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

[Laura Abbott]

On 07/15/2017 05:03 PM, Ard Biesheuvel wrote:
> On 14 July 2017 at 22:27, Mark Rutland <mark.rutland@xxxxxxx> 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 <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
>>>       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
> 

I used vmap-arm64-mark to compile kernels for a few days. It seemed to
work well enough.

Thanks,
Laura