Fix race conditions in /proc between execve() and ptrace_may_access()

[kees]

There are race conditions around ptrace_may_access() that can lead to various confidentiality exposures.

v1: https://lore.kernel.org/linux-fsdevel/1474211117-16674-1-git-send-email-jann@thejh.net/
v2: https://lore.kernel.org/linux-fsdevel/1474663238-22134-1-git-send-email-jann@thejh.net/
v3: https://lore.kernel.org/linux-fsdevel/1477863998-3298-1-git-send-email-jann@thejh.net/

It looks like these steps are needed:

• https://lore.kernel.org/linux-fsdevel/1477863998-3298-2-git-send-email-jann@thejh.net/ was implemented differently with task->signal->exec_update_lock in commit eea9673 (v5.7) and converted to a semaphore in commit f7cfd87 (v5.11)
• rewrite https://lore.kernel.org/linux-fsdevel/1477863998-3298-5-git-send-email-jann@thejh.net/ to use down_read_killable(&task->signal->exec_update_lock)
• rewrite https://lore.kernel.org/linux-fsdevel/1477863998-3298-4-git-send-email-jann@thejh.net/ to use the __mem_open opener and file->private_data != current->mm as the confused deputy check for each of at least:
  • do_task_stat
  • proc_pid_wchan
  • proc_pid_stack
• rewrite https://lore.kernel.org/linux-fsdevel/1477863998-3298-6-git-send-email-jann@thejh.net/ to use down_read_killable(&task->signal->exec_update_lock)
• https://lore.kernel.org/linux-fsdevel/1477863998-3298-7-git-send-email-jann@thejh.net/ was fixed (ultimately) with commit 94f0b2d (v5.13)
• figure out what's needed for https://lore.kernel.org/linux-fsdevel/1477863998-3298-8-git-send-email-jann@thejh.net/
• refresh https://lore.kernel.org/linux-fsdevel/1477863998-3298-9-git-send-email-jann@thejh.net/