Re: [PATCH 06/14] Fix CFLAGS for UBSAN_BOUNDS on Clang

From: Kees Cook <keescook@chromium.org>
To: George Popescu <georgepope@google.com>
Cc: Marco Elver <elver@google.com>,
	maz@kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	kvmarm@lists.cs.columbia.edu, LKML <linux-kernel@vger.kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	james.morse@arm.com, julien.thierry.kdev@gmail.com,
	suzuki.poulose@arm.com,
	Nathan Chancellor <natechancellor@gmail.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	David Brazdil <dbrazdil@google.com>,
	broonie@kernel.org, Fangrui Song <maskray@google.com>,
	Andrew Scull <ascull@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dmitry Vyukov <dvyukov@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Arnd Bergmann <arnd@arndb.de>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Alexander Potapenko <glider@google.com>
Subject: Re: [PATCH 06/14] Fix CFLAGS for UBSAN_BOUNDS on Clang
Date: Thu, 17 Sep 2020 15:21:47 -0700	[thread overview]
Message-ID: <202009171519.951D26DB@keescook> (raw)
In-Reply-To: <20200917113540.GA1742660@google.com>

On Thu, Sep 17, 2020 at 11:35:40AM +0000, George Popescu wrote:
> On Thu, Sep 17, 2020 at 08:37:07AM +0200, Marco Elver wrote:
> > So, it seems that local-bounds can still catch some rare OOB accesses,
> > where KASAN fails to catch it because the access might skip over the
> > redzone.
> > 
> > The other more interesting bit of history is that
> > -fsanitize=local-bounds used to be -fbounds-checking, and meant for
> > production use as a hardening feature:
> > http://lists.llvm.org/pipermail/llvm-dev/2012-May/049972.html
> > 
> > And local-bounds just does not behave like any other sanitizer as a
> > result, it just traps. The fact that it's enabled via
> > -fsanitize=local-bounds (or just bounds) but hasn't much changed in
> > behaviour is a little unfortunate.
> 
> > I suppose there are 3 options:
> > 
> > 1. George implements trap handling somehow. Is this feasible? If not,
> > why not? Maybe that should also have been explained in the commit
> > message.
> > 
> > 2. Only enable -fsanitize=local-bounds if UBSAN_TRAP was selected, at
> > least for as long as Clang traps for local-bounds. I think this makes
> > sense either way, because if we do not expect UBSAN to trap, it really
> > should not trap!
> > 
> > 3. Change the compiler. As always, this will take a while to implement
> > and then to reach whoever should have that updated compiler.
> > 
> > Preferences?
> Considering of what you said above, I find option 2 the most elegant.
> The first one doesn't sound doable for the moment, also the third.
> I will edit this patch considering your comments and resend it to the
> list.

I have a slightly different suggestion that is very nearly #2 above:
split local-bounds into a separate CONFIG that requires UBSAN_TRAP, and
then carefully document both:
- what does it catch that "bounds" doesn't
- why it only operates in trap mode

The rationale I have is that I don't like the coverage of some
mitigation or detection to "silently" vary between builds. e.g. someone
would build with/without UBSAN_TRAP and end up with unexpectedly
different coverage. I'd rather there be a separate CONFIG that appears.

-- 
Kees Cook