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,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 79289C433B4 for ; Fri, 7 May 2021 18:31:55 +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 0E07B61400 for ; Fri, 7 May 2021 18:31:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0E07B61400 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]:45934 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lf5GT-00019o-JS for qemu-devel@archiver.kernel.org; Fri, 07 May 2021 14:31:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:33686) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lf5Af-0003Jz-Pr for qemu-devel@nongnu.org; Fri, 07 May 2021 14:25:55 -0400 Received: from mail.kernel.org ([198.145.29.99]:48988) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lf5Aa-0000n7-Ia for qemu-devel@nongnu.org; Fri, 07 May 2021 14:25:53 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 2E468600D1; Fri, 7 May 2021 18:25:42 +0000 (UTC) Date: Fri, 7 May 2021 19:25:39 +0100 From: Catalin Marinas To: Steven Price Subject: Re: [PATCH v11 2/6] arm64: kvm: Introduce MTE VM feature Message-ID: <20210507182538.GF26528@arm.com> References: <20210416154309.22129-1-steven.price@arm.com> <20210416154309.22129-3-steven.price@arm.com> <20210428170705.GB4022@arm.com> <329286e8-a8f3-ea1a-1802-58813255a4a5@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <329286e8-a8f3-ea1a-1802-58813255a4a5@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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=0.249, 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, May 06, 2021 at 05:15:25PM +0100, Steven Price wrote: > On 04/05/2021 18:40, Catalin Marinas wrote: > > On Thu, Apr 29, 2021 at 05:06:41PM +0100, Steven Price wrote: > > > On 28/04/2021 18:07, Catalin Marinas wrote: > > > > 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... > > Shared MTE pages are certainly hard to use correctly (e.g. see the > discussions with the VMM accessing guest memory). But I guess that boat has > sailed. Digging out some old emails (two years ago), the Chrome people may have found a use for MTE in shared mappings (with memfd_create), though not sure they took advantage of this yet. > > 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(). > > For PG_dcache_clean AFAICS the solution is actually simple: split the test > and set parts. i.e..: > > if (!test_bit(PG_dcache_clean, &page->flags)) { > sync_icache_aliases(page_address(page), page_size(page)); > set_bit(PG_dcache_clean, &page->flags); > } > > There isn't a problem with repeating the sync_icache_aliases() call in the > case of a race. Or am I missing something? No, the fix is simple as you said. > > 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). > > This is the real issue I see - the race in PROT_MTE case is either a data > leak (syncing after setting the bit) or data loss (syncing before setting > the bit). For a moment I thought an mmap(PROT_MTE, MAP_SHARED) on memfd/tmpfs file may lead to the same situation but the mmap() itself won't directly cause allocating the page. The subsequent fault via filemap_map_pages() seems to take the page lock. > But without serialising through a spinlock (in mte_sync_tags()) I haven't > been able to come up with any way of closing the race. But with the change > to set_pte_at() to call mte_sync_tags() even if the PTE isn't PROT_MTE that > is likely to seriously hurt performance. Yeah. We could add another page flag as a lock though I think it should be the core code that prevents the race. If we are to do it in the arch code, maybe easier with a custom ptep_modify_prot_start/end() where we check if it's VM_SHARED and VM_MTE, take a (big) lock. In the core code, something like below (well, a partial hack, not tested and it doesn't handle huge pages but just to give an idea): diff --git a/mm/mprotect.c b/mm/mprotect.c index 94188df1ee55..6ba96ff141a6 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -76,14 +76,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, if (pte_present(oldpte)) { pte_t ptent; bool preserve_write = prot_numa && pte_write(oldpte); + struct page *page = NULL; /* * Avoid trapping faults against the zero or KSM * pages. See similar comment in change_huge_pmd. */ if (prot_numa) { - struct page *page; - /* Avoid TLB flush if possible */ if (pte_protnone(oldpte)) continue; @@ -114,6 +113,10 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, } oldpte = ptep_modify_prot_start(vma, addr, pte); + if (vma->vm_flags & VM_SHARED) { + page = vm_normal_page(vma, addr, oldpte); + lock_page(page); + } ptent = pte_modify(oldpte, newprot); if (preserve_write) ptent = pte_mk_savedwrite(ptent); @@ -138,6 +141,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, ptent = pte_mkwrite(ptent); } ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent); + if (page) + unlock_page(page); pages++; } else if (is_swap_pte(oldpte)) { swp_entry_t entry = pte_to_swp_entry(oldpte); -- Catalin