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

[Alexander Popov]

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 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/
>>>
>>> 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.