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

[Mark Rutland]

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.

> ... 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.