linux-kernel - Re: [PATCH RFC v9 2/7] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

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Message-ID: <b6abd009-8f64-c5a2-6496-6affb31ad75e@linux.com>
Date:   Tue, 6 Mar 2018 00:21:02 +0300
From:   Alexander Popov <alex.popov@...ux.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Laura Abbott <labbott@...hat.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        kernel-hardening@...ts.openwall.com,
        Kees Cook <keescook@...omium.org>,
        PaX Team <pageexec@...email.hu>,
        Brad Spengler <spender@...ecurity.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Borislav Petkov <bp@...en8.de>,
        Richard Sandiford <richard.sandiford@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Emese Revfy <re.emese@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Josef Bacik <jbacik@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>,
        Ding Tianhong <dingtianhong@...wei.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Juergen Gross <jgross@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Mathias Krause <minipli@...glemail.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Kyle Huey <me@...ehuey.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        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 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.

Best regards,
Alexander