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SubjectRe: [RFC PATCH 6/9] livepatch: create per-task consistency model
On Tue, 10 Feb 2015, Josh Poimboeuf wrote:

> On Tue, Feb 10, 2015 at 04:59:17PM +0100, Miroslav Benes wrote:
> >
> > On Mon, 9 Feb 2015, Josh Poimboeuf wrote:
> >
> > > Add a basic per-task consistency model. This is the foundation which
> > > will eventually enable us to patch those ~10% of security patches which
> > > change function prototypes and/or data semantics.
> > >
> > > When a patch is enabled, livepatch enters into a transition state where
> > > tasks are converging from the old universe to the new universe. If a
> > > given task isn't using any of the patched functions, it's switched to
> > > the new universe. Once all the tasks have been converged to the new
> > > universe, patching is complete.
> > >
> > > The same sequence occurs when a patch is disabled, except the tasks
> > > converge from the new universe to the old universe.
> > >
> > > The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
> > > is in transition. Only a single patch (the topmost patch on the stack)
> > > can be in transition at a given time. A patch can remain in the
> > > transition state indefinitely, if any of the tasks are stuck in the
> > > previous universe.
> > >
> > > A transition can be reversed and effectively canceled by writing the
> > > opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
> > > the transition is in progress. Then all the tasks will attempt to
> > > converge back to the original universe.
> >
> > Hi Josh,
> >
> > first, thanks a lot for great work. I'm starting to go through it and it's
> > gonna take me some time to do and send a complete review.
>
> I know there are a lot of details to look at, please take your time. I
> really appreciate your review. (And everybody else's, for that matter
> :-)
>
> > > + /* success! unpatch obsolete functions and do some cleanup */
> > > +
> > > + if (klp_universe_goal == KLP_UNIVERSE_OLD) {
> > > + klp_unpatch_objects(klp_transition_patch);
> > > +
> > > + /* prevent ftrace handler from reading old func->transition */
> > > + synchronize_rcu();
> > > + }
> > > +
> > > + pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
> > > + klp_universe_goal == KLP_UNIVERSE_NEW ? "patching" :
> > > + "unpatching");
> > > +
> > > + klp_complete_transition();
> > > +}
> >
> > ...synchronize_rcu() could be insufficient. There still can be some
> > process in our ftrace handler after the call.
> >
> > Consider the following scenario:
> >
> > When synchronize_rcu is called some process could have been preempted on
> > some other cpu somewhere at the start of the ftrace handler before
> > rcu_read_lock. synchronize_rcu waits for the grace period to pass, but that
> > does not mean anything for our process in the handler, because it is not
> > in rcu critical section. There is no guarantee that after synchronize_rcu
> > the process would be away from the handler.
> >
> > "Meanwhile" klp_try_complete_transition continues and calls
> > klp_complete_transition. This clears func->transition flags. Now the
> > process in the handler could be scheduled again. It reads the wrong value
> > of func->transition and redirection to the wrong function is done.
> >
> > What do you think? I hope I made myself clear.
>
> You really made me think. But I don't think there's a race here.
>
> Consider the two separate cases, patching and unpatching:
>
> 1. patching has completed: klp_universe_goal and all tasks'
> klp_universes are at KLP_UNIVERSE_NEW. In this case, the value of
> func->transition doesn't matter, because we want to use the func at
> the top of the stack, and if klp_universe is NEW, the ftrace handler
> will do that, regardless of the value of func->transition. This is
> why I didn't do the rcu_synchronize() in this case. But maybe you're
> not worried about this case anyway, I just described it for the sake
> of completeness :-)

Yes, this case shouldn't be a problem :)

> 2. unpatching has completed: klp_universe_goal and all tasks'
> klp_universes are at KLP_UNIVERSE_OLD. In this case, the value of
> func->transition _does_ matter. However, notice that
> klp_unpatch_objects() is called before rcu_synchronize(). That
> removes the "new" func from the klp_ops stack. Since the ftrace
> handler accesses the list _after_ calling rcu_read_lock(), it will
> never see the "new" func, and thus func->transition will never be
> set.

Hm, so indeed I messed it up. Let me rework the scenario a bit. We have a
function foo(), which has been already patched with foo_1() from patch_1
and foo_2() from patch_2. Now we would like to unpatch patch_2. It is
successfully completed and klp_try_complete_transition calls
klp_unpatch_objects and synchronize_rcu. Thus foo_2() is removed from the
RCU list in ops.

Now to the funny part. After synchronize_rcu() and before
klp_complete_transition some process might get to the ftrace handler (it
is still there because of the patch_1 still being present). It gets foo_1
from the list_first_or_null_rcu, sees that func->transition is 1 (it
hasn't been cleared yet), current->klp_universe is KLP_UNIVERSE_OLD... so
it tries to get previous function. There is none and foo() is called. This
is incorrect.

It is very similar scenario to the one in my other email earlier this day.
I think we need to clear func->transition before calling
klp_unpatch_objects. More or less.

> That said, I think there is a race where the WARN_ON_ONCE(!func)
> could trigger here, and it wouldn't be an error. So I think I'll
> remove the warning.
>
> Does that make sense?
>
> > There is the similar problem for dynamic trampolines in ftrace. You
> > cannot remove them unless there is no process in the handler. I think
> > rcu-tasks were merged a while ago for this purpose. However ftrace
> > does not use them yet and I don't know if we could exploit them to
> > solve this issue. I need to think more about it.
>
> Ok, sounds like that's an ftrace bug that could affect us.

Fortunately it is not. Steven knows about it and he does not allow dynamic
trampolines for CONFIG_PREEMPT and FTRACE_OPS_FL_DYNAMIC. Not yet. See the
comment in kernel/trace/ftrace.c for ftrace_update_trampoline.

Anyway the conclusion is that we need to be really careful with ftrace
handler. Especially in the future with dynamic trampolines and especially
with CONFIG_PREEMPT. Now the handler runs always in atomic context (at
least in cases relevant for our use) if I am not mistaken.

Miroslav


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