Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751257AbdGPADI (ORCPT ); Sat, 15 Jul 2017 20:03:08 -0400 Received: from mail-it0-f52.google.com ([209.85.214.52]:36397 "EHLO mail-it0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751198AbdGPADH (ORCPT ); Sat, 15 Jul 2017 20:03:07 -0400 MIME-Version: 1.0 In-Reply-To: <20170714212717.GB1086@leverpostej> References: <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> <20170714140605.GB16687@leverpostej> <20170714212717.GB1086@leverpostej> From: Ard Biesheuvel Date: Sun, 16 Jul 2017 01:03:05 +0100 Message-ID: Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP To: Mark Rutland Cc: 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: 2516 Lines: 59 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