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=-5.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS 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 2C899C4CECE for ; Mon, 14 Oct 2019 15:59:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F38CD2133F for ; Mon, 14 Oct 2019 15:59:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732492AbfJNP7u (ORCPT ); Mon, 14 Oct 2019 11:59:50 -0400 Received: from mx2a.mailbox.org ([80.241.60.219]:58387 "EHLO mx2a.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727038AbfJNP7t (ORCPT ); Mon, 14 Oct 2019 11:59:49 -0400 Received: from smtp1.mailbox.org (smtp1.mailbox.org [80.241.60.240]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mx2a.mailbox.org (Postfix) with ESMTPS id 42B24A12E1; Mon, 14 Oct 2019 17:59:45 +0200 (CEST) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp1.mailbox.org ([80.241.60.240]) by hefe.heinlein-support.de (hefe.heinlein-support.de [91.198.250.172]) (amavisd-new, port 10030) with ESMTP id rzhkEBTLWgtf; Mon, 14 Oct 2019 17:59:40 +0200 (CEST) Date: Tue, 15 Oct 2019 02:59:31 +1100 From: Aleksa Sarai To: Tejun Heo Cc: Li Zefan , Johannes Weiner , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit operations Message-ID: <20191014155931.jl7idjebhqxb3ck3@yavin.dot.cyphar.com> References: <20191012010539.6131-1-cyphar@cyphar.com> <20191014154136.GF18794@devbig004.ftw2.facebook.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="x7whfzqvnijraall" Content-Disposition: inline In-Reply-To: <20191014154136.GF18794@devbig004.ftw2.facebook.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --x7whfzqvnijraall Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-10-14, Tejun Heo wrote: > On Sat, Oct 12, 2019 at 12:05:39PM +1100, Aleksa Sarai wrote: > > Because pids->limit can be changed concurrently (but we don't want to > > take a lock because it would be needlessly expensive), use the > > appropriate memory barriers. >=20 > I can't quite tell what problem it's fixing. Can you elaborate a > scenario where the current code would break that your patch fixes? As far as I can tell, not using *_ONCE() here means that if you had a process changing pids->limit from A to B, a process might be able to temporarily exceed pids->limit -- because pids->limit accesses are not protected by mutexes and the C compiler can produce confusing intermediate values for pids->limit[1]. But this is more of a correctness fix than one fixing an actually exploitable bug -- given the kernel memory model work, it seems like a good idea to just use READ_ONCE() and WRITE_ONCE() for shared memory access. [1]: https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --x7whfzqvnijraall Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXaSbYAAKCRCdlLljIbnQ ErlGAPwNNvA0NEl3lea4jocc7jw2pPmGBL5rg/Ogyourf6PDpgD+IJJGCeNm9CFT vQo8yItkhrGR/QEne2SQK5v1XqPseg8= =ZrpX -----END PGP SIGNATURE----- --x7whfzqvnijraall--