From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754403AbdGNPD5 (ORCPT ); Fri, 14 Jul 2017 11:03:57 -0400 Received: from foss.arm.com ([217.140.101.70]:49742 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753968AbdGNPDz (ORCPT ); Fri, 14 Jul 2017 11:03:55 -0400 Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP From: Robin Murphy To: Mark Rutland , Ard Biesheuvel Cc: Kees Cook , Kernel Hardening , Catalin Marinas , Will Deacon , "linux-kernel@vger.kernel.org" , James Morse , Takahiro Akashi , Dave Martin , "linux-arm-kernel@lists.infradead.org" , Laura Abbott References: <1499898783-25732-7-git-send-email-mark.rutland@arm.com> <20170713104950.GB26194@leverpostej> <20170713161050.GG26194@leverpostej> <20170713175543.GA32528@leverpostej> <20170714103258.GA16128@leverpostej> <20170714140605.GB16687@leverpostej> <188731af-269c-4197-1c55-78e485e7af46@arm.com> Message-ID: <8f805a19-19d1-3c97-c85b-510664d22dad@arm.com> Date: Fri, 14 Jul 2017 16:03:51 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <188731af-269c-4197-1c55-78e485e7af46@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14/07/17 15:39, Robin Murphy wrote: > On 14/07/17 15:06, 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 > > Haven't you now forcibly cleared the top bit of x0 thanks to overflow? ...or maybe not. I still can't quite see it, but I suppose it must cancel out somewhere, since Mr. Helpful C Program[1] has apparently proven me mistaken :( I guess that means I approve! Robin. [1]: #include #include int main(void) { for (int i = 0; i < 256; i++) { for (int j = 0; j < 256; j++) { uint8_t x = i; uint8_t y = j; y = y + x; x = y - x; x = y - x; y = y - x; assert(x == i && y == j); } } } >> 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. >> >> _______________________________________________ >> linux-arm-kernel mailing list >> linux-arm-kernel@lists.infradead.org >> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel >> > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel >