[PATCH net] net: hsr: wait for pending node-free RCU callbacks on module exit

[Ali Ahmet Memis <ali@iusegentoo.com>]

HSR/PRP node teardown and the prune paths queue node frees with
call_rcu(&node->rcu_head, hsr_free_node_rcu). hsr_free_node_rcu() lives
in the hsr module text.

hsr_exit() tears down netlink, debugfs and the netdev notifier, but it
never calls rcu_barrier(). If a node-free callback is still pending when
the module is unloaded, RCU can invoke hsr_free_node_rcu() after the
module text has been freed, branching into freed memory.

Per Documentation/RCU/checklist.rst, a module that registers callbacks
with call_rcu() must call rcu_barrier() in its exit path before it is
unloaded; synchronize_rcu() is not sufficient because it only waits for
a grace period, not for the queued callbacks to actually run.

Add rcu_barrier() at the end of hsr_exit(), after netlink/link teardown
has stopped any new callbacks from being queued.

Fixes: 415e6367512b ("hsr: Implement more robust duplicate discard for PRP")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
---
 net/hsr/hsr_main.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/hsr/hsr_main.c b/net/hsr/hsr_main.c
index 33951d9bd..05d2e5750 100644
--- a/net/hsr/hsr_main.c
+++ b/net/hsr/hsr_main.c
@@ -179,6 +179,13 @@ static void __exit hsr_exit(void)
 	hsr_netlink_exit();
 	hsr_debugfs_remove_root();
 	unregister_netdevice_notifier(&hsr_nb);
+
+	/* Node teardown queues frees via call_rcu(hsr_free_node_rcu), whose
+	 * callback lives in this module. Wait for any still-pending callback
+	 * before the module text is freed. synchronize_rcu() is not enough:
+	 * it waits for a grace period, not for the callbacks to run.
+	 */
+	rcu_barrier();
 }
 
 module_init(hsr_init);
-- 
2.54.0