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Date: Thu, 26 Jul 2018 13:18:34 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Kees Cook <keescook@...gle.com>
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Subject: Re: [PATCH v14 7/7] stackleak, sysctl: Allow runtime disabling of
 kernel stack erasing

On 25.07.2018 02:59, Kees Cook wrote:
> On Tue, Jul 24, 2018 at 4:41 PM, Alexander Popov <alex.popov@...ux.com> wrote:
>> On 25.07.2018 01:56, Kees Cook wrote:
>>> On Thu, Jul 19, 2018 at 4:31 AM, Alexander Popov <alex.popov@...ux.com> wrote:
>>>> @@ -78,6 +113,11 @@ void __used stackleak_track_stack(void)
>>>>          */
>>>>         unsigned long sp = (unsigned long)&sp;
>>>>
>>>> +#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE
>>>> +       if (static_branch_unlikely(&stack_erasing_bypass))
>>>> +               return;
>>>> +#endif
>>>
>>> I would expect stackleak_erase() to be the expensive part, not the
>>> tracking part? Shouldn't timings be unchanged by leaving this in
>>> unconditionally, which would mean the sysctl could be re-enabled?
>>
>> Dropping the bypass in stackleak_track_stack() will not help against the
>> troubles with re-enabling stack erasing (tracking and erasing depend on each
> 
> Isn't the tracking checking "sp < current->lowest_stack", so if
> erasure was off, lowest_stack would only ever get further into the
> stack? And when erasure was turned back on, it would start getting
> reset correctly again. Or is the concern the poison searching could
> break? It seems like it would still work right? I must be missing
> something. :)

Umm.. You are right, that would be a solution. Let's assume that we:
 - allow stackleak_track_stack() to work,
 - skip stackleak_erase() giving most of performance penalty.
When we enable the 'stack_erasing_bypass', the 'lowest_stack' is not reset at
the end of syscall, it just continues to go down at next syscalls (because of
enabled tracking). In some sense it is similar to having a very long syscall.
Now if we re-enable erasing, the poison search in stackleak_erase() starts from
the _valid_ 'lowest_stack', which should work fine.

I'll send the improved version of the patch soon. Thanks!

Best regards,
Alexander

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