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Date: Tue, 20 Jun 2017 12:20:17 -0700
From: Kees Cook <keescook@...omium.org>
To: Alexander Popov <alex.popov@...ux.com>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>, Tycho Andersen <tycho@...ker.com>
Subject: Re: [PATCH RFC v2 1/1] gcc-plugins: Add stackleak feature erasing the
kernel stack at the end of syscalls
On Fri, Jun 9, 2017 at 4:00 PM, Alexander Popov <alex.popov@...ux.com> wrote:
> On 09.06.2017 20:28, Kees Cook wrote:
>> Awesome, and thanks for the benchmarks! That should really help people
>> understand the trade-offs for using this feature (and is likely worth
>> mentioning in the Kconfig). Seems like less than 4% overhead, maybe
>> much less? Real time on build times seems like a tiny difference, but
>> hackbench shows 4%.
>
> Yes, the performance penalty of STACKLEAK differs a lot depending on the
> kind of load. Do you have any idea which test can give a bigger slowdown?
> It should be some rapid syscall exhausting the kernel stack hard.
I can't think of anything off the top of my head. You could play with
CONFIG_FRAME_WARN[1] and related tools to find a deep call path and
try that?
[1] http://elinux.org/Kernel_Small_Stacks
>> Maybe specifically mention the -0xBEEF value?
>
> Ok. Should I create some macro for it?
Maybe? It's not really clear how useful that might be. If it's easy,
then yeah, use a common macro for the value, if it creates header
soup, leave it open-coded.
>> I would follow the naming of the others, and call this GCC_PLUGIN_STACKLEAK
>
> It seems to me that GCC_PLUGIN_STACKLEAK is not a right name since the whole
> feature consists of two parts: the arch-specific asm code actually cleaning
> the kernel stack and the gcc plugin which helps to do it faster and more
> reliable. What do you think?
It looks like the feature requires the plugin, so I think the common
naming (GCC_PLUGIN_STACKLEAK) would be preferred. But perhaps I'm
overlooking something where the plugin is not used?
Thanks!
-Kees
--
Kees Cook
Pixel Security
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