[PATCH] netrom: Prevent race conditions between multiple add route

Lizhi Xu
2025-10-20T08:14:30Z

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 Task3 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
===== =====
nr_add_node()
nr_neigh_get_dev() nr_add_node()
nr_node->routes[2].neighbour->count--
nr_neigh_put(nr_node->routes[2].neighbour);
nr_remove_neigh(nr_node->routes[2].neighbour)
nr_node->routes[2].neighbour = nr_neigh
nr_neigh_hold(nr_neigh);

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 <lizhi.xu@windriver.com>
---
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