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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,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 969E4C4CECE for ; Mon, 14 Oct 2019 16:33:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6FB37217D9 for ; Mon, 14 Oct 2019 16:33:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1571070792; bh=FcUTHcayL7M/TMVSM7QsI4LCJQgfA+E+GMaO89J0vJA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=ip6e6JMon++d8rK7zta/lIPaD/7EaEx2V0ObotewFDsgcCjlJrbJUyGJ0DvMF5rCJ LiVdn2PHV7JHxbj8VVuIg8ZcGHswYb2kMyqkgZ5f4EUJ97+trtWPWDvE0gNRYHQoyI YZuCawNGbLDMDG+UJy3T9ruIWM4W5NWPN8L5FPVk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388116AbfJNQdL (ORCPT ); Mon, 14 Oct 2019 12:33:11 -0400 Received: from mail-qk1-f195.google.com ([209.85.222.195]:37287 "EHLO mail-qk1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726637AbfJNQdL (ORCPT ); Mon, 14 Oct 2019 12:33:11 -0400 Received: by mail-qk1-f195.google.com with SMTP id u184so16453603qkd.4; Mon, 14 Oct 2019 09:33:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=4e/+dg0NVCCBrtHyCNhp9Ya4wF37EwRkEK+NeEl5+VU=; b=A31njF9Kt8jL9DxYdqt+AFERZ4GXgyF70roSgACytG3oPAM4SAiEfGJA0thhHyQsRE LDJwdWYGEuIpGGkNJedBJR9j2bImNS5IFxRfVCKSyf0ecjmUBeGfXkQhqHu0Q0/J9Z3S 6WvQvuglHPm37Ceukii41k6PvelrK0oe30D2phoqsH1fIzkYqooz65ZizMGS2Ae21r4u NQwH94TUGi4i8G9syVddwuPF4XeoBEdQbqtIcjw26FBbZcP0aibSAUHvEZE7EsZQDlcN rwgH1ijPQlLfswIvNQnaBq6wZuH+iOrUri5/E4uHH/RxDQQI8TCzgHFtRKVT9V2KgZL7 MY6A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to:user-agent; bh=4e/+dg0NVCCBrtHyCNhp9Ya4wF37EwRkEK+NeEl5+VU=; b=ZkbS568dRcdXYnh0M97wyMVgUFqEekU46ma0zvcwvIdfHOrYVIRGGBalrs5lLzY25o Jav3zSNQqxcuB010qOt/D2de3ze7cEHEeWqPBab5yfV57DFwidSjSGvg4Gs12dtJV0FI 31JEdcbb30HGCvGyKbcEkswZc96byhSlsmuYIfZXX8T19Qk71vLZKWcl/MYQXr3Po3Ea pJAiugIL/UPAgCzNXyLgBD2uszrQBSaVleBBEN17OI685omQP4NeLTpHq/OnSNP7eWNE LCoTb6+oimCB0yXtoPR7+ZSWlRt9XWnxqmwF2rzqn3f6kzzVM7glMR9EdNGYmAI+uueT SGoQ== X-Gm-Message-State: APjAAAWYpPBXVWivtuLm8iNO1rWer7dyVih3RddpS7271pt3lt9DWzi8 +Ilm2V+7LltSgX2PSeto5QY= X-Google-Smtp-Source: APXvYqz1ACcaWifQWUyV2g9FPNEuBZNZlzJ6t7epHU0KuMJ9cSqy7/zM89IFXIFaY8NMX42nGDVQtw== X-Received: by 2002:a37:484b:: with SMTP id v72mr31746062qka.206.1571070789784; Mon, 14 Oct 2019 09:33:09 -0700 (PDT) Received: from localhost ([2620:10d:c091:500::1:50c5]) by smtp.gmail.com with ESMTPSA id o124sm8220344qke.66.2019.10.14.09.33.08 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 14 Oct 2019 09:33:09 -0700 (PDT) Date: Mon, 14 Oct 2019 09:33:07 -0700 From: Tejun Heo To: Aleksa Sarai 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: <20191014163307.GG18794@devbig004.ftw2.facebook.com> References: <20191012010539.6131-1-cyphar@cyphar.com> <20191014154136.GF18794@devbig004.ftw2.facebook.com> <20191014155931.jl7idjebhqxb3ck3@yavin.dot.cyphar.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191014155931.jl7idjebhqxb3ck3@yavin.dot.cyphar.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Aleksa. On Tue, Oct 15, 2019 at 02:59:31AM +1100, Aleksa Sarai wrote: > 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. > > > > 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. READ/WRITE_ONCE provides protection against compiler generating multiple accesses for a single operation. It won't prevent split writes / reads of 64bit variables on 32bit machines. For that, you'd have to switch them to atomic64_t's. Thanks. -- tejun