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Date: Wed, 14 Mar 2018 08:52:06 -0700
From: Kees Cook <keescook@...omium.org>
To: Florian Weimer <fweimer@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Ingo Molnar <mingo@...nel.org>,
P J P <pjp@...oraproject.org>, Ard Biesheuvel <ard.biesheuvel@...aro.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: Re: Fully initialized stack usage (was Re: [PATCH RFC v9 4/7]
x86/entry: Erase kernel stack in syscall_trace_enter())
On Wed, Mar 14, 2018 at 7:21 AM, Florian Weimer <fweimer@...hat.com> wrote:
> On 03/12/2018 06:45 PM, Linus Torvalds wrote:
>>
>> On Mon, Mar 12, 2018 at 10:17 AM, Kees Cook<keescook@...omium.org> wrote:
>>>
>>> On Mon, Mar 12, 2018 at 10:09 AM, Linus Torvalds
>>> <torvalds@...ux-foundation.org> wrote:
>>>>
>>>> struct xyz var = { };
>>>>
>>>> I'm not sure what that will do with padding.
>>>
>>> AIUI, this does not guarantee padding initialization (yet another
>>> "undefined behavior"). This is why we've had to sprinkle memset(&var,
>>> 0, sizeof(var)) in places where a structure has padding and got
>>> leaked. :(
>>>
>>> I assume this may be orthogonal to -finit-local-vars, and maybe we'll
>>> need some -finit-padding or something. (Though, honestly, is there
>>> anyone that wants to get_padding_ correct, but not variable
>>> initialization?)
>
>
>> We would definitely have wanted it over the years, yes. And
>> conceptually it's a separate issue, so a separate flag makes sense.
>
>
> What would be the model for the kernel? Write zero to the padding
> initially, and on copying structs, make sure that you either copy the
> padding from the source, or clear the target?
>
> Clearing the padding while copying might be somewhat expensive.
I think the common expectation is that copying does a full copy from
the source. Zeroing of padding should be guaranteed by the constructor
only, IMO.
-Kees
--
Kees Cook
Pixel Security
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