Re: [PATCH v11 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

From: Kees Cook <keescook@chromium.org>
To: Alexander Popov <alex.popov@linux.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Ingo Molnar <mingo@kernel.org>, Laura Abbott <labbott@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: 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>,
	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>,
	Nicholas Piggin <npiggin@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"David S . Miller" <davem@davemloft.net>,
	Ding Tianhong <dingtianhong@huawei.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	Juergen Gross <jgross@suse.com>,
	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>,
	Florian Weimer <fweimer@redhat.com>,
	Boris Lukashev <blukashev@sempervictus.com>,
	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v11 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls
Date: Mon, 16 Apr 2018 11:29:13 -0700	[thread overview]
Message-ID: <CAGXu5jKm218fpxyV3vOM+2SO-ERG3tWPUV0x8+-S0MJAReFz+A@mail.gmail.com> (raw)
In-Reply-To: <1523024546-6150-3-git-send-email-alex.popov@linux.com>

On Fri, Apr 6, 2018 at 7:22 AM, Alexander Popov <alex.popov@linux.com> wrote:
> This commit introduces the architecture-specific code filling the used
> part of the kernel stack with a poison value before returning to the
> userspace. Full STACKLEAK feature also contains the gcc plugin which
> comes in a separate commit.

Thanks for sending this again! And thanks for the updated reasoning
for why this remains a valuable addition:

https://lkml.kernel.org/r/1523024546-6150-1-git-send-email-alex.popov@linux.com

I, too, remain convinced this is a good protection to have, even as we
slowly remove VLAs and try to improve the compiler's initialization of
stack variables.

Dave, Ingo, Linus: how does this look? With the assembly rewritten
into C, the entry changes are very small:

>  arch/x86/entry/entry_32.S        |  7 ++++++
>  arch/x86/entry/entry_64.S        |  3 +++
>  arch/x86/entry/entry_64_compat.S |  5 ++++
>  arch/x86/entry/erase.c           | 54 ++++++++++++++++++++++++++++++++++++++++

I'd really like to get people's Ack/Review. :)

Laura, can this C version work for arm64 as well?

Thanks,

-Kees

-- 
Kees Cook
Pixel Security