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Date: Thu, 24 May 2018 16:05:27 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Ingo Molnar <mingo@...nel.org>, Kees Cook <keescook@...omium.org>
Cc: kernel-hardening@...ts.openwall.com, PaX Team <pageexec@...email.hu>,
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 Peter Zijlstra <a.p.zijlstra@...llo.nl>, "Dmitry V . Levin"
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 Florian Weimer <fweimer@...hat.com>,
 Boris Lukashev <blukashev@...pervictus.com>,
 Andrey Konovalov <andreyknvl@...gle.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack
 at the end of syscalls

On 18.05.2018 09:53, Ingo Molnar wrote:
> * Alexander Popov <alex.popov@...ux.com> wrote:
>> +	if (on_thread_stack())
>> +		boundary = current_stack_pointer;
>> +	else
>> +		boundary = current_top_of_stack();
>> +
>> +	BUG_ON(boundary - p >= THREAD_SIZE);
> 
> Please make this:
> 
> 	if ( WARN_ON_ONCE())
> 		return;
> 
> ... or so, so that if this code is buggy we get actual useful user reports, not 
> just "my machine froze, help!"...

I've just double-checked that BUG() in erase_kstack() works fine when it's
called from the trampoline stack and thread stack.

But this "WARN_ON_ONCE() + return" logic would introduce an undesirable effect:
modifying 'lowest_stack' value would allow to silently skip stack erasing.

Best regards,
Alexander

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