Messages in this thread | | | Date | Fri, 27 May 2022 01:09:31 +0300 | Subject | Re: [PATCH v2 03/13] stackleak: remove redundant check | From | Alexander Popov <> |
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On 24.05.2022 13:03, Mark Rutland wrote: > On Sun, May 15, 2022 at 07:17:16PM +0300, Alexander Popov wrote: >> On 12.05.2022 12:14, Mark Rutland wrote: >>> 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. >> >> :) >> >> IMO, even if a kernel thread stack is moved somewhere for any weird reason, >> stackleak must erase it at the end of syscall and do its job. > > I'm not talking about the stack being *moved*. I'm talking about the pointers > to it being *corrupted* (wince we use those to determine the bounds). > > The problem is that we don't have a single source of truth here that we can > rely upon. > > We're stuck between a dichotomy: > > * If we assume an attacker *can* corrupt a word of memory, they can corrupt any > of the in-memory values we use to find the stack in the first place. If we > detect a mismatch we cannot know which is bad, and if the attacker can > corrupt the one(s) we choose to blindly trust, then they can weaponize the > erasing code to corrupt memory. > > That's *worse* than the info leak problem stackleak was originally trying to > solve. > > See below for one way we could avoid that. > > * If we assume the attacker *cannot* corrupt a word of memory, then we know the > values must always be within bounds, and there's no need for the check.
Mark, thanks for your reply.
Yes, I agree with this.
>>> So AFAICT we must *avoid* erasing when that goes wrong. Maybe we could WARN() >>> instead of BUG()? >> >> Mark, I think security features must not go out of service. >> >> The 'lowest_stack' value is for making stackleak faster. I believe if the >> 'lowest_stack' value is invalid, stackleak must not skip its main job and >> should erase the whole kernel thread stack. > > My point is that the conditions which permit `lowest_stack` to become invalid > (e.g. an attacker having an arbitrary or constrained write gadget) also permit > all the other stack boundariy values to become invalid. > > If we detect `lowest_stack` is out of bounds, we have no idea which of > `lowest_stack` or the bounds are corrupt -- so we *cannot* safely erase: if the > bounds are corrupt we'll corrupt arbitrary memory. > > We *could* do better by always deriving the bounds from an SP value (current > for on-stack, passed in by asm for off-stack). If we did that, we could more > reasonably treat the bounds values as more reliable than the `lowest_stack` > value, and with that I'd be happy with the bounds check (though I still think > we want to make this WARN()).
I would prefer erasing anyway over WARN_ON() + skipping it.
Ok, let's simply hope that the `lowest_stack` is correct. Let's erase the stack without additional checks.
>> When I developed 'stackleak_erase()' I tried adding 'WARN_ON()', but it >> didn't work properly there, as I remember. Warning handling code is very >> complex. So I dropped that fragile part. > > Do you recall any specific problem, or just that there were problems?
As I remember, the kernel was hanging without printing full warning.
> I ask because the entry code, and handling of BUG() and WARN() has changed > quite a bit over the last couple of years. We've fixed some latent issues > there, though IIUC this late in the exception return flow there are still some > potential issues with the RCU/lockdep/etc context that would need to be > saved/restored. > > We need to solve some of that in general anyuway, since there are other BUG() > and WARN() instances hidden in noinstr entry code. I'm happy to dig into that > (time permitting). > > Thanks, > Mark.
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