Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753064AbdGSIIn (ORCPT ); Wed, 19 Jul 2017 04:08:43 -0400 Received: from mail-io0-f171.google.com ([209.85.223.171]:35401 "EHLO mail-io0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752109AbdGSIIj (ORCPT ); Wed, 19 Jul 2017 04:08:39 -0400 MIME-Version: 1.0 In-Reply-To: <39a5ad84-4124-5b33-146a-cd4e48f3762f@redhat.com> References: <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> <20170714140605.GB16687@leverpostej> <20170714212717.GB1086@leverpostej> <39a5ad84-4124-5b33-146a-cd4e48f3762f@redhat.com> From: Ard Biesheuvel Date: Wed, 19 Jul 2017 09:08:37 +0100 Message-ID: Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP To: Laura Abbott Cc: Mark Rutland , Kernel Hardening , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Takahiro Akashi , Catalin Marinas , Dave Martin , James Morse , Laura Abbott , Will Deacon , Kees Cook Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2908 Lines: 69 On 18 July 2017 at 22:53, Laura Abbott wrote: > On 07/15/2017 05:03 PM, Ard Biesheuvel wrote: >> On 14 July 2017 at 22:27, Mark Rutland wrote: >>> On Fri, Jul 14, 2017 at 03:06:06PM +0100, Mark Rutland wrote: >>>> On Fri, Jul 14, 2017 at 01:27:14PM +0100, Ard Biesheuvel wrote: >>>>> On 14 July 2017 at 11:48, Ard Biesheuvel wrote: >>>>>> On 14 July 2017 at 11:32, Mark Rutland wrote: >>>>>>> On Thu, Jul 13, 2017 at 07:28:48PM +0100, Ard Biesheuvel wrote: >>>> >>>>>>>> OK, so here's a crazy idea: what if we >>>>>>>> a) carve out a dedicated range in the VMALLOC area for stacks >>>>>>>> b) for each stack, allocate a naturally aligned window of 2x the stack >>>>>>>> size, and map the stack inside it, leaving the remaining space >>>>>>>> unmapped >>>> >>>>>>> The logical ops (TST) and conditional branches (TB(N)Z, CB(N)Z) operate >>>>>>> on XZR rather than SP, so to do this we need to get the SP value into a >>>>>>> GPR. >>>>>>> >>>>>>> Previously, I assumed this meant we needed to corrupt a GPR (and hence >>>>>>> stash that GPR in a sysreg), so I started writing code to free sysregs. >>>>>>> >>>>>>> However, I now realise I was being thick, since we can stash the GPR >>>>>>> in the SP: >>>>>>> >>>>>>> sub sp, sp, x0 // sp = orig_sp - x0 >>>>>>> add x0, sp, x0 // x0 = x0 - (orig_sp - x0) == orig_sp >>>> >>>> That comment is off, and should say x0 = x0 + (orig_sp - x0) == orig_sp >>>> >>>>>>> sub x0, x0, #S_FRAME_SIZE >>>>>>> tb(nz) x0, #THREAD_SHIFT, overflow >>>>>>> add x0, x0, #S_FRAME_SIZE >>>>>>> sub x0, sp, x0 >>>>> >>>>> You need a neg x0, x0 here I think >>>> >>>> Oh, whoops. I'd mis-simplified things. >>>> >>>> We can avoid that by storing orig_sp + orig_x0 in sp: >>>> >>>> add sp, sp, x0 // sp = orig_sp + orig_x0 >>>> sub x0, sp, x0 // x0 = orig_sp >>>> < check > >>>> sub x0, sp, x0 // x0 = orig_x0 >>>> sub sp, sp, x0 // sp = orig_sp >>>> >>>> ... which works in a locally-built kernel where I've aligned all the >>>> stacks. >>> >>> FWIW, I've pushed out a somewhat cleaned-up (and slightly broken!) >>> version of said kernel source to my arm64/vmap-stack-align branch [1]. >>> That's still missing the backtrace handling, IRQ stack alignment is >>> broken at least on 64K pages, and there's still more cleanup and rework >>> to do. >>> >> >> I have spent some time addressing the issues mentioned in the commit >> log. Please take a look. >> >> git://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git vmap-arm64-mark >> > > I used vmap-arm64-mark to compile kernels for a few days. It seemed to > work well enough. > Thanks for giving this a spin. Any comments on the performance impact? (if you happened to notice any)