In the Linux kernel, the following vulnerability has been resolved:

procfs: fix missing RCU protection when reading real_parent in do_task_stat()

When reading /proc/[pid]/stat, do_task_stat() accesses task->real_parent
without proper RCU protection, which leads to:

  cpu 0                               cpu 1
  -----                               -----
  do_task_stat
    var = task->real_parent
                                      release_task
                                        call_rcu(delayed_put_task_struct)
    task_tgid_nr_ns(var)
      rcu_read_lock   <--- Too late to protect task->real_parent!
      task_pid_ptr    <--- UAF!
      rcu_read_unlock

This patch uses task_ppid_nr_ns() instead of task_tgid_nr_ns() to add
proper RCU protection for accessing task->real_parent.