From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752038AbdF3Olh (ORCPT ); Fri, 30 Jun 2017 10:41:37 -0400 Received: from mail-it0-f44.google.com ([209.85.214.44]:37426 "EHLO mail-it0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751675AbdF3Olg (ORCPT ); Fri, 30 Jun 2017 10:41:36 -0400 MIME-Version: 1.0 In-Reply-To: References: <1495829844-69341-1-git-send-email-keescook@chromium.org> <1495829844-69341-5-git-send-email-keescook@chromium.org> From: Kees Cook Date: Fri, 30 Jun 2017 07:41:34 -0700 X-Google-Sender-Auth: pseZ4SMbxcUsUv5sZlOoPCfDeQ8 Message-ID: Subject: Re: [PATCH v2 04/20] gcc-plugins: Add the randstruct plugin To: Arnd Bergmann Cc: Ard Biesheuvel , Kernel Hardening , Laura Abbott , "the arch/x86 maintainers" , Linux Kernel Mailing List , Russell King - ARM Linux , Nicolas Pitre , Will Deacon Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 30, 2017 at 1:27 AM, Arnd Bergmann wrote: > On Fri, Jun 30, 2017 at 9:55 AM, Ard Biesheuvel > wrote: >> On 30 June 2017 at 07:35, Arnd Bergmann wrote: >>> On Fri, Jun 30, 2017 at 12:53 AM, Kees Cook wrote: >>>> The first obviously won't fly. The second just bypasses the problem >>>> forcing it to be exposed by other people later. The third is likely >>>> easiest to do now, but reduces the effectiveness of randomization for >>>> architectures that don't have sensitive immediate values. The fourth >>>> sounds not generally useful. The fifth may be unacceptable to arm >>>> maintainers due to performance impacts. >>> >>> I was thinking of the fifth solution, but don't know exactly how to >>> do it. If performance is a concern, I guess we could have separate >>> implementations for randstruct and traditional builds. >>> >> >> Does this not apply to *all* entries in asm-offsets? If so, I don't >> see how it is tractable to fix this in the code, unless we add some >> instrumentation to asm-offsets to whitelist some huge structs and >> error out on new ones. Or perhaps there's really only a handful? > > I think the other structs are all small enough: > > * thread_info is at most 720 bytes (including crunch+vfp3, which > you wouldn't find in one combined kernel) and not randomized > at the moment > * pt_regs is 72 bytes and I don't see how that would be randomized > * machine_desc would be a candidate for randomizing, but is only > 108 bytes > * proc_info_list is 52 bytes and not currently randomized > * vm_area_struct is randomized but only 96 bytes. > * task_struct is clearly large enough, but we only use TSK_ACTIVE_MM > and TSK_STACK_CANARY, both can be fixed with your trick. Yup, that matches what I found. task_struct is the only truly giant struct. >> In any case, these particular examples are fairly straightforward, >> since there is no need to preserve the register's value. >> >> ldr r7, [r7, #TSK_STACK_CANARY] >> >> could be replaced with >> >> .if TSK_STACK_CANARAY >= PAGE_SIZE >> add r7, r7, #TSK_STACK_CANARY & PAGE_MASK >> .endif >> ldr r7, [r7, #TSK_STACK_CANARY & ~PAGE_MASK] > > Nice! Oh, very cool. This'll make it only an asm change in the case where it's required for randstruct. Perfect. I'll send a patch and carry it in the randstruct tree. (In looking at this, it seems tsk_mm is unused in mm/proc-macros.S, so I'll remove that code unless someone sees something I don't.) -Kees -- Kees Cook Pixel Security