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Date: Mon, 16 Jul 2018 10:24:26 +0300
From: Alexander Popov <>
To: Ingo Molnar <>, Kees Cook <>
Cc: Linus Torvalds <>,
 Kernel Hardening <>,
 Pax Team <>, Brad Spengler <>,
 Andrew Lutomirski <>, Tycho Andersen <>,
 Laura Abbott <>, Mark Rutland <>,
 Ard Biesheuvel <>, Borislav Petkov <>,
 Richard Sandiford <>,
 Thomas Gleixner <>, Peter Anvin <>,
 Peter Zijlstra <>, "Dmitry V. Levin"
 <>, Emese Revfy <>,
 Jonathan Corbet <>, Andrey Ryabinin <>,
 "Kirill A. Shutemov" <>,
 Thomas Garnier <>,
 Andrew Morton <>,
 Alexei Starovoitov <>, Josef Bacik <>,
 Masami Hiramatsu <>, Nick Piggin <>,
 Al Viro <>, David Miller <>,
 dingtianhong <>, David Woodhouse <>,
 Josh Poimboeuf <>, Steven Rostedt <>,
 Dominik Brodowski <>, Jürgen Groß <>,
 Greg Kroah-Hartman <>,
 Dan Williams <>,
 Dave Hansen <>,
 Mathias Krause <>,
 Vikas Shivappa <>, Kyle Huey
 <>, Dmitry Safonov <>,
 Will Deacon <>, Arnd Bergmann <>,
 Florian Weimer <>,
 Boris Lukashev <>,
 Andrey Konovalov <>,
 the arch/x86 maintainers <>,
 Linux Kernel Mailing List <>
Subject: Re: [PATCH v14 0/6] Introduce the STACKLEAK feature and a test for it

On 16.07.2018 01:44, Ingo Molnar wrote:
> * Kees Cook <> wrote:
>> On Thu, Jul 12, 2018 at 2:22 PM, Alexander Popov <> wrote:
>>> On 12.07.2018 23:50, Ingo Molnar wrote:
>>>> Let's make sure informed users have an easy runtime way out
>>>> from the worst of the overhead that doesn't involve "recompile your distro
>>>> kernel". Also, make it easier to measure and fingerpoint the overhead...
>>> Would you like the following solution?
>>> I'll create the CONFIG_STACKLEAK_RUNTIME_DISABLE config option, which would be
>>> similar to CONFIG_SECURITY_SELINUX_DISABLE. That option will provide a sysctl
>>> switch disabling stackleak_erase(), which provides the most of performance penalty.
>> I don't think CONFIG/sysctl is the way to go. I'd recommend making it
>> a boot arg and using a static key, similar to what's happening to
>> hardened_usercopy:
> Why no sysctl? It's a PITA to reboot systems just to turn a stupid knob off.
> Also, it's _much_ easier to measure performance impact when there's a sysctl.

Yes, you are right.

But I looked carefully and now see the troubles which sysctl would bring us.
Each 'task_struct' has 'lowest_stack', which must be initialized before enabling
STACKLEAK. So runtime enabling via sysctl is not plain and may bring race

On the other hand, a boot param + static key that can only disable STACKLEAK
during __init are much more robust. I'm preparing the patch and performance

Best regards,

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