Messages in this thread | | | Date | Thu, 12 May 2022 10:14:53 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2 03/13] stackleak: remove redundant check |
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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@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.
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