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_MUTT 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 D2454C04AB4 for ; Fri, 17 May 2019 17:11:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A2ABE20848 for ; Fri, 17 May 2019 17:11:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1558113078; bh=7UZuJXocK6xbm1x1t2BXecJQo5/K2Y27siX9jHLU0lw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=rQfH7BTZW2HQNc2beniUIvYKaK7wdW4+hqmoPvsE71oAtb9fBARquuBg/U3Q4x79I vrAi7OwFp4wQYOJAWLOh1jNYEOlilWpWEk2ovYwNSg9XcRYNU9s3ChWQ/n+vINLuXa vqLSdOHF5sRjEa27Tr9sHjNhZjvKc72hiZM4E93k= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728050AbfEQRLS (ORCPT ); Fri, 17 May 2019 13:11:18 -0400 Received: from mx2.suse.de ([195.135.220.15]:51658 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725933AbfEQRLS (ORCPT ); Fri, 17 May 2019 13:11:18 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 29A57AEB0; Fri, 17 May 2019 17:11:16 +0000 (UTC) Date: Fri, 17 May 2019 19:11:15 +0200 From: Michal Hocko To: Kees Cook Cc: Alexander Potapenko , Andrew Morton , Christoph Lameter , Kernel Hardening , Masahiro Yamada , James Morris , "Serge E. Hallyn" , Nick Desaulniers , Kostya Serebryany , Dmitry Vyukov , Sandeep Patil , Laura Abbott , Randy Dunlap , Jann Horn , Mark Rutland , Linux Memory Management List , linux-security-module Subject: Re: [PATCH v2 1/4] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options Message-ID: <20190517170805.GS6836@dhcp22.suse.cz> References: <20190514143537.10435-1-glider@google.com> <20190514143537.10435-2-glider@google.com> <20190517140446.GA8846@dhcp22.suse.cz> <20190517142048.GM6836@dhcp22.suse.cz> <201905170928.A8F3BEC1B1@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201905170928.A8F3BEC1B1@keescook> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: On Fri 17-05-19 09:36:36, Kees Cook wrote: > On Fri, May 17, 2019 at 04:20:48PM +0200, Michal Hocko wrote: > > On Fri 17-05-19 16:11:32, Alexander Potapenko wrote: > > > On Fri, May 17, 2019 at 4:04 PM Michal Hocko wrote: > > > > > > > > On Tue 14-05-19 16:35:34, Alexander Potapenko wrote: > > > > > The new options are needed to prevent possible information leaks and > > > > > make control-flow bugs that depend on uninitialized values more > > > > > deterministic. > > > > > > > > > > init_on_alloc=1 makes the kernel initialize newly allocated pages and heap > > > > > objects with zeroes. Initialization is done at allocation time at the > > > > > places where checks for __GFP_ZERO are performed. > > > > > > > > > > init_on_free=1 makes the kernel initialize freed pages and heap objects > > > > > with zeroes upon their deletion. This helps to ensure sensitive data > > > > > doesn't leak via use-after-free accesses. > > > > > > > > Why do we need both? The later is more robust because even free memory > > > > cannot be sniffed and the overhead might be shifted from the allocation > > > > context (e.g. to RCU) but why cannot we stick to a single model? > > > init_on_free appears to be slower because of cache effects. It's > > > several % in the best case vs. <1% for init_on_alloc. > > > > This doesn't really explain why we need both. > > There are a couple reasons. The first is that once we have hardware with > memory tagging (e.g. arm64's MTE) we'll need both on_alloc and on_free > hooks to do change the tags. With MTE, zeroing comes for "free" with > tagging (though tagging is as slow as zeroing, so it's really the tagging > that is free...), so we'll need to re-use the init_on_free infrastructure. I am not sure I follow, but ... > > The second reason is for very paranoid use-cases where in-memory > data lifetime is desired to be minimized. There are various arguments > for/against the realism of the associated threat models, but given that > we'll need the infrastructre for MTE anyway, and there are people who > want wipe-on-free behavior no matter what the performance cost, it seems > reasonable to include it in this series. > > All that said, init_on_alloc looks desirable enough that distros will > likely build with it enabled by default (I hope), and the very paranoid > users will switch to (or additionally enable) init_on_free for their > systems. ... this should all be part of the changelog. -- Michal Hocko SUSE Labs