Message-ID: <188731af-269c-4197-1c55-78e485e7af46@arm.com>
Date:   Fri, 14 Jul 2017 15:39:39 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        James Morse <james.morse@....com>,
        Takahiro Akashi <akashi.takahiro@...aro.org>,
        Dave Martin <dave.martin@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Laura Abbott <labbott@...oraproject.org>
Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and
 detect out-of-bounds SP

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 <ard.biesheuvel@...aro.org> wrote:
>>> On 14 July 2017 at 11:32, Mark Rutland <mark.rutland@....com> 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?

Robin.

> 	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.
> 
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