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Message-ID: <CAGXu5jKNsp4WKUJQZ81NWciOGYBGAdVhiSQy9Qa-JAY84LiwmQ@mail.gmail.com>
Date: Mon, 5 Mar 2018 13:36:01 -0800
From: Kees Cook <keescook@...omium.org>
To: Alexander Popov <alex.popov@...ux.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Laura Abbott <labbott@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>, Ingo Molnar <mingo@...nel.org>,
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Richard Sandiford <richard.sandiford@....com>, Thomas Gleixner <tglx@...utronix.de>,
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Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>, X86 ML <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v9 2/7] x86/entry: Add STACKLEAK erasing the kernel
stack at the end of syscalls
On Mon, Mar 5, 2018 at 1:21 PM, Alexander Popov <alex.popov@...ux.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
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