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Date:   Thu, 17 Sep 2020 11:35:40 +0000
From:   George Popescu <georgepope@google.com>
To:     Marco Elver <elver@google.com>
Cc:     Kees Cook <keescook@chromium.org>, 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>,
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        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
Message-ID: <20200917113540.GA1742660@google.com>
References: <202009141509.CDDC8C8@keescook>
 <20200915102458.GA1650630@google.com>
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 <20200916074027.GA2946587@google.com>
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 <20200916121401.GA3362356@google.com>
 <20200916134029.GA1146904@elver.google.com>
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On Thu, Sep 17, 2020 at 08:37:07AM +0200, Marco Elver wrote:
> On Wed, 16 Sep 2020 at 15:40, Marco Elver <elver@google.com> wrote:
> > On Wed, Sep 16, 2020 at 12:14PM +0000, George Popescu wrote:
> > > On Wed, Sep 16, 2020 at 10:32:40AM +0200, Marco Elver wrote:
> > > > On Wed, 16 Sep 2020 at 09:40, George Popescu <georgepope@google.com> wrote:
> > > > > On Tue, Sep 15, 2020 at 07:32:28PM +0200, Marco Elver wrote:
> > > > > > On Tue, 15 Sep 2020 at 14:01, George Popescu <georgepope@google.com> wrote:
> > > > > > > On Tue, Sep 15, 2020 at 01:18:11PM +0200, Marco Elver wrote:
> > > > > > > > On Tue, 15 Sep 2020 at 12:25, George Popescu <georgepope@google.com> wrote:
> > > > > > > > > On Mon, Sep 14, 2020 at 03:13:14PM -0700, Kees Cook wrote:
> > > > > > > > > > On Mon, Sep 14, 2020 at 05:27:42PM +0000, George-Aurelian Popescu wrote:
> > > > > > > > > > > From: George Popescu <georgepope@google.com>
> > > > > > > > > > >
> > > > > > > > > > > When the kernel is compiled with Clang, UBSAN_BOUNDS inserts a brk after
> > > > > > > > > > > the handler call, preventing it from printing any information processed
> > > > > > > > > > > inside the buffer.
> > > > > > > > > > > For Clang -fsanitize=bounds expands to -fsanitize=array-bounds and
> > > > > > > > > > > -fsanitize=local-bounds, and the latter adds a brk after the handler
> > > > > > > > > > > call
> > > > > > > > > >
> > > > > > > > > This would mean losing the local-bounds coverage. I tried to  test it without
> > > > > > > > > local-bounds and with a locally defined array on the stack and it works fine
> > > > > > > > > (the handler is called and the error reported). For me it feels like
> > > > > > > > > --array-bounds and --local-bounds are triggered for the same type of
> > > > > > > > > undefined_behaviours but they are handling them different.
> > > > > > > >
> > > > > > > > Does -fno-sanitize-trap=bounds help?
> > [...]
> > > > Your full config would be good, because it includes compiler version etc.
> > > My full config is:
> >
> > Thanks. Yes, I can reproduce, and the longer I keep digging I start
> > wondering why we have local-bounds at all.
> >
> > It appears that local-bounds finds a tiny subset of the issues that
> > KASAN finds:
> >
> >         http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20131021/091536.html
> >         http://llvm.org/viewvc/llvm-project?view=revision&revision=193205
> >
> > fsanitize=undefined also does not include local-bounds:
> >
> >         https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#available-checks
> >
> > And the reason is that we do want to enable KASAN and UBSAN together;
> > but local-bounds is useless overhead if we already have KASAN.
> >
> > I'm inclined to say that what you propose is reasonable (but the commit
> > message needs to be more detailed explaining the relationship with
> > KASAN) -- but I have no idea if this is going to break somebody's
> > usecase (e.g. find some OOB bugs, but without KASAN -- but then why not
> > use KASAN?!)
> 
> 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.
Thank you for your support.

Thanks,
George