From nobody Mon Apr 6 19:57:23 2026 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 E6A8FC54EE9 for ; Fri, 2 Sep 2022 23:31:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229910AbiIBXbN (ORCPT ); Fri, 2 Sep 2022 19:31:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33832 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229517AbiIBXbJ (ORCPT ); Fri, 2 Sep 2022 19:31:09 -0400 Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 43C7DD34C1 for ; Fri, 2 Sep 2022 16:31:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1662161467; x=1693697467; h=from:to:cc:subject:date:message-id; bh=qLq0yeIaO/mvA3cm8hCykqt3PIzVE68HdER9lamMWV8=; b=XiTAure7RJkNoDskjPK37b/gyeh9Kxu4JrxoiiMzBbr8Ueu+R1BcZUzE mmxQ9/FKmw10UsqjO3swVXAipRlEgi54w93MpwdXhP1yvW1MOvJAxF4DJ pncE1C0bnFKwchyWwcXYou8/HtU81RrY8znyK/lgk8YDnw9cYV/KAVPnf +YVaCLUD/tPD1Z6wpCazpPmcFCdguQuqXsxzGEJVyisLwQhLQNgz5/nQv B0NgqTFbuSqd+Stos+zH7QK0nPxz6Z5brhLIjvK6AtQVBmdgBva2R4NDt lh0vh7qdocIgVD7epaNYiN2spdAqELabFCwry8mwj4ljaaXlWO+U5bNiw A==; X-IronPort-AV: E=McAfee;i="6500,9779,10458"; a="294848359" X-IronPort-AV: E=Sophos;i="5.93,285,1654585200"; d="scan'208";a="294848359" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Sep 2022 16:31:07 -0700 X-IronPort-AV: E=Sophos;i="5.93,285,1654585200"; d="scan'208";a="643103273" Received: from gescaler-mobl1.amr.corp.intel.com (HELO rpedgeco-desk.amr.corp.intel.com) ([10.209.116.199]) by orsmga008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Sep 2022 16:31:06 -0700 From: Rick Edgecombe To: pasha.tatashin@soleen.com, akpm@linux-foundation.org, linux-mm@kvack.org Cc: rick.p.edgecombe@intel.com, linux-kernel@vger.kernel.org Subject: [PATCH] mm: Check writable zero page in page table check Date: Fri, 2 Sep 2022 16:27:32 -0700 Message-Id: <20220902232732.12358-1-rick.p.edgecombe@intel.com> X-Mailer: git-send-email 2.17.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" The zero page should remain all zero, so that it can be mapped as read-only for read faults of memory that should be zeroed. If it is ever mapped writable to userspace, it could become non-zero and so other apps would unexpectedly get non-zero data. So the zero page should never be mapped writable to userspace. Check for this condition in page_table_check_set(). Signed-off-by: Rick Edgecombe --- Hi, CONFIG_PAGE_TABLE_CHECK is pretty explicit about what it checks (and doesn't mention the zero page), but this condition seems to fit with the general category of "pages mapped wrongly to userspace". I added it locally to help me debug something. Maybe it's more widely useful. mm/page_table_check.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/mm/page_table_check.c b/mm/page_table_check.c index e2062748791a..665ece0d55d4 100644 --- a/mm/page_table_check.c +++ b/mm/page_table_check.c @@ -102,6 +102,8 @@ static void page_table_check_set(struct mm_struct *mm, = unsigned long addr, if (!pfn_valid(pfn)) return; =20 + BUG_ON(is_zero_pfn(pfn) && rw); + page =3D pfn_to_page(pfn); page_ext =3D lookup_page_ext(page); anon =3D PageAnon(page); base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5 --=20 2.17.1