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Date: Mon, 12 Mar 2018 09:42:23 -0700
From: Kees Cook <keescook@...omium.org>
To: Ingo Molnar <mingo@...nel.org>, P J P <pjp@...oraproject.org>,
Florian Weimer <fweimer@...hat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Linus Torvalds <torvalds@...ux-foundation.org>, Steven Rostedt <rostedt@...dmis.org>,
Arnd Bergmann <arnd@...db.de>, Daniel Micay <danielmicay@...il.com>,
Dave Hansen <dave.hansen@...ux.intel.com>, Alexander Popov <alex.popov@...ux.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>, Andy Lutomirski <luto@...nel.org>,
Tycho Andersen <tycho@...ho.ws>, Laura Abbott <labbott@...hat.com>, Mark Rutland <mark.rutland@....com>,
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>, 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>, X86 ML <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Fully initialized stack usage (was Re: [PATCH RFC v9 4/7] x86/entry:
Erase kernel stack in syscall_trace_enter())
On Mon, Mar 12, 2018 at 2:21 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
>> > Is it possible to implement this "safe automatic variable initialization" language
>> > feature via a GCC plugin robustly, while still keeping code generation sane? (i.e.
>> > no forced allocation of stack slots, etc.) It should be a superset of
>> > CONFIG_GCC_PLUGIN_STRUCTLEAK=y.
>>
>> I think that should be feasible, yes.
>>
>> It would be worth trying whether the current code can be simplified, though: it
>> currently takes care not to add such an initialization if it can already spot
>> one, but I wonder whether just blindly adding one at the start and letting the
>> compiler optimize it away again is safer.
>
> Absolutely - followed by:
>
> - a good look at the resulting code generation with a reasonably uptodate GCC
>
> - a look at the resulting code with older GCCs, to make sure there's no
> pathological code generation
>
> ... because IMHO in the end it is the practical effects that will make (or break)
> any such attempt.
>
> ( BTW., initializing all automatic variables might even speed up certain
> optimization passes, FWIIW. )
[attempting to merge threads...]
On Tue, Feb 27, 2018 at 3:15 AM, P J P <ppandit@...hat.com> wrote:
> Please see:
> -> https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00615.html
>
> This experimental patch by Florian Weimer(CC'd) adds an option
> '-finit-local-vars' to gcc(1) compiler. When a program(or kernel)
> is built using this option, its automatic(local) variables are
> initialised with zero(0). This could significantly reduce the kernel
> information leakage issues.
>
> A dnf(8) repository of the latest gcc-7.3.1 package built with the above
> patch and kernel-4.15.5 package built using '-finit-local-vars' option
> on Fedora-27 is available below
>
> -> https://pjp.fedorapeople.org/init-vars/
>
> This same kernel is running on my F27 test machine as I write this.
> There is no slowness or notice-able performance impact as such.
Getting this implemented directly in the compiler would be preferred
over a plugin. (Prototyping may be easier in the plugin, though.)
Considerations:
- the kernel needs a way to "opt out" of this for places where later
functions will do the initialization. Something like
__attribute__((no_automatic_variable_init)) ?
- initialization _must include structure padding_. Without this, we're
only solving part of the exposure. Does -finit-local-vars do this?
- Can we retain the uninitialized variable usage warning? (with an
updated text; maybe "variable used without explicit initialization,
using zero-initialization"?)
It sounds like Fedora-27's default kernel build is already using this
option, is that correct?
-Kees
--
Kees Cook
Pixel Security
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