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=-3.8 required=3.0 tests=BAYES_00, 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 C2367C433B4 for ; Tue, 4 May 2021 17:45:33 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 5765061166 for ; Tue, 4 May 2021 17:45:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5765061166 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:36604 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldz6y-0000zW-9U for qemu-devel@archiver.kernel.org; Tue, 04 May 2021 13:45:32 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:51532) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldz2A-0008Ao-D4 for qemu-devel@nongnu.org; Tue, 04 May 2021 13:40:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:53740) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldz26-0005Xi-Il for qemu-devel@nongnu.org; Tue, 04 May 2021 13:40:34 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 3B4F6613CB; Tue, 4 May 2021 17:40:21 +0000 (UTC) Date: Tue, 4 May 2021 18:40:18 +0100 From: Catalin Marinas To: Steven Price Subject: Re: [PATCH v11 2/6] arm64: kvm: Introduce MTE VM feature Message-ID: References: <20210416154309.22129-1-steven.price@arm.com> <20210416154309.22129-3-steven.price@arm.com> <20210428170705.GB4022@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=198.145.29.99; envelope-from=cmarinas@kernel.org; helo=mail.kernel.org X-Spam_score_int: -66 X-Spam_score: -6.7 X-Spam_bar: ------ X-Spam_report: (-6.7 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.248, RCVD_IN_DNSWL_HI=-5, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Peter Maydell , "Dr. David Alan Gilbert" , Andrew Jones , Haibo Xu , Suzuki K Poulose , qemu-devel@nongnu.org, Marc Zyngier , Juan Quintela , Richard Henderson , linux-kernel@vger.kernel.org, Dave Martin , James Morse , linux-arm-kernel@lists.infradead.org, Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu, Julien Thierry Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Apr 29, 2021 at 05:06:41PM +0100, Steven Price wrote: > On 28/04/2021 18:07, Catalin Marinas wrote: > > I probably asked already but is the only way to map a standard RAM page > > (not device) in stage 2 via the fault handler? One case I had in mind > > was something like get_user_pages() but it looks like that one doesn't > > call set_pte_at_notify(). There are a few other places where > > set_pte_at_notify() is called and these may happen before we got a > > chance to fault on stage 2, effectively populating the entry (IIUC). If > > that's an issue, we could move the above loop and check closer to the > > actual pte setting like kvm_pgtable_stage2_map(). > > The only call sites of kvm_pgtable_stage2_map() are in mmu.c: > > * kvm_phys_addr_ioremap() - maps as device in stage 2 > > * user_mem_abort() - handled above > > * kvm_set_spte_handler() - ultimately called from the .change_pte() > callback of the MMU notifier > > So the last one is potentially a problem. It's called via the MMU notifiers > in the case of set_pte_at_notify(). The users of that are: > > * uprobe_write_opcode(): Allocates a new page and performs a > copy_highpage() to copy the data to the new page (which with MTE includes > the tags and will copy across the PG_mte_tagged flag). > > * write_protect_page() (KSM): Changes the permissions on the PTE but it's > still the same page, so nothing to do regarding MTE. My concern here is that the VMM had a stage 1 pte but we haven't yet faulted in at stage 2 via user_mem_abort(), so we don't have any stage 2 pte set. write_protect_page() comes in and sets the new stage 2 pte via the callback. I couldn't find any check in kvm_pgtable_stage2_map() for the old pte, so it will set the new stage 2 pte regardless. A subsequent guest read would no longer fault at stage 2. > * replace_page() (KSM): If the page has MTE tags then the MTE version of > memcmp_pages() will return false, so the only caller > (try_to_merge_one_page()) will never call this on a page with tags. > > * wp_page_copy(): This one is more interesting - if we go down the > cow_user_page() path with an old page then everything is safe (tags are > copied over). The is_zero_pfn() case worries me a bit - a new page is > allocated, but I can't instantly see anything to zero out the tags (and set > PG_mte_tagged). True, I think tag zeroing happens only if we map it as PROT_MTE in the VMM. > * migrate_vma_insert_page(): I think migration should be safe as the tags > should be copied. > > So wp_page_copy() looks suspicious. > > kvm_pgtable_stage2_map() looks like it could be a good place for the checks, > it looks like it should work and is probably a more obvious place for the > checks. That would be my preference. It also matches the stage 1 set_pte_at(). > > While the set_pte_at() race on the page flags is somewhat clearer, we > > may still have a race here with the VMM's set_pte_at() if the page is > > mapped as tagged. KVM has its own mmu_lock but it wouldn't be held when > > handling the VMM page tables (well, not always, see below). > > > > gfn_to_pfn_prot() ends up calling get_user_pages*(). At least the slow > > path (hva_to_pfn_slow()) ends up with FOLL_TOUCH in gup and the VMM pte > > would be set, tags cleared (if PROT_MTE) before the stage 2 pte. I'm not > > sure whether get_user_page_fast_only() does the same. > > > > The race with an mprotect(PROT_MTE) in the VMM is fine I think as the > > KVM mmu notifier is invoked before set_pte_at() and racing with another > > user_mem_abort() is serialised by the KVM mmu_lock. The subsequent > > set_pte_at() would see the PG_mte_tagged set either by the current CPU > > or by the one it was racing with. > > Given the changes to set_pte_at() which means that tags are restored from > swap even if !PROT_MTE, the only race I can see remaining is the creation of > new PROT_MTE mappings. As you mention an attempt to change mappings in the > VMM memory space should involve a mmu notifier call which I think serialises > this. So the remaining issue is doing this in a separate address space. > > So I guess the potential problem is: > > * allocate memory MAP_SHARED but !PROT_MTE > * fork() > * VM causes a fault in parent address space > * child does a mprotect(PROT_MTE) > > With the last two potentially racing. Sadly I can't see a good way of > handling that. Ah, the mmap lock doesn't help as they are different processes (mprotect() acquires it as a writer). I wonder whether this is racy even in the absence of KVM. If both parent and child do an mprotect(PROT_MTE), one of them may be reading stale tags for a brief period. Maybe we should revisit whether shared MTE pages are of any use, though it's an ABI change (not bad if no-one is relying on this). However... Thinking about this, we have a similar problem with the PG_dcache_clean and two processes doing mprotect(PROT_EXEC). One of them could see the flag set and skip the I-cache maintenance while the other executes stale instructions. change_pte_range() could acquire the page lock if the page is VM_SHARED (my preferred core mm fix). It doesn't immediately solve the MTE/KVM case but we could at least take the page lock via user_mem_abort(). Or maybe we just document this behaviour as racy both for PROT_EXEC and PROT_MTE mappings and be done with this. The minor issue with PROT_MTE is the potential leaking of tags (it's harder to leak information through the I-cache). -- Catalin