Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP

From: Robin Murphy
Date: Fri Jul 14 2017 - 10:39:48 EST


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@xxxxxxxxxx> wrote:
>>> On 14 July 2017 at 11:32, Mark Rutland <mark.rutland@xxxxxxx> 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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