[PATCH v2 03/13] stackleak: remove redundant check

[Mark Rutland]

On Wed, May 11, 2022 at 07:44:41AM -0700, Kees Cook wrote:
> 
> 
> On May 11, 2022 1:02:45 AM PDT, Mark Rutland <mark.rutland at arm.com> wrote:
> >On Tue, May 10, 2022 at 08:00:38PM -0700, Kees Cook wrote:
> >> On Tue, May 10, 2022 at 12:46:48PM +0100, Mark Rutland wrote:
> >> > On Sun, May 08, 2022 at 09:17:01PM +0300, Alexander Popov wrote:
> >> > > On 27.04.2022 20:31, Mark Rutland wrote:
> >> > > > In __stackleak_erase() we check that the `erase_low` value derived from
> >> > > > `current->lowest_stack` is above the lowest legitimate stack pointer
> >> > > > value, but this is already enforced by stackleak_track_stack() when
> >> > > > recording the lowest stack value.
> >> > > > 
> >> > > > Remove the redundant check.
> >> > > > 
> >> > > > There should be no functional change as a result of this patch.
> >> > > 
> >> > > Mark, I can't agree here. I think this check is important.
> >> > > The performance profit from dropping it is less than the confidence decrease :)
> >> > > 
> >> > > With this check, if the 'lowest_stack' value is corrupted, stackleak doesn't
> >> > > overwrite some wrong kernel memory, but simply clears the whole thread
> >> > > stack, which is safe behavior.
> >> > 
> >> > If you feel strongly about it, I can restore the check, but I struggle to
> >> > believe that it's worthwhile. The `lowest_stack` value lives in the
> >> > task_struct, and if you have the power to corrupt that you have the power to do
> >> > much more interesting things.
> >> > 
> >> > If we do restore it, I'd like to add a big fat comment explaining the
> >> > rationale (i.e. that it only matter if someone could corrupt
> >> > `current->lowest_stack`, as otherwise that's guarnateed to be within bounds).
> >> 
> >> Yeah, let's restore it and add the comment. While I do agree it's likely
> >> that such an corruption would likely mean an attacker had significant
> >> control over kernel memory already, it is not uncommon that an attack
> >> only has a limited index from a given address, etc. Or some manipulation
> >> is possible via weird gadgets, etc. It's unlikely, but not impossible,
> >> and a bounds-check for that value is cheap compared to the rest of the
> >> work happening. :)
> >
> >Fair enough; I can go spin a patch restoring this. I'm somewhat unhappy with
> >silently fixing that up, though -- IMO it'd be better to BUG() or similar in
> >that case.
> 
> I share your desires, and this was exactly what Alexander originally proposed, but Linus rejected it violently. :(
> https://lore.kernel.org/lkml/CA+55aFy6jNLsywVYdGp83AMrXBo_P-pkjkphPGrO=82SPKCpLQ@mail.gmail.com/

I see. :/

Thinking about this some more, if we assume someone can corrupt *some* word of
memory, then we need to consider that instead of corrupting
task_struct::lowest_stack, they could corrupt task_struct::stack (or x86's
cpu_current_top_of_stack prior to this series).

With that in mind, if we detect that task_struct::lowest_stack is
out-of-bounds, we have no idea whether it has been corrupted or the other bound
values have been corrupted, and so we can't do the erase safely anyway.

So AFAICT we must *avoid* erasing when that goes wrong. Maybe we could WARN()
instead of BUG()?

Thanks,
Mark.