rtnetlink: Protect link_ops by mutex.

rtnl_link_unregister() holds RTNL and calls synchronize_srcu(),
but rtnl_newlink() will acquire SRCU frist and then RTNL.

Then, we need to unlink ops and call synchronize_srcu() outside
of RTNL to avoid the deadlock.

   rtnl_link_unregister()       rtnl_newlink()
   ----                         ----
   lock(rtnl_mutex);
                                lock(&ops->srcu);
                                lock(rtnl_mutex);
   sync(&ops->srcu);

Let's move as such and add a mutex to protect link_ops.

Now, link_ops is protected by its dedicated mutex and
rtnl_link_register() no longer needs to hold RTNL.

While at it, we move the initialisation of ops->dellink and
ops->srcu out of the mutex scope.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241108004823.29419-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 20 insertions, 13 deletions

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index f0246ecec7fa..21154ef0048f 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -466,6 +466,7 @@ void __rtnl_unregister_many(const struct rtnl_msg_handler *handlers, int n)
 }
 EXPORT_SYMBOL_GPL(__rtnl_unregister_many);
 
+static DEFINE_MUTEX(link_ops_mutex);
 static LIST_HEAD(link_ops);
 
 static struct rtnl_link_ops *rtnl_link_ops_get(const char *kind, int *srcu_index)
@@ -508,14 +509,6 @@ int __rtnl_link_register(struct rtnl_link_ops *ops)
 	struct rtnl_link_ops *tmp;
 	int err;
 
-	/* When RTNL is removed, add lock for link_ops. */
-	ASSERT_RTNL();
-
-	list_for_each_entry(tmp, &link_ops, list) {
-		if (!strcmp(ops->kind, tmp->kind))
-			return -EEXIST;
-	}
-
 	/* The check for alloc/setup is here because if ops
 	 * does not have that filled up, it is not possible
 	 * to use the ops for creating device. So do not
@@ -528,9 +521,20 @@ int __rtnl_link_register(struct rtnl_link_ops *ops)
 	if (err)
 		return err;
 
+	mutex_lock(&link_ops_mutex);
+
+	list_for_each_entry(tmp, &link_ops, list) {
+		if (!strcmp(ops->kind, tmp->kind)) {
+			err = -EEXIST;
+			goto unlock;
+		}
+	}
+
 	list_add_tail_rcu(&ops->list, &link_ops);
+unlock:
+	mutex_unlock(&link_ops_mutex);
 
-	return 0;
+	return err;
 }
 EXPORT_SYMBOL_GPL(__rtnl_link_register);
 
@@ -598,14 +602,17 @@ void rtnl_link_unregister(struct rtnl_link_ops *ops)
 {
 	struct net *net;
 
-	/* Close the race with setup_net() and cleanup_net() */
-	down_write(&pernet_ops_rwsem);
-	rtnl_lock_unregistering_all();
-
+	mutex_lock(&link_ops_mutex);
 	list_del_rcu(&ops->list);
+	mutex_unlock(&link_ops_mutex);
+
 	synchronize_srcu(&ops->srcu);
 	cleanup_srcu_struct(&ops->srcu);
 
+	/* Close the race with setup_net() and cleanup_net() */
+	down_write(&pernet_ops_rwsem);
+	rtnl_lock_unregistering_all();
+
 	for_each_net(net)
 		__rtnl_kill_links(net, ops);