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 F1387C43461 for ; Tue, 27 Apr 2021 17:50:29 +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 71965613F3 for ; Tue, 27 Apr 2021 17:50:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 71965613F3 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]:51630 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lbRqu-0003Sp-Ee for qemu-devel@archiver.kernel.org; Tue, 27 Apr 2021 13:50:28 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:45762) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lbRks-0008Ex-G6 for qemu-devel@nongnu.org; Tue, 27 Apr 2021 13:44:16 -0400 Received: from mail.kernel.org ([198.145.29.99]:38498) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lbRkk-0007vg-GU for qemu-devel@nongnu.org; Tue, 27 Apr 2021 13:44:13 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 81EE6613E5; Tue, 27 Apr 2021 17:44:00 +0000 (UTC) Date: Tue, 27 Apr 2021 18:43:58 +0100 From: Catalin Marinas To: Steven Price Subject: Re: [PATCH v11 1/6] arm64: mte: Sync tags for pages where PTE is untagged Message-ID: <20210427174357.GA17872@arm.com> References: <20210416154309.22129-1-steven.price@arm.com> <20210416154309.22129-2-steven.price@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210416154309.22129-2-steven.price@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 Fri, Apr 16, 2021 at 04:43:04PM +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 which are !pte_valid_user() as these will > not have been swapped out. You should remove the pte_valid_user() mention from the commit log as well. > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index e17b96d0e4b5..cf4b52a33b3c 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr, > __sync_icache_dcache(pte); > > if (system_supports_mte() && > - pte_present(pte) && pte_tagged(pte) && !pte_special(pte)) > + pte_present(pte) && (pte_val(pte) & PTE_USER) && !pte_special(pte)) I would add a pte_user() macro here or, if we restore the tags only when the page is readable, use pte_access_permitted(pte, false). Also add a comment why we do this. There's also the pte_user_exec() case which may not have the PTE_USER set (exec-only permission) but I don't think it matters. We don't do tag checking on instruction fetches, so if the user adds a PROT_READ to it, it would go through set_pte_at() again. I'm not sure KVM does anything special with exec-only mappings at stage 2, I suspect they won't be accessible by the guest (but needs checking). > 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 b3c70a612c7a..e016ab57ea36 100644 > --- a/arch/arm64/kernel/mte.c > +++ b/arch/arm64/kernel/mte.c > @@ -26,17 +26,23 @@ u64 gcr_kernel_excl __ro_after_init; > > static bool report_fault_once = true; > > -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) > { > pte_t old_pte = READ_ONCE(*ptep); > > if (check_swap && is_swap_pte(old_pte)) { > swp_entry_t entry = pte_to_swp_entry(old_pte); > > - if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) > + if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) { > + set_bit(PG_mte_tagged, &page->flags); > return; > + } > } > > + if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags)) > + return; I don't think we need another test_bit() here, it was done in the caller (bar potential races which need more thought). > + > page_kasan_tag_reset(page); > /* > * We need smp_wmb() in between setting the flags and clearing the > @@ -54,11 +60,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte) > struct page *page = pte_page(pte); > long i, nr_pages = compound_nr(page); > bool check_swap = nr_pages == 1; > + bool pte_is_tagged = pte_tagged(pte); > > /* if PG_mte_tagged is set, tags have already been initialised */ > for (i = 0; i < nr_pages; i++, page++) { > - if (!test_and_set_bit(PG_mte_tagged, &page->flags)) > - mte_sync_page_tags(page, ptep, check_swap); > + if (!test_bit(PG_mte_tagged, &page->flags)) > + mte_sync_page_tags(page, ptep, check_swap, > + pte_is_tagged); > } > } You were right in the previous thread that if we have a race, it's already there even without your patches KVM patches. If it's the same pte in a multithreaded app, we should be ok as the core code holds the ptl (the arch code also holds the mmap_lock during exception handling but only as a reader, so you can have multiple holders). If there are multiple ptes to the same page, for example mapped with MAP_ANONYMOUS | MAP_SHARED, metadata recovery is done via arch_swap_restore() before we even set the pte and with the page locked. So calling lock_page() again in mte_restore_tags() would deadlock. I can see that do_swap_page() also holds the page lock around set_pte_at(), so I think we are covered. Any other scenario I may have missed? My understanding is that if the pte is the same, we have the ptl. Otherwise we have the page lock for shared pages. -- Catalin