntl.h>
+#include <linux/list.h>
 #include <linux/mm.h>
 #include <linux/interrupt.h>
 #include <linux/init.h>
 
-ax25_dev *ax25_dev_list;
 DEFINE_SPINLOCK(ax25_dev_lock);
 
 ax25_dev *ax25_addr_ax25dev(ax25_address *addr)

@@ -34,11 +34,13 @@ ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
 	ax25_dev *ax25_dev, *res = NULL;
 
 	spin_lock_bh(&ax25_dev_lock);
-	for (ax25_dev = ax25_dev_list; ax25_dev != NULL; ax25_dev = ax25_dev->next)
+	list_for_each_entry(ax25_dev, &ax25_dev_list, list) {
 		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);
 
 	return res;

@@ -52,6 +54,7 @@ void ax25_dev_device_up(struct net_device *dev)
 {
 	ax25_dev *ax25_dev;
 
+	INIT_LIST_HEAD(&ax25_dev_list);

 	ax25_dev = kzalloc(sizeof(*ax25_dev), GFP_KERNEL);
 	if (!ax25_dev) {
 		printk(KERN_ERR "AX.25: ax25_dev_device_up - out of memory\n");

@@ -59,7 +62,6 @@ void ax25_dev_device_up(struct net_device *dev)
 	}
 
 	refcount_set(&ax25_dev->refcount, 1);
-	dev->ax25_ptr     = ax25_dev;
 	ax25_dev->dev     = dev;
 	netdev_hold(dev, &ax25_dev->dev_tracker, GFP_KERNEL);
 	ax25_dev->forward = NULL;

@@ -85,10 +87,9 @@ void ax25_dev_device_up(struct net_device *dev)
 #endif
 
 	spin_lock_bh(&ax25_dev_lock);
-	ax25_dev->next = ax25_dev_list;
-	ax25_dev_list  = ax25_dev;
+	list_add(&ax25_dev->list, &ax25_dev_list);
 	spin_unlock_bh(&ax25_dev_lock);
-	ax25_dev_hold(ax25_dev);
+	dev->ax25_ptr     = ax25_dev;