On Mon, Oct 20, 2025 at 03:25:40PM +0300, Dan Carpenter wrote:

On Mon, Oct 20, 2025 at 07:02:44PM +0800, Lizhi Xu wrote:

quoted

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.

This chart is still not right.  Let me add line numbers to your chart:

netrom/nr_route.c
   214          nr_node_lock(nr_node);
   215  
   216          if (quality != 0)
   217                  strscpy(nr_node->mnemonic, mnemonic);
   218  
   219          for (found = 0, i = 0; i < nr_node->count; i++) {
   220                  if (nr_node->routes[i].neighbour == nr_neigh) {
   221                          nr_node->routes[i].quality   = quality;
   222                          nr_node->routes[i].obs_count = obs_count;

Should we call nr_neigh->count++ if we found it?  I guess I don't
really understand what nr_neigh->count is counting...  It would be
really nice if someone added some comments explaining how the ref
counting worked.

   223                          found = 1;
   224                          break;
   225                  }
   226          }

regards,
dan carpenter