Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT.

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 11, 2022 at 08:44:24AM -0500, Eric W. Biederman wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> > On Fri, Apr 08, 2022 at 10:06:30PM +0200, Peter Zijlstra wrote:
> >
> >> I'll ponder if wait_task_inactive() can simplify things..
> >
> > This,.. so ptrace_check_attach(), which does ptrace_freeze_traced()
> > already does wait_task_inactive(), but on the 'wrong' side of things.
> >
> > AFAICT, if we move that up, we're almost there, except that opens up a
> > detach+attach race. That could be fixed by doing another
> > wait_task_inactive(), but we can't due to locking :/
> >
> > Let's see if I can make that work without making a mess of things.
> > Because ensuring the task is stuck in schedule() makes the whole
> > saved_state thing go away -- as you noted.
> 
> The code can perhaps synchronize on a bit using the the full locking and
> then drop the locks and call the wait_task_inactive or whatever.
> 
> The challenge as I see it is after the traced task is inactive to allow
> "wake_up_state(t, TASK_WAKEKILL)" on the traced task, have the traced
> tasks state change to TASK_RUNNING and not allow the traced task to run
> until what is today ptrace_unfreeze_task is called.
> 
> I just don't know how to get something stuck and not allow it to run.

Same as today? Clear TASK_WAKEKILL from __state and check
__fatal_signal_pending() before putting it back again.

The thing is, once we hit schedule() with TASK_TRACED, there's only two
possible ways for that task to wake up:

  wake_up_state(t, TASK_WAKEKILL)

and

  wake_up_state(t, __TASK_TRACED)

both are issued while holding sighand lock, so provided we hold sighand
lock, we can actually frob __state as we do today, we just need to know
the task really has scheduled out first.

That is, the problem today, for PREEMPT_RT, is:

ptrace_stop():					ptrace_freeze_traced()

  set_special_state(TASK_TRACING)

  ...

  spin_lock(&foo)
    current->saved_state = current->__state;
    current->__state = TASK_RTLOCK_WAIT

						READ_ONCE(t->__state)
						  // whoopsie, not
						  // TRACED

  ...

  schedule()


But if wait_task_inactive() ensures our @t is actually in schedule(),
we're good again, nothing will then ever change __state as long as we
hold sighand lock.

The only fun bit is that wait_task_inactive() likes to schedule so we
want do that with sighand lock held. What we need to do is call it
first, and then re-check stuff is still sane once we (re)acquire all the
locks.

This is certainly possible -- and not in fact too hard; the only thing
I'm really concerned about is not making it more ugly than dealing with
saved_state in the first place (and *that* is turning out to be somewhat
hard).

But while going over all this I think there might be an additional
problem; wait_task_inactive() is stubbed for SMP=n...