linux-kernel - Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace

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Message-ID: <abc02704-304e-2604-6297-75e6d2757c1e@linux.com>
Date:   Tue, 6 Mar 2018 00:02:49 +0300
From:   Alexander Popov <alex.popov@...ux.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Cc:     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>,
        Andy Lutomirski <luto@...nel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Laura Abbott <labbott@...hat.com>,
        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>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        "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 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in
 syscall_trace_enter()

Hello Linus,

Thanks for your reply (despite some strong words).

On 05.03.2018 23:15, Linus Torvalds wrote:
> This is the first I see of any of this, it was apparently not actually
> posted to lkml or anything like that.

I described that below.

> Honestly, what I see just makes me go "this is security-masturbation".

Let me quote the cover letter of this patch series.

STACKLEAK (initially developed by PaX Team):
 - reduces the information that can be revealed through kernel stack leak bugs;
 - blocks some uninitialized stack variable attacks (e.g. CVE-2017-17712,
CVE-2010-2963);
 - introduces some runtime checks for kernel stack overflow detection. It blocks
the Stack Clash attack against the kernel.

So it seems to be a useful feature.

> It doesn't actually seem to help *find* bugs at all. As such, it's
> another "paper over and forget" thing that just adds fairly high
> overhead when it's enabled.

The cover letter also contains the information about the performance impact.
It's 0.6% on the compiling the kernel (with Ubuntu config) and approx 4% on a
very intensive hackbench test.

> I'm NAK'ing it sight-unseen (see above) just because I'm tired of
> these kinds of pointless things that don't actually strive to improve
> on the kernel, just add more and more overhead for nebulous "things
> may happen", and that just make the code uglier.
> 
> Why wasn't it even posted to lkml?

That's my mistake. I started to learn that feature 9 month ago, just before
Qualys published the Stack Clash attack (which is blocked by STACKLEAK). I sent
first WIP versions to a short list of people (and had a lot of feedback to work
with). But later unfortunately I didn't adjust the list of recipients.

That was not done intentionally.

> And why isn't the focus of security people on tools to _analyse_ and
> find problems?

You know, I like KASAN and kernel fuzzing as well:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=82f2341c94d270421f383641b7cd670e474db56b

Best regards,
Alexander