Re: [RFC][PATCH v2 09/11] context_tracking,livepatch: Dont disturb NOHZ_FULL

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 06, 2021 at 10:12:17AM +0200, Petr Mladek wrote:

> > @@ -180,8 +186,10 @@ void klp_update_patch_state(struct task_
> >  	 *    of func->transition, if klp_ftrace_handler() is called later on
> >  	 *    the same CPU.  See __klp_disable_patch().
> >  	 */
> > -	if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
> > +	if (test_tsk_thread_flag(task, TIF_PATCH_PENDING)) {
> 
> This would require smp_rmb() here. It will make sure that we will
> read the right @klp_target_state when TIF_PATCH_PENDING is set.

Bah, yes.

> >  		task->patch_state = READ_ONCE(klp_target_state);
> 
> Note that smp_wmb() is not needed here because
> klp_complete_transition() calls klp_synchronize_transition()
> aka synchronize_rcu() before clearing klp_target_state.
> This is why the original code worked.
> 
> 
> > +		clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > +	}
> >  
> >  	preempt_enable_notrace();
> >  }
> > @@ -270,15 +278,30 @@ static int klp_check_and_switch_task(str
> >  {
> >  	int ret;
> >  
> > -	if (task_curr(task))
> > +	if (task_curr(task)) {
> > +		/*
> > +		 * This only succeeds when the task is in NOHZ_FULL user
> > +		 * mode, the true return value guarantees any kernel entry
> > +		 * will call klp_update_patch_state().
> > +		 *
> > +		 * XXX: ideally we'd simply return 0 here and leave
> > +		 * TIF_PATCH_PENDING alone, to be fixed up by
> > +		 * klp_update_patch_state(), except livepatching goes wobbly
> > +		 * with 'pending' TIF bits on.
> > +		 */
> > +		if (context_tracking_set_cpu_work(task_cpu(task), CT_WORK_KLP))
> > +			goto clear;
> 
> If I get it correctly, this will clear TIF_PATCH_PENDING immediately
> but task->patch_state = READ_ONCE(klp_target_state) will be
> done later by ct_exit_user_work().
> 
> This is a bit problematic:
> 
>   1. The global @klp_target_state is set to KLP_UNDEFINED when all
>      processes have TIF_PATCH_PENDING is cleared. This is actually
>      still fine because func->transition is cleared as well.
>      As a result, current->patch_state is ignored in klp_ftrace_handler.
> 
>   2. The real problem happens when another livepatch is enabled.
>      The global @klp_target_state is set to new value and
>      func->transition is set again. In this case, the delayed
>      ct_exit_user_work() might assign wrong value that might
>      really be used by klp_ftrace_handler().

Urgghh.. totally missed that.

> IMHO, the original solution from v1 was better. We only needed to

It was also terribly broken in other 'fun' ways. See below.

> be careful when updating task->patch_state and clearing
> TIF_PATCH_PENDING to avoid the race.
> 
> The following might work:
> 
> static int klp_check_and_switch_task(struct task_struct *task, void *arg)
> {
> 	int ret;
> 
> 	/*
> 	 * Stack is reliable only when the task is not running on any CPU,
> 	 * except for the task running this code.
> 	 */
> 	if (task_curr(task) && task != current) {
> 		/*
> 		 * This only succeeds when the task is in NOHZ_FULL user
> 		 * mode. Such a task might be migrated immediately. We
> 		 * only need to be careful to set task->patch_state before
> 		 * clearing TIF_PATCH_PENDING so that the task migrates
> 		 * itself when entring kernel in the meatime.
> 		 */
> 		if (is_ct_user(task)) {
> 			klp_update_patch_state(task);
> 			return 0;
> 		}
> 
> 		return -EBUSY;
> 	}
> 
> 	ret = klp_check_stack(task, arg);
> 	if (ret)
> 		return ret;
> 
> 	/*
> 	 * The task neither is running on any CPU and nor it can get
> 	 * running. As a result, the ordering is not important and
> 	 * barrier is not needed.
> 	 */
> 	task->patch_state = klp_target_state;
> 	clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> 
> 	return 0;
> }
> 
> , where is_ct_user(task) would return true when task is running in
> CONTEXT_USER. If I get the context_tracking API correctly then
> it might be implemeted the following way:

That's not sufficient, you need to tag the remote task with a ct_work
item to also runs klp_update_patch_state(), otherwise the remote CPU can
enter kernel space between checking is_ct_user() and doing
klp_update_patch_state():

	CPU0				CPU1

					<user>

	if (is_ct_user()) // true
					<kernel-entry>
					  // run some kernel code
	  klp_update_patch_state()
	  *WHOOPSIE*


So it needs to be something like:


	CPU0				CPU1

					<user>

	if (context_tracking_set_cpu_work(task_cpu(), CT_WORK_KLP))

					<kernel-entry>
	  klp_update_patch_state	  klp_update_patch_state()


So that CPU0 and CPU1 race to complete klp_update_patch_state() *before*
any regular (!noinstr) code gets run.

Which then means it needs to look something like:

noinstr void klp_update_patch_state(struct task_struct *task)
{
	struct thread_info *ti = task_thread_info(task);

	preempt_disable_notrace();
	if (arch_test_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags)) {
		/*
		 * Order loads of TIF_PATCH_PENDING vs klp_target_state.
		 * See klp_init_transition().
		 */
		smp_rmb();
		task->patch_state = __READ_ONCE(klp_target_state);
		/*
		 * Concurrent against self; must observe updated
		 * task->patch_state if !TIF_PATCH_PENDING.
		 */
		smp_mb__before_atomic();
		arch_clear_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags);
	} else {
		/*
		 * Concurrent against self, see smp_mb__before_atomic()
		 * above.
		 */
		smp_rmb();
	}
	preempt_enable_notrace();
}