The root cause of the problem is that multiple different tasks initiate
NETROM_NODE commands to add new routes, there is no lock between them to
protect the same nr_neigh.
Task0 may add the nr_neigh.refcount value of 1 on Task1 to routes[2].

When Task2 executes nr_neigh_put(nr_node->routes[2].neighbour), it will
release the neighbour because its refcount value is 1.

In this case, the following situation causes a UAF:

Task0					Task1						Task2
=====					=====						=====
nr_add_node()
nr_neigh_get_dev()			nr_add_node()
					nr_node_lock()
					nr_node->routes[2].neighbour->count--
					nr_neigh_put(nr_node->routes[2].neighbour);
					nr_remove_neigh(nr_node->routes[2].neighbour)
					nr_node_unlock()
nr_node_lock()
nr_node->routes[2].neighbour = nr_neigh
nr_neigh_hold(nr_neigh);								nr_add_node()
											nr_neigh_put()

The solution to the problem is to use a lock to synchronize each add a route
to node.

syzbot reported:
BUG: KASAN: slab-use-after-free in nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248
Read of size 4 at addr ffff888051e6e9b0 by task syz.1.2539/8741

Call Trace:
 <TASK>
 nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248

Reported-by: syzbot+2860e75836a08b172755@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2860e75836a08b172755
Signed-off-by: Lizhi Xu <redacted>
---
V1 -> V2: update comments for cause uaf

 net/netrom/nr_route.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/netrom/nr_route.c b/net/netrom/nr_route.c
index b94cb2ffbaf8..ae1e5ee1f52f 100644
--- a/net/netrom/nr_route.c
+++ b/net/netrom/nr_route.c

@@ -102,7 +102,9 @@ static int __must_check nr_add_node(ax25_address *nr, const char *mnemonic,
 	struct nr_neigh *nr_neigh;
 	int i, found;
 	struct net_device *odev;
+	static DEFINE_MUTEX(add_node_lock);
 
+	guard(mutex)(&add_node_lock);
 	if ((odev=nr_dev_get(nr)) != NULL) {	/* Can't add routes to ourself */
 		dev_put(odev);
 		return -EINVAL;

-- 
2.43.0