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

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Date: Mon, 12 Mar 2018 09:22:26 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Arnd Bergmann <arnd@...db.de>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Daniel Micay <danielmicay@...il.com>,
	Kees Cook <keescook@...omium.org>,
	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: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in
 syscall_trace_enter()


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Mar 6, 2018 at 1:41 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > The warning would remain for the case where you don't enable this
> > hardening feature, so it wouldn't go away.
> 
> Side note: if in ten years we'd have a minimum gcc version that we
> could  just unconditionally say "auto (scalars) initialize to zero",
> then we'd just make that be the *semantics*, and the warning would
> obviously simply not ever be an issue.

Btw., I'd suggest we initialize aggregate types to zero as well, and then work 
from there by marking exceptions via attributes.

>From what I've seen over 90% of 'tricky' initialization sequences either don't 
matter to performance, or are unnecessarily complicated.

I.e. let's eliminate VLAs and let's also make the object initialization aspect of 
the C language reliably and broadly safe by default (via a GCC plugin) with no 
exceptions, and allow an opt-in mechanism for more fragile (but faster if coded 
correctly) constructs.

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.

Plugin support is present in GCC version 4.5 and higher, correct? So if such a 
plugin is possible we could raise the minimum GCC version to support it 
unconditionally.

I suspect a fair chunk of all kernel CVEs would go away if we fixed the C language 
this way.

Thanks,

	Ingo
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