From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757259AbbLWV0X (ORCPT ); Wed, 23 Dec 2015 16:26:23 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55689 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756310AbbLWV0S (ORCPT ); Wed, 23 Dec 2015 16:26:18 -0500 Subject: Re: [PATCH v2] ARM: mm: flip priority of CONFIG_DEBUG_RODATA To: Russell King - ARM Linux , Kees Cook References: <20151202202725.GA794@www.outflux.net> <20151223201529.GX8644@n2100.arm.linux.org.uk> Cc: Laura Abbott , Catalin Marinas , linux-arm-kernel@lists.infradead.org, LKML , Linux-MM , Ard Biesheuvel , Will Deacon , Nicolas Pitre , Arnd Bergmann , kernel-hardening@lists.openwall.com From: Laura Abbott Message-ID: <567B1176.4000106@redhat.com> Date: Wed, 23 Dec 2015 13:26:14 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 MIME-Version: 1.0 In-Reply-To: <20151223201529.GX8644@n2100.arm.linux.org.uk> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/23/2015 12:15 PM, Russell King - ARM Linux wrote: > On Wed, Dec 02, 2015 at 12:27:25PM -0800, Kees Cook wrote: >> The use of CONFIG_DEBUG_RODATA is generally seen as an essential part of >> kernel self-protection: >> http://www.openwall.com/lists/kernel-hardening/2015/11/30/13 >> Additionally, its name has grown to mean things beyond just rodata. To >> get ARM closer to this, we ought to rearrange the names of the configs >> that control how the kernel protects its memory. What was called >> CONFIG_ARM_KERNMEM_PERMS is really doing the work that other architectures >> call CONFIG_DEBUG_RODATA. > > Kees, > > There is a subtle problem with the kernel memory permissions and the > DMA debugging. > > DMA debugging checks whether we're trying to do DMA from the kernel > mappings (text, rodata, data etc). It checks _text.._etext. However, > when RODATA is enabled, we have about one section between _text and > _stext which are freed into the kernel's page pool, and then become > available for allocation and use for DMA. > > This then causes the DMA debugging sanity check to fire. > > So, I think I'll revert this change for the time being as it seems to > be causing many people problems, and having this enabled is creating > extra warnings when kernel debug options are enabled along with it. > > Sorry. > in include/asm-generic/sections.h: /* * Usage guidelines: * _text, _data: architecture specific, don't use them in arch-independent code * [_stext, _etext]: contains .text.* sections, may also contain .rodata.* * and/or .init.* sections So based on that comment it seems like the dma-debug should be checking for _stext not _text since only _stext is guaranteed across all architectures. I'll submit a patch to dma-debug.c if this seems appropriate or if you haven't done so already. Thanks, Laura