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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 77B96C433ED for ; Thu, 29 Apr 2021 16:06:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3F28561463 for ; Thu, 29 Apr 2021 16:06:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240811AbhD2QHd (ORCPT ); Thu, 29 Apr 2021 12:07:33 -0400 Received: from foss.arm.com ([217.140.110.172]:54868 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233480AbhD2QHc (ORCPT ); Thu, 29 Apr 2021 12:07:32 -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 DB6151FB; Thu, 29 Apr 2021 09:06:45 -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 025393F70D; Thu, 29 Apr 2021 09:06:42 -0700 (PDT) Subject: Re: [PATCH v11 2/6] arm64: kvm: Introduce MTE VM feature To: Catalin Marinas 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 References: <20210416154309.22129-1-steven.price@arm.com> <20210416154309.22129-3-steven.price@arm.com> <20210428170705.GB4022@arm.com> From: Steven Price Message-ID: Date: Thu, 29 Apr 2021 17:06:41 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <20210428170705.GB4022@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28/04/2021 18:07, Catalin Marinas wrote: > On Fri, Apr 16, 2021 at 04:43:05PM +0100, Steven Price wrote: >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c >> index 77cb2d28f2a4..5f8e165ea053 100644 >> --- a/arch/arm64/kvm/mmu.c >> +++ b/arch/arm64/kvm/mmu.c >> @@ -879,6 +879,26 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, >> if (vma_pagesize == PAGE_SIZE && !force_pte) >> vma_pagesize = transparent_hugepage_adjust(memslot, hva, >> &pfn, &fault_ipa); >> + >> + if (fault_status != FSC_PERM && kvm_has_mte(kvm) && !device && >> + pfn_valid(pfn)) { > > In the current implementation, device == !pfn_valid(), so we could skip > the latter check. Thanks, I'll drop that check. >> + /* >> + * VM will be able to see the page's tags, so we must ensure >> + * they have been initialised. if PG_mte_tagged is set, tags >> + * have already been initialised. >> + */ >> + unsigned long i, nr_pages = vma_pagesize >> PAGE_SHIFT; >> + struct page *page = pfn_to_online_page(pfn); >> + >> + if (!page) >> + return -EFAULT; > > I think that's fine, though maybe adding a comment that otherwise it > would be mapped at stage 2 as Normal Cacheable and we cannot guarantee > that the memory supports MTE tags. That's what I intended by "be able to see the page's tags", but I'll reword to be explicit about it being Normal Cacheable. >> + >> + for (i = 0; i < nr_pages; i++, page++) { >> + if (!test_and_set_bit(PG_mte_tagged, &page->flags)) >> + mte_clear_page_tags(page_address(page)); >> + } >> + } >> + >> if (writable) >> prot |= KVM_PGTABLE_PROT_W; > > 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. * 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). * 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. > 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. Steve