[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/kernel/kthread.c b/kernel/kthread.c
index f97fd01a2932..1335c89c6225 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -336,7 +336,6 @@ static int kthread(void *_create)
 	void *data = create->data;
 	struct completion *done;
 	struct kthread *self;
-	int ret;
 
 	self = to_kthread(current);
 
@@ -365,17 +364,20 @@ static int kthread(void *_create)
 	 * or the creator may spend more time in wait_task_inactive().
 	 */
 	preempt_disable();
+	if (test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
+		preempt_enable();
+		create->result = ERR_PTR(-EINTR);
+		complete(done);
+		do_exit(0);
+	}
 	complete(done);
 	schedule_preempt_disabled();
 	preempt_enable();
 
-	ret = -EINTR;
-	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
-		cgroup_kthread_ready();
-		__kthread_parkme(self);
-		ret = threadfn(data);
-	}
-	kthread_exit(ret);
+	cgroup_kthread_ready();
+	__kthread_parkme(self);
+	/* Run the actual thread function. */
+	kthread_exit(threadfn(data));
 }
 
 /* called from kernel_clone() to get node information for about to be created task */
diff --git a/net/rxrpc/local_object.c b/net/rxrpc/local_object.c
index 44222923c0d1..24ee585d9aaf 100644
--- a/net/rxrpc/local_object.c
+++ b/net/rxrpc/local_object.c
@@ -357,10 +357,11 @@ 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);