proc_fork_connector: a lockless ->real_parent usage is not safe

proc_fork_connector() uses ->real_parent lockless. This is not safe if
copy_process() was called with CLONE_THREAD or CLONE_PARENT, in this case
the parent != current can go away at any moment.

Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Vladimir Zapolskiy <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Evgeniy Polyakov <[e-mail]>
Cc: Evgeniy Polyakov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/connector/cn_proc.c

···
 	struct proc_event *ev;
 	__u8 buffer[CN_PROC_MSG_SIZE];
 	struct timespec ts;
+	struct task_struct *parent;
 
 	if (atomic_read(&proc_event_num_listeners) < 1)
 		return;
···
 	ktime_get_ts(&ts); /* get high res monotonic timestamp */
 	put_unaligned(timespec_to_ns(&ts), (__u64 *)&ev->timestamp_ns);
 	ev->what = PROC_EVENT_FORK;
-	ev->event_data.fork.parent_pid = task->real_parent->pid;
-	ev->event_data.fork.parent_tgid = task->real_parent->tgid;
+	rcu_read_lock();
+	parent = rcu_dereference(task->real_parent);
+	ev->event_data.fork.parent_pid = parent->pid;
+	ev->event_data.fork.parent_tgid = parent->tgid;
+	rcu_read_unlock();
 	ev->event_data.fork.child_pid = task->pid;
 	ev->event_data.fork.child_tgid = task->tgid;