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 1CDF4C433B4 for ; Thu, 20 May 2021 15:05:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 00AED61246 for ; Thu, 20 May 2021 15:05:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242213AbhETPHS (ORCPT ); Thu, 20 May 2021 11:07:18 -0400 Received: from foss.arm.com ([217.140.110.172]:53438 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239490AbhETPHQ (ORCPT ); Thu, 20 May 2021 11:07:16 -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 B333D11B3; Thu, 20 May 2021 08:05:54 -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 F08993F73B; Thu, 20 May 2021 08:05:51 -0700 (PDT) Subject: Re: [PATCH v12 4/8] 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: <20210517123239.8025-1-steven.price@arm.com> <20210517123239.8025-5-steven.price@arm.com> <20210520115426.GB12251@arm.com> From: Steven Price Message-ID: <5f0996d6-0a6e-ebcd-afcd-8290faba6780@arm.com> Date: Thu, 20 May 2021 16:05:46 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210520115426.GB12251@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 20/05/2021 12:54, Catalin Marinas wrote: > On Mon, May 17, 2021 at 01:32:35PM +0100, Steven Price wrote: >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c >> index c5d1f3c87dbd..8660f6a03f51 100644 >> --- a/arch/arm64/kvm/mmu.c >> +++ b/arch/arm64/kvm/mmu.c >> @@ -822,6 +822,31 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot, >> return PAGE_SIZE; >> } >> >> +static int sanitise_mte_tags(struct kvm *kvm, unsigned long size, >> + kvm_pfn_t pfn) >> +{ >> + if (kvm_has_mte(kvm)) { >> + /* >> + * The page will be mapped in stage 2 as Normal Cacheable, so >> + * the VM will be able to see the page's tags and therefore >> + * they must be initialised first. If PG_mte_tagged is set, >> + * tags have already been initialised. >> + */ >> + unsigned long i, nr_pages = size >> PAGE_SHIFT; >> + struct page *page = pfn_to_online_page(pfn); >> + >> + if (!page) >> + return -EFAULT; > > IIRC we ended up with pfn_to_online_page() to reject ZONE_DEVICE pages > that may be mapped into a guest and we have no idea whether they support > MTE. It may be worth adding a comment, otherwise, as Marc said, the page > wouldn't disappear. I'll add a comment. >> + >> + 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)); > > We started the page->flags thread and ended up fixing it for the host > set_pte_at() as per the first patch: > > https://lore.kernel.org/r/c3293d47-a5f2-ea4a-6730-f5cae26d8a7e@arm.com > > Now, can we have a race between the stage 2 kvm_set_spte_gfn() and a > stage 1 set_pte_at()? Only the latter takes a lock. Or between two > kvm_set_spte_gfn() in different VMs? I think in the above thread we > concluded that there's only a problem if the page is shared between > multiple VMMs (MAP_SHARED). How realistic is this and what's the > workaround? > > Either way, I think it's worth adding a comment here on the race on > page->flags as it looks strange that here it's just a test_and_set_bit() > while set_pte_at() uses a spinlock. > Very good point! I should have thought about that. I think splitting the test_and_set_bit() in two (as with the cache flush) is sufficient. While there technically still is a race which could lead to user space tags being clobbered: a) It's very odd for a VMM to be doing an mprotect() after the fact to add PROT_MTE, or to be sharing the memory with another process which sets PROT_MTE. b) The window for the race is incredibly small and the VMM (generally) needs to be robust against the guest changing tags anyway. But I'll add a comment here as well: /* * There is a potential race between sanitising the * flags here and user space using mprotect() to add * PROT_MTE to access the tags, however by splitting * the test/set the only risk is user space tags * being overwritten by the mte_clear_page_tags() call. */ Thanks, Steve