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

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Date: Tue, 6 Mar 2018 09:08:55 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.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>,
	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()


* Kees Cook <keescook@...omium.org> wrote:

> [...] We've already seen multiple cases of this just with the by-ref 
> initialization plugin, where a stack content leak goes away because we asked the 
> compiler to please initialize the memory for us when we forgot to do it 
> ourselves. Getting the compiler to help us seems like the obviously correct 
> thing to do, since we're using such a memory-safety-unfriendly language. :)

So the question is, are there any known classes of stack content leak that the 
following .config options:

  CONFIG_GCC_PLUGIN_STRUCTLEAK=y
  CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y

... do not handle, and for which leaks the best solution is such runtime stack 
clearing?

Because the GCC plugins clearing automatic local variables are _way_ superior:

- it's probably a heck of a lot cheaper to clear structs than it is to clear the 
  stack in every system call...

- it's probably also safer statistically, because if there _is_ a reference to an 
  uninitialized piece of memory in the kernel it will be to zero, not to a user 
  controlled value...

Thanks,

	Ingo

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