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Message-ID: <CA+DvKQJrqk6hBKkN3eObgnAwy6i1QjHOBt-LvfjV7nL4m6aJYw@mail.gmail.com>
Date: Tue, 6 Mar 2018 10:28:22 -0500
From: Daniel Micay <danielmicay@...il.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Kees Cook <keescook@...omium.org>, 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()
The kernel could use -Werror=vla-larger-than=2048 as a stop-gap and
work around all the errors by making sure it can't be larger and
reporting an error or adding `BUG_ON(size > limit)` to prove it to the
compiler if necessary. It warns if the VLA *can be* larger than the
limit, unlike the frame size warning the kernel uses only warning when
the frame is *guaranteed* to be larger than the limit. It wouldn't be
a guaranteed to work solution since a function can have other VLAs,
etc. but in practice it could probably be good enough. I've tried it
and I don't think it would be that much work. It might save someone
from a vulnerability that isn't using VMAP_STACK even if the issue
gets fixed for that.
Android kernels are likely mostly going to be compiled with Clang now
for 4.4+ and it doesn't have a similar warning though. I mentioned
last year that there were some real vulnerabilities caused by
unbounded VLAs:
http://openwall.com/lists/kernel-hardening/2017/05/12/33. It's fairly
easy to flip up and accidentally use one especially for people that
work with other languages. Maybe the kernel should already be using
-Werror=vla and overriding it for the few cases that exist to stop
letting people introduce more.
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