Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()

From: Ingo Molnar <mingo@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Alexander Popov <alex.popov@linux.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	PaX Team <pageexec@freemail.hu>,
	Brad Spengler <spender@grsecurity.net>,
	Andy Lutomirski <luto@kernel.org>,
	Tycho Andersen <tycho@tycho.ws>,
	Laura Abbott <labbott@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Borislav Petkov <bp@alien8.de>,
	Richard Sandiford <richard.sandiford@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	"Dmitry V . Levin" <ldv@altlinux.org>,
	Emese Revfy <re.emese@gmail.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Thomas Garnier <thgarnie@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>, Josef Bacik <jbacik@fb.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
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	Al Viro <viro@zeniv.linux.org.uk>,
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	Ding Tianhong <dingtianhong@huawei.com>,
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	Steven Rostedt <rostedt@goodmis.org>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
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	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Mathias Krause <minipli@googlemail.com>,
	Vikas Shivappa <vikas.shivappa@linux.intel.com>,
	Kyle Huey <me@kylehuey.com>,
	Dmitry Safonov <dsafonov@virtuozzo.com>,
	Will Deacon <will.deacon@arm.com>, Arnd Bergmann <arnd@arndb.de>,
	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()
Date: Tue, 6 Mar 2018 09:08:55 +0100	[thread overview]
Message-ID: <20180306080855.phtgl2bzqm5hnthu@gmail.com> (raw)
In-Reply-To: <CAGXu5j+-AjMKxOYDOpbmJ4X357JRGWViAOmqyqd6bu_w47mffw@mail.gmail.com>

* Kees Cook <keescook@chromium.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