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

[Alexander Popov <alex.popov@linux.com>]

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.

> 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.

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.

Best regards,
Alexander