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Date: Fri, 6 Jul 2018 14:46:10 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>,
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 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 (resend) 2/6] x86/entry: Add STACKLEAK erasing the
 kernel stack at the end of syscalls

On 06.07.2018 01:20, Ingo Molnar wrote:
> 
> * Alexander Popov <alex.popov@...ux.com> wrote:
> 
>> Hello Ingo,
>>
>> Thanks for your review! I'll fix the style issues you pointed at.
>>
>> Please also see my answers below.
>>
>> On 05.07.2018 11:12, Ingo Molnar wrote:
>>>> +	  The tradeoff is the performance impact: on a single CPU system kernel
>>>> +	  compilation sees a 1% slowdown, other systems and workloads may vary
>>>> +	  and you are advised to test this feature on your expected workload
>>>> +	  before deploying it.
>>>
>>> Is there a way to patch this out runtime? I.e. if a distro enabled it, is there an 
>>> easy way to disable much of the overhead without rebooting the kernel?
>>
>> Hm. We can't completely disable STACKLEAK in runtime, since STACKLEAK gcc plugin
>> performs compile-time instrumentation of the kernel code. So we can only chop
>> off a part of functionality, for example, by introducing some variable and
>> checking it before every stack erasing (additional performance impact), but the
>> kernel will stay uselessly instrumented. It doesn't look reasonable to me.
> 
> Or we could use what every other performance critical instrumentation feature uses 
> to reduce overhead (ftrace, perf): kernel patching.

I see. It would be a big separate research - how to combine those different
kinds of instrumentation. I would propose to postpone it until we have a request
for STACKLEAK runtime disabling.

>>> If so then please make this:
>>>
>>> 	if (WARN_ON(boundary - kstack_ptr >= THREAD_SIZE))
>>> 		return;
>>>
>>> or so, to make it non-fatal and to allow users to report it, should it trigger 
>>> against all expectations.
>>
>> I've made an experiment. The results:
>>  1. BUG_ON() here doesn't freeze the kernel output - I see a full 'PANIC: double
>> fault' report;
> 
> Only in text mode - very few users are using text mode.
> 
>>  2. WARN_ON() here gives absolutely same 'PANIC: double fault' here.
> 
> that should only happen if the kernel is otherwise already fatally corrupted, 
> right?

No, I mean WARN_ON() in stackleak_erase_kstack() gives the double fault just
like BUG_ON() (without any corruption). In my experiment I've made the following
change:

-       BUG_ON(boundary - kstack_ptr >= THREAD_SIZE);
+//     BUG_ON(boundary - kstack_ptr >= THREAD_SIZE);
+       WARN_ON(1);

It might be caused by the fact, that stackleak_erase_kstack() is called from the
trampoline stack just before returning to the userspace.

So I mean 'WARN_ON() + return' here wouldn't give any profit over a single
BUG_ON() check.

Best regards,
Alexander

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