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

[Mark Rutland]

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.

Thanks,
Mark.