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

[Kees Cook]

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/


-- 
Kees Cook