Re: [PATCH v4 13/15] livepatch: change to a per-task consistency model

Petr Mladek
2017-02-08T15:47:59Z

On Mon 2017-02-06 13:51:48, Josh Poimboeuf wrote:
> 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;

Great catch! Looks good to me.

> 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.

I agree that it is impossible with the current code. In fact, I cannot
imagine a way to migrate the task where the barrier would be needed.
The question if we could/should somehow document it. Something like

* The barrier is not really needed when the patch is being
* disabled. The value of func->transition would change
* the result of this handled only after the task is migrated.
* But the conditions for the migration are very limited
* and practically include a full barrier, see
* klp_update_patch_state().

> > 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.

Sounds good to me.

I am sorry for the late reply. I am ill and work only limited
time.

Best Regards,
Petr