From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 583B3C3F2D2 for ; Mon, 2 Mar 2020 10:36:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 36C9F20863 for ; Mon, 2 Mar 2020 10:36:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727030AbgCBKgw convert rfc822-to-8bit (ORCPT ); Mon, 2 Mar 2020 05:36:52 -0500 Received: from mx2.suse.de ([195.135.220.15]:42344 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726674AbgCBKgv (ORCPT ); Mon, 2 Mar 2020 05:36:51 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 72D6CAC37; Mon, 2 Mar 2020 10:36:49 +0000 (UTC) From: Nicolai Stange To: JeffleXu Cc: jpoimboe@redhat.com, live-patching@vger.kernel.org Subject: Re: [help] Confusion on livepatch's per-task consistency model References: <315f87a7-eb40-67a7-4ab9-4b384fde752a@linux.alibaba.com> Date: Mon, 02 Mar 2020 11:36:48 +0100 In-Reply-To: <315f87a7-eb40-67a7-4ab9-4b384fde752a@linux.alibaba.com> (jefflexu@linux.alibaba.com's message of "Mon, 2 Mar 2020 16:45:24 +0800") Message-ID: <87mu8z6o1r.fsf@suse.de> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: live-patching-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org Hi, JeffleXu writes: > According to the model, there will be scenario where old function and new > > function can co-exist, though for a single thread, it sees either all new > > functions or all old functions. That's correct. > I can't understand why Vojtech said that 'old func processing new > data' was impossible. Just to make it explicit: Vojtech was talking about data layout changes. That is, consider you have something like e.g. this in the unmodified kernel sources: struct my_driver_work { struct work_struct work; struct list_head works_list; void *some_payload; }; In general, you can't change that struct definition from a live patch. So simply extending it like this struct my_driver_work { struct work_struct work; struct list_head works_list; unsigned long flags /* New */ void *some_payload; }; won't work. > Assuming a scenario where a process calls func-A to submit a > > work request (inserted into a global list), and then a kthread is > responsible > > for calling func-B to process all work requests in the list. What if > this process > > has finished the transition (sees new func-A) while kthread still sees > the old func-B? Going with the example from above, the patched func-A would submit instances of the new struct my_driver_work whereas the unpatched func-B would expect the ->some_payload pointer at where ->flags is stored now, which is bad, obviously. In this specific example, it could perhaps be possible to make the patched func-A associate a shadow variable corresponding to the new ->flags member with newly created struct my_driver_work instances (of original, unmodified layout). Any unpatched func-B would obviously ignore it and consider the shadow only when it becomes patched. It very much depends on the specific situation whether or not this is acceptable. If not, the ->post_patch() can sometimes be used to achieve some notion of a "global consistency" state (in this context, c.f. Documentation/livepatch/system-state.rst). Note however, that the patched func-B must always be able to handle the situation where a struct my_driver_work instance does not have such a ->flags shadow attached to it, either because the instance had been created when the live patch has not been applied at all or because it has been submitted from a not yet transitioned func-A. There's another subtlety: the deallocation code for struct my_driver_work needs to get livepatched as well to make it free the ->flags shadow. Consider the case where func-A has been transitioned, but the deallocation code hasn't yet. Without any extra measures in func-A, it could happen that a stale ->flags shadow from a deallocated struct my_driver_work gets (wrongly) reassociated with a fresh struct my_driver_work instance allocated at the same address as the old one (because shadow variables are keyed on addresses of the data they're associated with). Sometimes that's acceptable, sometimes it's not. In the latter case you probably had to check for this situation and work around it in the allocation code, i.e. the live-patched func-A. Finally, let me remark that from my experience, most CVEs (>95%) can be fixed via live patching without having to resort to either of shadow variables, callbacks or the state API. For the rest, things usually tend to become really non-trivial, hackish and subtle. > In this case, old func-B has to process new data. If there's some lock > semantic > > changes in func-A and func-B, then old func-B has no way identifying > the shadow > > variable labeled by new func-A. I don't understand what you mean by "variable labeled by new func-A"? Anyway, it's correct that an unpatched func-B would not consider any shadow variables instantiated by patched func-A. And it's also correct that changing locking semantics is difficult, if not impossible. Thanks, Nicolai -- SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg), GF: Felix Imendörffer