linux-kernel - Re: [PATCH V3] netrom: Prevent race conditions between neighbor operations

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Message-ID: <aPtYvPq12Txu9JCG@stanley.mountain>
Date: Fri, 24 Oct 2025 13:45:16 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Lizhi Xu <lizhi.xu@...driver.com>, davem@...emloft.net,
	edumazet@...gle.com, horms@...nel.org, kuba@...nel.org,
	linux-hams@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	syzbot+2860e75836a08b172755@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH V3] netrom: Prevent race conditions between neighbor
 operations

On Thu, Oct 23, 2025 at 01:44:18PM +0200, Paolo Abeni wrote:
> Why reordering the statements as:
> 
> 	if (nr_node->routes[2].neighbour->count == 0 &&
> !nr_node->routes[2].neighbour->locked)
> 		nr_remove_neigh(nr_node->routes[2].neighbour);
> 	nr_neigh_put(nr_node->routes[2].neighbour);
> 
> is not enough?

There are so many unfortunate things like this:

net/netrom/nr_route.c
   243                          /* It must be better than the worst */
   244                          if (quality > nr_node->routes[2].quality) {
   245                                  nr_node->routes[2].neighbour->count--;

++/-- are not atomic.

   246                                  nr_neigh_put(nr_node->routes[2].neighbour);
   247  
   248                                  if (nr_node->routes[2].neighbour->count == 0 && !nr_node->routes[2].neighbour->locked)
   249                                          nr_remove_neigh(nr_node->routes[2].neighbour);
   250  
   251                                  nr_node->routes[2].quality   = quality;
   252                                  nr_node->routes[2].obs_count = obs_count;
   253                                  nr_node->routes[2].neighbour = nr_neigh;

This line should come after the next two lines.

   254  
   255                                  nr_neigh_hold(nr_neigh);
   256                                  nr_neigh->count++;
   257                          }

regards,
dan carpenter