From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754495AbdGNOHI (ORCPT ); Fri, 14 Jul 2017 10:07:08 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:49038 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754430AbdGNOHF (ORCPT ); Fri, 14 Jul 2017 10:07:05 -0400 Date: Fri, 14 Jul 2017 15:06:06 +0100 From: Mark Rutland To: Ard Biesheuvel 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 Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP Message-ID: <20170714140605.GB16687@leverpostej> References: <1499898783-25732-7-git-send-email-mark.rutland@arm.com> <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > ... only, this requires a dedicated stack region, and so we'd need to > check whether sp is inside that window as well. > > The easieast way would be to use a window whose start address is base2 > aligned, but that means the beginning of the kernel VA range (where > KASAN currently lives, and cannot be moved afaik), or a window at the > top of the linear region. Neither look very appealing > > So that means arbitrary low and high limits to compare against in this > entry path. That means more GPRs I'm afraid. Could you elaborate on that? I'm not sure that I follow. My understanding was that the comprimise with this approach is that we only catch overflow/underflow within THREAD_SIZE of the stack, and can get false-negatives elsewhere. Otherwise, IIUC this is sufficient Are you after a more stringent check (like those from the two existing proposals that caught all out-of-bounds accesses)? Or am I missing something else? Thanks, Mark. From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Fri, 14 Jul 2017 15:06:06 +0100 Subject: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP In-Reply-To: References: <1499898783-25732-7-git-send-email-mark.rutland@arm.com> <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> Message-ID: <20170714140605.GB16687@leverpostej> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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. > ... only, this requires a dedicated stack region, and so we'd need to > check whether sp is inside that window as well. > > The easieast way would be to use a window whose start address is base2 > aligned, but that means the beginning of the kernel VA range (where > KASAN currently lives, and cannot be moved afaik), or a window at the > top of the linear region. Neither look very appealing > > So that means arbitrary low and high limits to compare against in this > entry path. That means more GPRs I'm afraid. Could you elaborate on that? I'm not sure that I follow. My understanding was that the comprimise with this approach is that we only catch overflow/underflow within THREAD_SIZE of the stack, and can get false-negatives elsewhere. Otherwise, IIUC this is sufficient Are you after a more stringent check (like those from the two existing proposals that caught all out-of-bounds accesses)? Or am I missing something else? Thanks, Mark. From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 14 Jul 2017 15:06:06 +0100 From: Mark Rutland Message-ID: <20170714140605.GB16687@leverpostej> References: <1499898783-25732-7-git-send-email-mark.rutland@arm.com> <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP To: Ard Biesheuvel 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 List-ID: 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. > ... only, this requires a dedicated stack region, and so we'd need to > check whether sp is inside that window as well. > > The easieast way would be to use a window whose start address is base2 > aligned, but that means the beginning of the kernel VA range (where > KASAN currently lives, and cannot be moved afaik), or a window at the > top of the linear region. Neither look very appealing > > So that means arbitrary low and high limits to compare against in this > entry path. That means more GPRs I'm afraid. Could you elaborate on that? I'm not sure that I follow. My understanding was that the comprimise with this approach is that we only catch overflow/underflow within THREAD_SIZE of the stack, and can get false-negatives elsewhere. Otherwise, IIUC this is sufficient Are you after a more stringent check (like those from the two existing proposals that caught all out-of-bounds accesses)? Or am I missing something else? Thanks, Mark.