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

[Ard Biesheuvel]

On 14 July 2017 at 15:06, Mark Rutland <mark.rutland at arm.com> 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 at linaro.org> wrote:
>> > On 14 July 2017 at 11:32, Mark Rutland <mark.rutland at arm.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
>         sub     sp, sp, x0      // sp = orig_sp
>
> ... which works in a locally-built kernel where I've aligned all the
> stacks.
>

Yes, that looks correct to me now.

>> ... 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?
>

No, not at all. I managed to confuse myself into thinking that we need
to validate the value of SP in some way, i.e., as we would when
dealing with an arbitrary faulting address.