[syzbot] KASAN: use-after-free Read in rxrpc_lookup_local

[David Howells]

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git master

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index e7dccab7b741..37f3aec784cc 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -287,6 +287,7 @@ struct rxrpc_local {
 	struct hlist_node	link;
 	struct socket		*socket;	/* my UDP socket */
 	struct task_struct	*io_thread;
+	struct completion	io_thread_ready; /* Indication that the I/O thread started */
 	struct rxrpc_sock __rcu	*service;	/* Service(s) listening on this endpoint */
 	struct rw_semaphore	defrag_sem;	/* control re-enablement of IP DF bit */
 	struct sk_buff_head	rx_queue;	/* Received packets */
diff --git a/net/rxrpc/io_thread.c b/net/rxrpc/io_thread.c
index d83ae3193032..e460e4151c16 100644
--- a/net/rxrpc/io_thread.c
+++ b/net/rxrpc/io_thread.c
@@ -426,6 +426,8 @@ int rxrpc_io_thread(void *data)
 	struct rxrpc_call *call;
 	struct sk_buff *skb;
 
+	complete(&local->io_thread_ready);
+
 	skb_queue_head_init(&rx_queue);
 
 	set_user_nice(current, MIN_NICE);
diff --git a/net/rxrpc/local_object.c b/net/rxrpc/local_object.c
index 44222923c0d1..d8dfd5459f50 100644
--- a/net/rxrpc/local_object.c
+++ b/net/rxrpc/local_object.c
@@ -96,6 +96,7 @@ static struct rxrpc_local *rxrpc_alloc_local(struct rxrpc_net *rxnet,
 		atomic_set(&local->active_users, 1);
 		local->rxnet = rxnet;
 		INIT_HLIST_NODE(&local->link);
+		init_completion(&local->io_thread_ready);
 		init_rwsem(&local->defrag_sem);
 		skb_queue_head_init(&local->rx_queue);
 		INIT_LIST_HEAD(&local->call_attend_q);
@@ -189,6 +190,7 @@ static int rxrpc_open_socket(struct rxrpc_local *local, struct net *net)
 		goto error_sock;
 	}
 
+	wait_for_completion(&local->io_thread_ready);
 	local->io_thread = io_thread;
 	_leave(" = 0");
 	return 0;
@@ -357,10 +359,11 @@ struct rxrpc_local *rxrpc_use_local(struct rxrpc_local *local,
  */
 void rxrpc_unuse_local(struct rxrpc_local *local, enum rxrpc_local_trace why)
 {
-	unsigned int debug_id = local->debug_id;
+	unsigned int debug_id;
 	int r, u;
 
 	if (local) {
+		debug_id = local->debug_id;
 		r = refcount_read(&local->ref);
 		u = atomic_dec_return(&local->active_users);
 		trace_rxrpc_local(debug_id, why, r, u);