Messages in this thread | | | Date | Wed, 11 May 2022 09:02:45 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2 03/13] stackleak: remove redundant check |
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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.
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