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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 01476C47089 for ; Thu, 27 May 2021 13:08:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D56D9613AC for ; Thu, 27 May 2021 13:08:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236373AbhE0NKa (ORCPT ); Thu, 27 May 2021 09:10:30 -0400 Received: from mail.kernel.org ([198.145.29.99]:38316 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236099AbhE0NK0 (ORCPT ); Thu, 27 May 2021 09:10:26 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 107246128B; Thu, 27 May 2021 13:08:50 +0000 (UTC) Date: Thu, 27 May 2021 14:08:48 +0100 From: Catalin Marinas To: Steven Price Cc: Marc Zyngier , Will Deacon , James Morse , Julien Thierry , Suzuki K Poulose , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave Martin , Mark Rutland , Thomas Gleixner , qemu-devel@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Richard Henderson , Peter Maydell , Haibo Xu , Andrew Jones Subject: Re: [PATCH v12 7/8] KVM: arm64: ioctl to fetch/store tags in a guest Message-ID: <20210527130848.GA8661@arm.com> References: <20210517123239.8025-1-steven.price@arm.com> <20210517123239.8025-8-steven.price@arm.com> <20210520120556.GC12251@arm.com> <20210520172713.GF12251@arm.com> <5eec330f-63c0-2af8-70f8-ba9b643e2558@arm.com> <20210524181129.GI14645@arm.com> <58345eca-6e5f-0faa-e47d-e9149d73f6c5@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <58345eca-6e5f-0faa-e47d-e9149d73f6c5@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 27, 2021 at 08:50:30AM +0100, Steven Price wrote: > On 24/05/2021 19:11, Catalin Marinas wrote: > > I had some (random) thoughts on how to make things simpler, maybe. I > > think most of these races would have been solved if we required PROT_MTE > > in the VMM but this has an impact on the VMM if it wants to use MTE > > itself. If such requirement was in place, all KVM needed to do is check > > PG_mte_tagged. > > > > So what we actually need is a set_pte_at() in the VMM to clear the tags > > and set PG_mte_tagged. Currently, we only do this if the memory type is > > tagged (PROT_MTE) but it's not strictly necessary. > > > > As an optimisation for normal programs, we don't want to do this all the > > time but the visible behaviour wouldn't change (well, maybe for ptrace > > slightly). However, it doesn't mean we couldn't for a VMM, with an > > opt-in via prctl(). This would add a MMCF_MTE_TAG_INIT bit (couldn't > > think of a better name) to mm_context_t.flags and set_pte_at() would > > behave as if the pte was tagged without actually mapping the memory in > > user space as tagged (protection flags not changed). Pages that don't > > support tagging are still safe, just some unnecessary ignored tag > > writes. This would need to be set before the mmap() for the guest > > memory. > > > > If we want finer-grained control we'd have to store this information in > > the vma flags, in addition to VM_MTE (e.g. VM_MTE_TAG_INIT) but without > > affecting the actual memory type. The easiest would be another pte bit, > > though we are short on them. A more intrusive (not too bad) approach is > > to introduce a set_pte_at_vma() and read the flags directly in the arch > > code. In most places where set_pte_at() is called on a user mm, the vma > > is also available. > > > > Anyway, I'm not saying we go this route, just thinking out loud, get > > some opinions. > > Does get_user_pages() actually end up calling set_pte_at() normally? Not always, at least as how it's called from hva_to_pfn(). My reading of the get_user_page_fast_only() is that it doesn't touch the pte, just walks the page tables and pins the page. Of course, it expects a valid pte to have been set in the VMM already, otherwise it doesn't pin any page and the caller falls back to the slow path. The slow path, get_user_pages_unlocked(), passes FOLL_TOUCH and set_pte_at() will be called either in follow_pfn_pte() if it was valid or via faultin_page() -> handle_mm_fault(). > If not then on the normal user_mem_abort() route although we can > easily check VM_MTE_TAG_INIT there's no obvious place to hook in to > ensure that the pages actually allocated have the PG_mte_tagged flag. I don't think it helps if we checked such vma flag in user_mem_abort(), we'd still have the race with set_pte_at() on the page flags. What I was trying to avoid is touching the page flags in too many places, so deferring this always to set_pte_at() in the VMM. > I'm also not sure how well this would work with the MMU notifiers path > in KVM. With MMU notifiers (i.e. the VMM replacing a page in the > memslot) there's not even an obvious hook to enforce the VMA flag. So I > think we'd end up with something like the sanitise_mte_tags() function > to at least check that the PG_mte_tagged flag is set on the pages > (assuming that the trigger for the MMU notifier has done the > corresponding set_pte_at()). Admittedly this might close the current > race documented there. If we kept this check to the VMM set_pte_at(), I think we can ignore the notifiers. > It also feels wrong to me to tie this to a process with prctl(), it > seems much more normal to implement this as a new mprotect() flag as > this is really a memory property not a process property. And I think > we'll find some scary corner cases if we try to associate everything > back to a process - although I can't instantly think of anything that > will actually break. I agree, tying it to the process looks wrong, only that it's less intrusive. I don't think it would break anything, only potential performance regression. A process would still need to pass PROT_MTE to be able to get tag checking. That's basically what I had in an early MTE implementation with clear_user_page() always zeroing the tags. I agree with you that a vma flag would be better but it's more complicated without an additional pte bit. We could also miss some updates as mprotect() for example checks for pte_same() before calling set_pte_at() (it would need to check the updated vma flags). I'll review the latest series but I'm tempted to move the logic in santise_mte_tags() to mte.c and take the big lock in there if PG_mte_tagged is not already set. If we hit performance issues, we can optimise this later to have the page flag set already on creation (new PROT flag, prctl etc.). -- Catalin