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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, 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 4BA04C433B4 for ; Wed, 19 May 2021 09:32:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 24380611BF for ; Wed, 19 May 2021 09:32:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345509AbhESJd1 (ORCPT ); Wed, 19 May 2021 05:33:27 -0400 Received: from foss.arm.com ([217.140.110.172]:56086 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229668AbhESJdZ (ORCPT ); Wed, 19 May 2021 05:33:25 -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 C9B69101E; Wed, 19 May 2021 02:32:05 -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 402773F73D; Wed, 19 May 2021 02:32:03 -0700 (PDT) Subject: Re: [PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is untagged To: Marc Zyngier Cc: Catalin Marinas , 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> <20210517123239.8025-4-steven.price@arm.com> <87y2cdtk09.wl-maz@kernel.org> From: Steven Price Message-ID: Date: Wed, 19 May 2021 10:32:01 +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: <87y2cdtk09.wl-maz@kernel.org> 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 17/05/2021 17:14, Marc Zyngier wrote: > On Mon, 17 May 2021 13:32:34 +0100, > Steven Price wrote: >> >> A KVM guest could store tags in a page even if the VMM hasn't mapped >> the page with PROT_MTE. So when restoring pages from swap we will >> need to check to see if there are any saved tags even if !pte_tagged(). >> >> However don't check pages for which pte_access_permitted() returns false >> as these will not have been swapped out. >> >> Signed-off-by: Steven Price >> --- >> arch/arm64/include/asm/pgtable.h | 9 +++++++-- >> arch/arm64/kernel/mte.c | 16 ++++++++++++++-- >> 2 files changed, 21 insertions(+), 4 deletions(-) >> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h >> index 0b10204e72fc..275178a810c1 100644 >> --- a/arch/arm64/include/asm/pgtable.h >> +++ b/arch/arm64/include/asm/pgtable.h >> @@ -314,8 +314,13 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr, >> if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte)) >> __sync_icache_dcache(pte); >> >> - if (system_supports_mte() && >> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte)) >> + /* >> + * If the PTE would provide user space access to the tags associated >> + * with it then ensure that the MTE tags are synchronised. Exec-only >> + * mappings don't expose tags (instruction fetches don't check tags). > > I'm not sure I understand this comment. Of course, execution doesn't > match tags. But the memory could still have tags associated with > it. Does this mean such a page would lose its tags is swapped out? Hmm, I probably should have reread that - the context of the comment is lost. I added the comment when changing to pte_access_permitted(), and the comment on pte_access_permitted() explains a potential gotcha: * p??_access_permitted() is true for valid user mappings (PTE_USER * bit set, subject to the write permission check). For execute-only * mappings, like PROT_EXEC with EPAN (both PTE_USER and PTE_UXN bits * not set) must return false. PROT_NONE mappings do not have the * PTE_VALID bit set. So execute-only mappings return false even though that is effectively a type of user access. However, because MTE checks are not performed by the PE for instruction fetches this doesn't matter. I'll update the comment, how about: /* * If the PTE would provide user space access to the tags associated * with it then ensure that the MTE tags are synchronised. Although * pte_access_permitted() returns false for exec only mappings, they * don't expose tags (instruction fetches don't check tags). */ Thanks, Steve > Thanks, > > M. > >> + */ >> + if (system_supports_mte() && pte_present(pte) && >> + pte_access_permitted(pte, false) && !pte_special(pte)) >> mte_sync_tags(ptep, pte); >> >> __check_racy_pte_update(mm, ptep, pte); >> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c >> index c88e778c2fa9..a604818c52c1 100644 >> --- a/arch/arm64/kernel/mte.c >> +++ b/arch/arm64/kernel/mte.c >> @@ -33,11 +33,15 @@ DEFINE_STATIC_KEY_FALSE(mte_async_mode); >> EXPORT_SYMBOL_GPL(mte_async_mode); >> #endif >> >> -static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap) >> +static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap, >> + bool pte_is_tagged) >> { >> unsigned long flags; >> pte_t old_pte = READ_ONCE(*ptep); >> >> + if (!is_swap_pte(old_pte) && !pte_is_tagged) >> + return; >> + >> spin_lock_irqsave(&tag_sync_lock, flags); >> >> /* Recheck with the lock held */ >> @@ -53,6 +57,9 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap) >> } >> } >> >> + if (!pte_is_tagged) >> + goto out; >> + >> page_kasan_tag_reset(page); >> /* >> * We need smp_wmb() in between setting the flags and clearing the >> @@ -76,10 +83,15 @@ void mte_sync_tags(pte_t *ptep, pte_t pte) >> bool check_swap = nr_pages == 1; >> bool pte_is_tagged = pte_tagged(pte); >> >> + /* Early out if there's nothing to do */ >> + if (!check_swap && !pte_is_tagged) >> + return; >> + >> /* if PG_mte_tagged is set, tags have already been initialised */ >> for (i = 0; i < nr_pages; i++, page++) { >> if (!test_bit(PG_mte_tagged, &page->flags)) >> - mte_sync_page_tags(page, ptep, check_swap); >> + mte_sync_page_tags(page, ptep, check_swap, >> + pte_is_tagged); >> } >> } >> >> -- >> 2.20.1 >> >> >