linux-kernel - Re: [PATCH 06/14] Fix CFLAGS for UBSAN

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

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