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

[Ard Biesheuvel]

On 14 July 2017 at 22:27, Mark Rutland <mark.rutland at arm.com> 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 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.
>
> 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