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Message-ID: <CAGXu5j+4mEyhE6shkxWqY4AVQGcq2U7nzf4j_n2+CdW2t6CzqQ@mail.gmail.com> 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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