From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755819AbbCEOT3 (ORCPT ); Thu, 5 Mar 2015 09:19:29 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37725 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751670AbbCEOT1 (ORCPT ); Thu, 5 Mar 2015 09:19:27 -0500 Date: Thu, 5 Mar 2015 08:18:45 -0600 From: Josh Poimboeuf To: Masami Hiramatsu Cc: Petr Mladek , Seth Jennings , Jiri Kosina , Vojtech Pavlik , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org, Rusty Russell , Miroslav Benes , mingo@kernel.org, mathieu.desnoyers@efficios.com, oleg@redhat.com, paulmck@linux.vnet.ibm.com, andi@firstfloor.org, rostedt@goodmis.org, tglx@linutronix.de Subject: Re: [PATCH 2/2] livepatch: fix patched module loading race Message-ID: <20150305141845.GB1870@treble.redhat.com> References: <297cb260437910c43bf0ec85f53578f6cfef6d88.1425423640.git.jpoimboe@redhat.com> <20150304131752.GD15177@pathway.suse.cz> <54F7A8D9.7090808@hitachi.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <54F7A8D9.7090808@hitachi.com> 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 On Thu, Mar 05, 2015 at 09:52:41AM +0900, Masami Hiramatsu wrote: > (2015/03/04 22:17), Petr Mladek wrote: > > On Tue 2015-03-03 17:02:22, Josh Poimboeuf wrote: > >> It's possible for klp_register_patch() to see a module before the COMING > >> notifier is called, or after the GOING notifier is called. > >> > >> That can cause all kinds of ugly races. As Pter Mladek reported: > >> > >> "The problem is that we do not keep the klp_mutex lock all the time when > >> the module is being added or removed. > >> > >> First, the module is visible even before ftrace is ready. If we enable a patch > >> in this time frame, adding ftrace ops will fail and the patch will get rejected > >> just because bad timing. > > > > Ah, this is not true after all. I did not properly check when > > MODULE_STATE_COMING was set. I though that it was before ftrace was > > initialized but it was not true. > > > > > >> Second, if we are "lucky" and enable the patch for the coming module when the > >> ftrace is ready but before the module notifier has been called. The notifier > >> will try to enable the patch as well. It will detect that it is already patched, > >> return error, and the module will get rejected just because bad > >> timing. The more serious problem is that it will not call the notifier for > >> going module, so that the mess will stay there and we wont be able to load > >> the module later. > > > > Ah, the race is there but the effect is not that serious in the > > end. It seems that errors from module notifiers are ignored. In fact, > > we do not propagate the error from klp_module_notify_coming(). It means > > that WARN() from klp_enable_object() will be printed but the module > > will be loaded and patched. > > > > I am sorry, I was confused by kGraft where kgr_module_init() was > > called directly from module_load(). The errors were propagated. It > > means that kGraft rejects module when the patch cannot be applied. > > > > Note that the current solution is perfectly fine for the simple > > consistency model. > > > > > >> Third, similar problems are there for going module. If a patch is enabled after > >> the notifier finishes but before the module is removed from the list of modules, > >> the new patch will be applied to the module. The module might disappear at > >> anytime when the patch enabling is in progress, so there might be an access out > >> of memory. Or the whole patch might be applied and some mess will be left, > >> so it will not be possible to load/patch the module again." > > > > This is true. > > No, that's not true if you try_get_module() before patching. After the > module state goes GOING (more correctly say, after try_release_module_ref() > succeeded), all try_get_module() must fail :) > So, please make sure to get module when applying patches. Hi Masami, As Jikos pointed out elsewhere, try_get_module() won't solve all the GOING races. The module can be in GOING before mod->exit() is called. If we apply a patch between GOING getting set and mod->exit(), try_module_get() will fail and the module won't be patched. But module code can still run before or during mod->exit(), so the unpatched module code might interact badly with new patched code elsewhere. -- Josh