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

From: Daniel Micay <danielmicay@gmail.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Kees Cook <keescook@chromium.org>,
	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>,
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	Dan Williams <dan.j.williams@intel.com>,
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	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 10:28:22 -0500	[thread overview]
Message-ID: <CA+DvKQJrqk6hBKkN3eObgnAwy6i1QjHOBt-LvfjV7nL4m6aJYw@mail.gmail.com> (raw)
In-Reply-To: <CA+DvKQ+uQD0qyMvwqaRd_sMLBHE5q9pcfOQ+VknBW2==q0NJJA@mail.gmail.com>

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.