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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 30 Jun 2020 15:57:54 -0700
Message-ID: <CABCJKudxmrSNNzgPkc4NHt71rfdjAqFbb9n49S4QBDZPQ52e0w@mail.gmail.com>
Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y
To:     Marco Elver <elver@google.com>
Cc:     Will Deacon <will@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Matt Turner <mattst88@gmail.com>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Richard Henderson <rth@twiddle.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Boqun Feng <boqun.feng@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-alpha@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Android Kernel Team <kernel-team@android.com>
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On Tue, Jun 30, 2020 at 12:47 PM Marco Elver <elver@google.com> wrote:
>
> On Tue, 30 Jun 2020 at 19:39, Will Deacon <will@kernel.org> wrote:
> >
> > When building with LTO, there is an increased risk of the compiler
> > converting an address dependency headed by a READ_ONCE() invocation
> > into a control dependency and consequently allowing for harmful
> > reordering by the CPU.
> >
> > Ensure that such transformations are harmless by overriding the generic
> > READ_ONCE() definition with one that provides acquire semantics when
> > building with LTO.
> >
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  arch/arm64/include/asm/rwonce.h   | 63 +++++++++++++++++++++++++++++++
> >  arch/arm64/kernel/vdso/Makefile   |  2 +-
> >  arch/arm64/kernel/vdso32/Makefile |  2 +-
> >  3 files changed, 65 insertions(+), 2 deletions(-)
> >  create mode 100644 arch/arm64/include/asm/rwonce.h
>
> This seems reasonable, given we can't realistically tell the compiler
> about dependent loads. What (if any), is the performance impact? I
> guess this also heavily depends on the actual silicon.
>
> I do wonder, though, if there is some way to make the compiler do
> something better for us. Clearly, implementing real
> memory_order_consume hasn't worked out until today. But maybe the
> compiler could promote dependent loads to acquires if it recognizes it
> lost dependencies during optimizations. Just thinking out loud, it
> probably still has some weird corner case that will break. ;-)
>
> The other thing is that I'd be cautious blaming LTO, as I tried to
> summarize here:
> https://lore.kernel.org/kernel-hardening/20200630191931.GA884155@elver.google.com/
>
> The main thing is that, yes, this might be something to be worried
> about, but if we are worried about it, we need to be worried about it
> in *all* builds (LTO or not). My guess is that's not acceptable. Would
> it be better to just guard the promotion of READ_ONCE() to acquire
> behind a config option like CONFIG_ACQUIRE_READ_DEPENDENCIES, and then
> make LTO select that (or maybe leave it optional?). In future, for
> very aggressive non-LTO compilers even, one may then also select that
> if there is substantiated worry things do actually break.

I agree, a separate config option would be better here.

Also Will, the LTO patches use CONFIG_LTO_CLANG instead of CLANG_LTO.

Sami