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From:   Kees Cook <keescook@chromium.org>
Date:   Mon, 5 Mar 2018 13:36:01 -0800
Message-ID: <CAGXu5jKNsp4WKUJQZ81NWciOGYBGAdVhiSQy9Qa-JAY84LiwmQ@mail.gmail.com>
Subject: Re: [PATCH RFC v9 2/7] x86/entry: Add STACKLEAK erasing the kernel
 stack at the end of syscalls
To:     Alexander Popov <alex.popov@linux.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Laura Abbott <labbott@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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On Mon, Mar 5, 2018 at 1:21 PM, Alexander Popov <alex.popov@linux.com> wrote:
> On 05.03.2018 23:25, Peter Zijlstra wrote:
>> On Mon, Mar 05, 2018 at 11:43:19AM -0800, Laura Abbott wrote:
>>> On 03/05/2018 08:41 AM, Dave Hansen wrote:
>>>> On 03/03/2018 12:00 PM, Alexander Popov wrote:
>>>>>   Documentation/x86/x86_64/mm.txt  |   2 +
>>>>>   arch/Kconfig                     |  27 ++++++++++
>>>>>   arch/x86/Kconfig                 |   1 +
>>>>>   arch/x86/entry/entry_32.S        |  88 +++++++++++++++++++++++++++++++
>>>>>   arch/x86/entry/entry_64.S        | 108 +++++++++++++++++++++++++++++++++++++++
>>>>>   arch/x86/entry/entry_64_compat.S |  11 ++++
>>>>
>>>> This is a *lot* of assembly.  I wonder if you tried at all to get more
>>>> of this into C or whether you just inherited the assembly from the
>>>> original code?
>>>>
>>>
>>> This came up previously http://www.openwall.com/lists/kernel-hardening/2017/10/23/5
>>> there were concerns about trusting C to do the right thing as well as
>>> speed.
>>
>> And therefore the answer to this obvious question should've been part of
>> the Changelog :-)
>>
>> Dave is last in a long line of people asking this same question.
>
> Yes, actually the changelog in the cover letter contains that:
>
>   After some experiments, kept the asm implementation of erase_kstack(),
>   because it gives a full control over the stack for clearing it neatly
>   and doesn't offend KASAN.
>
> Moreover, later erase_kstack() on x86_64 became different from one on x86_32.

Maybe explicitly mention the C experiments in future change log?

-Kees

-- 
Kees Cook
Pixel Security