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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4DFA8C4332F for ; Tue, 22 Nov 2022 08:13:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232696AbiKVIN0 (ORCPT ); Tue, 22 Nov 2022 03:13:26 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49710 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232371AbiKVINY (ORCPT ); Tue, 22 Nov 2022 03:13:24 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id CBACE1B78C for ; Tue, 22 Nov 2022 00:13:22 -0800 (PST) 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 AC6F01FB; Tue, 22 Nov 2022 00:13:28 -0800 (PST) Received: from [10.162.40.16] (unknown [10.162.40.16]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 636BE3F73D; Tue, 22 Nov 2022 00:13:20 -0800 (PST) Message-ID: <879e561c-e834-196c-b9c5-6e44ac2c0296@arm.com> Date: Tue, 22 Nov 2022 13:43:17 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.2 Subject: Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at() Content-Language: en-US To: Will Deacon Cc: linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, Mark Rutland , Andrew Morton , linux-kernel@vger.kernel.org References: <20221116031001.292236-1-anshuman.khandual@arm.com> <20221118141317.GF4046@willie-the-truck> From: Anshuman Khandual In-Reply-To: <20221118141317.GF4046@willie-the-truck> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/18/22 19:43, Will Deacon wrote: > On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote: >> Changing pfn on a user page table mapped entry, without first going through >> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at() >> to intercept such changes, via an updated pgattr_change_is_safe(). This new >> check happens via __check_racy_pte_update(), which has now been renamed as >> __check_safe_pte_update(). >> >> Cc: Catalin Marinas >> Cc: Will Deacon >> Cc: Mark Rutland >> Cc: Andrew Morton >> Cc: linux-arm-kernel@lists.infradead.org >> Cc: linux-kernel@vger.kernel.org >> Signed-off-by: Anshuman Khandual >> --- >> This applies on v6.1-rc4 >> >> arch/arm64/include/asm/pgtable.h | 8 ++++++-- >> arch/arm64/mm/mmu.c | 8 +++++++- >> 2 files changed, 13 insertions(+), 3 deletions(-) > > I remember Mark saying that BBM is sometimes violated by the core code in > cases where the pte isn't actually part of a live pgtable (e.g. if it's on > the stack or part of a newly allocated table). Won't that cause false > positives here? Could you please elaborate ? If the pte is not on a live page table, then pte_valid() will return negative on such entries. So any update there will be safe. I am wondering, how this change will cause false positives which would not have been possible earlier.