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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 BCF61C433E2 for ; Thu, 10 Sep 2020 21:21:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8104D221E2 for ; Thu, 10 Sep 2020 21:21:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728217AbgIJVVB (ORCPT ); Thu, 10 Sep 2020 17:21:01 -0400 Received: from foss.arm.com ([217.140.110.172]:37390 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731130AbgIJOZ5 (ORCPT ); Thu, 10 Sep 2020 10:25:57 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 747EB113E; Thu, 10 Sep 2020 07:15:50 -0700 (PDT) Received: from [192.168.1.179] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 92E7F3F66E; Thu, 10 Sep 2020 07:15:48 -0700 (PDT) Subject: Re: [PATCH v2 0/2] MTE support for KVM guest To: Andrew Jones Cc: Peter Maydell , linux-kernel@vger.kernel.org, Juan Quintela , Catalin Marinas , Richard Henderson , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Marc Zyngier , Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, Dave Martin References: <20200904160018.29481-1-steven.price@arm.com> <20200909152540.ylnrljd6aelxoxrf@kamzik.brq.redhat.com> <857566df-1b98-84f7-9268-d092722dc749@arm.com> <20200910062958.o55apuvdxmf3uiqb@kamzik.brq.redhat.com> <37663bb6-d3a7-6f53-d0cd-88777633a2b2@arm.com> <20200910135618.cvnlrgvhuy3amv6s@kamzik.brq.redhat.com> From: Steven Price Message-ID: <17efa848-9bda-26b2-b70f-040c9fa3f2da@arm.com> Date: Thu, 10 Sep 2020 15:14:47 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200910135618.cvnlrgvhuy3amv6s@kamzik.brq.redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/09/2020 14:56, Andrew Jones wrote: > On Thu, Sep 10, 2020 at 10:21:04AM +0100, Steven Price wrote: >> On 10/09/2020 07:29, Andrew Jones wrote: >>> But if userspace created the memslots with memory already set with >>> PROT_MTE, then this wouldn't be necessary, right? And, as long as >>> there's still a way to access the memory with tag checking disabled, >>> then it shouldn't be a problem. >> >> Yes, so one option would be to attempt to validate that the VMM has provided >> memory pages with the PG_mte_tagged bit set (e.g. by mapping with PROT_MTE). >> The tricky part here is that we support KVM_CAP_SYNC_MMU which means that >> the VMM can change the memory backing at any time - so we could end up in >> user_mem_abort() discovering that a page doesn't have PG_mte_tagged set - at >> that point there's no nice way of handling it (other than silently upgrading >> the page) so the VM is dead. >> >> So since enforcing that PG_mte_tagged is set isn't easy and provides a >> hard-to-debug foot gun to the VMM I decided the better option was to let the >> kernel set the bit automatically. >> > > The foot gun still exists when migration is considered, no? If userspace > is telling a guest it can use MTE on its normal memory, but then doesn't > prepare that memory correctly, or remember to migrate the tags correctly > (which requires knowing the memory has tags and knowing how to get them), > then I guess the VM is in trouble one way or another. Well not all VMMs support migration, and it's only migration that is affected by this for a simple VMM (e.g. the changes to kvmtool are minimal for MTE). But yes fundamentally if a VMM enables MTE it needs to know how to deal with the extra tags everywhere. > I feel like we should trust the VMM to ensure MTE will work on any memory > the guest could use it on, and change the action in user_mem_abort() to > abort the guest with a big error message if it sees the flag is missing. I'm happy to change it, if you feel this is easier to debug. >>>>> >>>>> If userspace needs to write to guest memory then it should be due to >>>>> a device DMA or other specific hardware emulation. Those accesses can >>>>> be done with tag checking disabled. >>>> >>>> Yes, the question is can the VMM (sensibly) wrap the accesses with a >>>> disable/renable tag checking for the process sequence. The alternative at >>>> the moment is to maintain a separate (untagged) mapping for the purpose >>>> which might present it's own problems. >>> >>> Hmm, so there's no easy way to disable tag checking when necessary? If we >>> don't map the guest ram with PROT_MTE and continue setting the attribute >>> in KVM, as this series does, then we don't need to worry about it tag >>> checking when accessing the memory, but then we can't access the tags for >>> migration. >> >> There's a "TCO" (Tag Check Override) bit in PSTATE which allows disabling >> tag checking, so if it's reasonable to wrap accesses to the memory you can >> simply set the TCO bit, perform the memory access and then unset TCO. That >> would mean a single mapping with MTE enabled would work fine. What I don't >> have a clue about is whether it's practical in the VMM to wrap guest >> accesses like this. >> > > At least QEMU goes through many abstractions to get to memory already. > There may already be a hook we could use, if not, it probably wouldn't > be too hard to add one (famous last words). Sounds good. My hope was that the abstractions were already in there. Thanks, Steve