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Date: Mon, 16 Jul 2018 12:13:38 +0200
From: Ingo Molnar <>
To: Alexander Popov <>
Cc: Kees Cook <>,
	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

* Alexander Popov <> wrote:

> 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
> conditions.

Firstly, a sysctl could still allow it to be *disabled*, once - which is the most 
important usecase of the sysctl anyway.

Secondly, in the first iteration this could be kept included unconditionally:

	current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;

... which would keep it initialized and wouldn't be racy, right?

I.e. only the most expensive part of the function, the scanning, would be turned 
off via the sysctl. I submit that this will avoid all measurable aspects of the 1% 
kbuild performance overhead.

A more involved approach can be done in the future if warranted, and the feature 
could be disabled/enabled more thoroughly - but the runtime sysctl would be 
acceptable for me for now, as a first iteration.

> 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 
> measurements.

A boot parameter does *not* address the concern I outlined. Why force admins 
reboot a box just to disable something that causes overhead?



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