Date:   Mon, 6 Feb 2017 13:51:48 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        Vojtech Pavlik <vojtech@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Chris J Arges <chris.j.arges@...onical.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH v4 13/15] livepatch: change to a per-task consistency
 model

On Mon, Feb 06, 2017 at 05:44:31PM +0100, Petr Mladek wrote:
> > > > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch)
> > > >  
> > > >  	pr_notice("enabling patch '%s'\n", patch->mod->name);
> > > >  
> > > > +	klp_init_transition(patch, KLP_PATCHED);
> > > > +
> > > > +	/*
> > > > +	 * Enforce the order of the func->transition writes in
> > > > +	 * klp_init_transition() and the ops->func_stack writes in
> > > > +	 * klp_patch_object(), so that klp_ftrace_handler() will see the
> > > > +	 * func->transition updates before the handler is registered and the
> > > > +	 * new funcs become visible to the handler.
> > > > +	 */
> > > > +	smp_wmb();
> > > > +
> > > >  	klp_for_each_object(patch, obj) {
> > > >  		if (!klp_is_object_loaded(obj))
> > > >  			continue;
> > > >  
> > > >  		ret = klp_patch_object(obj);
> > > > -		if (ret)
> > > > -			goto unregister;
> > > > +		if (ret) {
> > > > +			pr_warn("failed to enable patch '%s'\n",
> > > > +				patch->mod->name);
> > > > +
> > > > +			klp_unpatch_objects(patch);
> > > 
> > > We should call here synchronize_rcu() here as we do
> > > in klp_try_complete_transition(). Some of the affected
> > > functions might have more versions on the stack and we
> > > need to make sure that klp_ftrace_handler() will _not_
> > > see the removed patch on the stack.
> > 
> > Even if the handler sees the new func on the stack, the
> > task->patch_state is still KLP_UNPATCHED, so it will still choose the
> > previous version of the function.  Or did I miss your point?
> 
> The barrier is needed from exactly the same reason as the one
> in klp_try_complete_transition()
> 
> CPU0					CPU1
> 
> __klp_enable_patch()
>   klp_init_transition()
> 
>     for_each...
>       task->patch_state = KLP_UNPATCHED
> 
>     for_each...
>       func->transition = true
> 
>   klp_for_each_object()
>     klp_patch_object()
>       list_add_rcu()
> 
> 					klp_ftrace_handler()
> 					  func = list_first_...()
> 
> 					  if (func->transition)
> 
> 
>     ret = klp_patch_object()
>     /* error */
>     if (ret) {
>       klp_unpatch_objects()
> 
> 	list_remove_rcu()
> 
>       klp_complete_transition()
> 
> 	for_each_....
> 	  func->transition = true
> 
> 	for_each_....
> 	  task->patch_state = PATCH_UNDEFINED
> 
> 					    patch_state = current->patch_state;
> 					    WARN_ON_ONCE(patch_state
> 							==
> 							 KLP_UNDEFINED);
> 
> BANG: The warning is triggered.
> 
> => we need to call rcu_synchronize(). It will make sure that
> no ftrace handled will see the removed func on the stack
> and we could clear all the other values.

Makes sense.

Notice in this case that klp_target_state is KLP_PATCHED.  Which means
that klp_complete_transition() would not call synchronize_rcu() at the
right time, nor would it call module_put().  It can be fixed with:

@@ -387,7 +389,7 @@ static int __klp_enable_patch(struct klp_patch *patch)
 			pr_warn("failed to enable patch '%s'\n",
 				patch->mod->name);
 
-			klp_unpatch_objects(patch);
+			klp_target_state = KLP_UNPATCHED;
 			klp_complete_transition();
 
 			return ret;

This assumes that the 'if (klp_target_state == KLP_UNPATCHED)' clause in
klp_try_complete_transition() gets moved to klp_complete_transition() as
you suggested.

> > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > > > index 5efa262..1a77f05 100644
> > > > --- a/kernel/livepatch/patch.c
> > > > +++ b/kernel/livepatch/patch.c
> > > > @@ -29,6 +29,7 @@
> > > >  #include <linux/bug.h>
> > > >  #include <linux/printk.h>
> > > >  #include "patch.h"
> > > > +#include "transition.h"
> > > >  
> > > >  static LIST_HEAD(klp_ops);
> > > >  
> > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > > >  {
> > > >  	struct klp_ops *ops;
> > > >  	struct klp_func *func;
> > > > +	int patch_state;
> > > >  
> > > >  	ops = container_of(fops, struct klp_ops, fops);
> > > >  
> > > >  	rcu_read_lock();
> > > > +
> > > >  	func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
> > > >  				      stack_node);
> > > > +
> > > > +	/*
> > > > +	 * func should never be NULL because preemption should be disabled here
> > > > +	 * and unregister_ftrace_function() does the equivalent of a
> > > > +	 * synchronize_sched() before the func_stack removal.
> > > > +	 */
> > > > +	if (WARN_ON_ONCE(!func))
> > > > +		goto unlock;
> > > > +
> > > > +	/*
> > > > +	 * Enforce the order of the ops->func_stack and func->transition reads.
> > > > +	 * The corresponding write barrier is in __klp_enable_patch().
> > > > +	 */
> > > > +	smp_rmb();
> > > 
> > > I was curious why the comment did not mention __klp_disable_patch().
> > > It was related to the hours of thinking. I would like to avoid this
> > > in the future and add a comment like.
> > > 
> > > 	 * This barrier probably is not needed when the patch is being
> > > 	 * disabled. The patch is removed from the stack in
> > > 	 * klp_try_complete_transition() and there we need to call
> > > 	 * rcu_synchronize() to prevent seeing the patch on the stack
> > > 	 * at all.
> > > 	 *
> > > 	 * Well, it still might be needed to see func->transition
> > > 	 * when the patch is removed and the task is migrated. See
> > > 	 * the write barrier in __klp_disable_patch().
> > 
> > Agreed, though as you mentioned earlier, there's also the implicit
> > barrier in klp_update_patch_state(), which would execute first in such a
> > scenario.  So I think I'll update the barrier comments in
> > klp_update_patch_state().
> 
> You inspired me to a scenario with 3 CPUs:
> 
> CPU0			CPU1			CPU2
> 
> __klp_disable_patch()
> 
>   klp_init_transition()
> 
>     func->transition = true
> 
>   smp_wmb()
> 
>   klp_start_transition()
> 
>     set TIF_PATCH_PATCHPENDING
> 
> 			klp_update_patch_state()
> 
> 			  task->patch_state
> 			     = KLP_UNPATCHED
> 
> 			  smp_mb()
> 
> 						klp_ftrace_handler()
> 						  func = list_...
> 
> 						  smp_rmb() /*needed?*/
> 
> 						  if (func->transition)
> 

I think this isn't possible.  Remember the comment I added to
klp_update_patch_state():

 * NOTE: If task is not 'current', the caller must ensure the task is inactive.
 * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.

Right now klp_update_patch_state() is only called for current.
klp_ftrace_handler() on CPU2 would be running in the context of a
different task.

> We need to make sure the CPU3 sees func->transition set. Otherwise,
> it would wrongly use the function from the patch.
> 
> So, the description might be:
> 
> 	 * Enforce the order of the ops->func_stack and
> 	 * func->transition reads when the patch is enabled.
> 	 * The corresponding write barrier is in __klp_enable_patch().
> 	 *
> 	 * Also make sure that func->transition is visible before
> 	 * TIF_PATCH_PENDING_FLAG is set and the task might get
> 	 * migrated to KLP_UNPATCHED state. The corresponding
> 	 * write barrier is in __klp_disable_patch().
> 
> 
> By other words, the read barrier here is needed from the same
> reason as the write barrier in __klp_disable_patch().
> > > > +void klp_reverse_transition(void)
> > > > +{
> > > > +	unsigned int cpu;
> > > > +	struct task_struct *g, *task;
> > > > +
> > > > +	klp_transition_patch->enabled = !klp_transition_patch->enabled;
> > > > +
> > > > +	klp_target_state = !klp_target_state;
> > > > +
> > > > +	/*
> > > > +	 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
> > > > +	 * klp_update_patch_state() running in parallel with
> > > > +	 * klp_start_transition().
> > > > +	 */
> > > > +	read_lock(&tasklist_lock);
> > > > +	for_each_process_thread(g, task)
> > > > +		clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > > > +	read_unlock(&tasklist_lock);
> > > > +
> > > > +	for_each_possible_cpu(cpu)
> > > > +		clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
> > > > +
> > > > +	/* Let any remaining calls to klp_update_patch_state() complete */
> > > > +	synchronize_rcu();
> > > > +
> > > > +	klp_start_transition();
> > > 
> > > Hmm, we should not call klp_try_complete_transition() when
> > > klp_start_transition() is called from here. I can't find a safe
> > > way to cancel klp_transition_work() when we own klp_mutex.
> > > It smells with a possible deadlock.
> > > 
> > > I suggest to move move klp_try_complete_transition() outside
> > > klp_start_transition() and explicitely call it from
> > >  __klp_disable_patch() and __klp_enabled_patch().
> > > This would fix also the problem with immediate patches, see
> > > klp_start_transition().
> > 
> > Agreed.  I'll fix it as you suggest and I'll put the mod_delayed_work()
> > call in klp_reverse_transition() again.
> 
> There is one small catch. The mod_delayed_work() might cause that two
> works might be scheduled:
> 
>   + one already running that is waiting for the klp_mutex
>   + another one scheduled by that mod_delayed_work()
>
> A solution would be to cancel the work from klp_transition_work_fn()
> if the transition succeeds.
> 
> Another possibility would be to do nothing here. The work is
> scheduled very often anyway.

Yes, I think I'll do this, for the sake of simplicity.

-- 
Josh

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