Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752884AbbBJQ4r (ORCPT ); Tue, 10 Feb 2015 11:56:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:47336 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752688AbbBJQ4p (ORCPT ); Tue, 10 Feb 2015 11:56:45 -0500 Date: Tue, 10 Feb 2015 10:56:39 -0600 From: Josh Poimboeuf To: Miroslav Benes Cc: Seth Jennings , Jiri Kosina , Vojtech Pavlik , Masami Hiramatsu , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 6/9] livepatch: create per-task consistency model Message-ID: <20150210165639.GF21643@treble.redhat.com> References: <2c3d1e685dae5cccc2dfdb1b24c241b2f1c89348.1423499826.git.jpoimboe@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5050 Lines: 115 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//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//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 :-) 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. 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. -- Josh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/