From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753657AbbCEAwu (ORCPT ); Wed, 4 Mar 2015 19:52:50 -0500 Received: from mail7.hitachi.co.jp ([133.145.228.42]:33367 "EHLO mail7.hitachi.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751171AbbCEAwt (ORCPT ); Wed, 4 Mar 2015 19:52:49 -0500 Message-ID: <54F7A8D9.7090808@hitachi.com> Date: Thu, 05 Mar 2015 09:52:41 +0900 From: Masami Hiramatsu Organization: Hitachi, Ltd., Japan User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20120614 Thunderbird/13.0.1 MIME-Version: 1.0 To: Petr Mladek CC: Josh Poimboeuf , 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 References: <297cb260437910c43bf0ec85f53578f6cfef6d88.1425423640.git.jpoimboe@redhat.com> <20150304131752.GD15177@pathway.suse.cz> In-Reply-To: <20150304131752.GD15177@pathway.suse.cz> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (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. Thank you, -- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Research Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu.pt@hitachi.com