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=-7.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 50CAFC4338F for ; Wed, 25 Aug 2021 14:59:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 34A4F610CF for ; Wed, 25 Aug 2021 14:59:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240644AbhHYPAj (ORCPT ); Wed, 25 Aug 2021 11:00:39 -0400 Received: from mga12.intel.com ([192.55.52.136]:47909 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231975AbhHYPAi (ORCPT ); Wed, 25 Aug 2021 11:00:38 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10086"; a="197109061" X-IronPort-AV: E=Sophos;i="5.84,350,1620716400"; d="scan'208";a="197109061" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Aug 2021 07:59:52 -0700 X-IronPort-AV: E=Sophos;i="5.84,350,1620716400"; d="scan'208";a="444213104" Received: from cschaef-mobl1.amr.corp.intel.com (HELO [10.212.141.45]) ([10.212.141.45]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Aug 2021 07:59:51 -0700 Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables To: Andy Lutomirski , Mike Rapoport , linux-mm@kvack.org Cc: Andrew Morton , Dave Hansen , Ira Weiny , Kees Cook , Mike Rapoport , "Peter Zijlstra (Intel)" , Rick P Edgecombe , Vlastimil Babka , the arch/x86 maintainers , Linux Kernel Mailing List References: <20210823132513.15836-1-rppt@kernel.org> <20210823132513.15836-5-rppt@kernel.org> <1cccc2b6-8b5b-4aee-483d-f10e64a248a5@intel.com> <8b50991d-2ab5-4577-83e9-a2d74135c5f5@www.fastmail.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <8b50991d-2ab5-4577-83e9-a2d74135c5f5@www.fastmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/23/21 8:34 PM, Andy Lutomirski wrote: >> I would expected this to have leveraged the pte_offset_map/unmap() code >> to enable/disable write access. Granted, it would enable write access >> even when only a read is needed, but that could be trivially fixed with >> having a variant like: >> >> pte_offset_map_write() >> pte_offset_unmap_write() > I would also like to see a discussion of how races in which multiple > threads or CPUs access ptes in the same page at the same time. Yeah, the 32-bit highmem code has a per-cpu mapping area for kmap_atomic() that lies underneath the pte_offset_map(). Although it's in the shared kernel mapping, only one CPU uses it. I didn't look at it until you mentioned it, but the code in this set is just plain buggy if two CPUs do a enable_pgtable_write()/disable_pgtable_write(). They'll clobber each other: > +static void pgtable_write_set(void *pg_table, bool set) > +{ > + int level = 0; > + pte_t *pte; > + > + /* > + * Skip the page tables allocated from pgt_buf break area and from > + * memblock > + */ > + if (!after_bootmem) > + return; > + if (!PageTable(virt_to_page(pg_table))) > + return; > + > + pte = lookup_address((unsigned long)pg_table, &level); > + if (!pte || level != PG_LEVEL_4K) > + return; > + > + if (set) { > + if (pte_write(*pte)) > + return; > + > + WRITE_ONCE(*pte, pte_mkwrite(*pte)); > + } else { > + if (!pte_write(*pte)) > + return; > + > + WRITE_ONCE(*pte, pte_wrprotect(*pte)); > + } > +}