Re: [PATCH] Kbuild: disable FORTIFY_SOURCE on clang-10

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Kees Cook <keescook@chromium.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	"open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
	<linux-crypto@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Arnd Bergmann <arnd@arndb.de>, George Burgess <gbiv@google.com>
Subject: Re: [PATCH] Kbuild: disable FORTIFY_SOURCE on clang-10
Date: Tue, 5 May 2020 17:22:01 -0600	[thread overview]
Message-ID: <CAHmME9q3zFe4e1xnpptJ27zywGqngZK2K7LCVzDSoG__ht=fNA@mail.gmail.com> (raw)
In-Reply-To: <202005051617.F9B32B5526@keescook>

On Tue, May 5, 2020 at 5:19 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Tue, May 05, 2020 at 04:37:38PM -0600, Jason A. Donenfeld wrote:
> > On Tue, May 5, 2020 at 4:25 PM Nathan Chancellor
> > <natechancellor@gmail.com> wrote:
> > > I believe these issues are one in the same. I did a reverse bisect with
> > > Arnd's test case and converged on George's first patch:
> > >
> > > https://github.com/llvm/llvm-project/commit/2dd17ff08165e6118e70f00e22b2c36d2d4e0a9a
> > >
> > > I think that in lieu of this patch, we should have that patch and its
> > > follow-up fix merged into 10.0.1.
> >
> > If this is fixed in 10.0.1, do we even need to patch the kernel at
> > all? Or can we just leave it be, considering most organizations using
> > clang know what they're getting into? I'd personally prefer the
> > latter, so that we don't clutter things.
>
> I agree: I'd rather this was fixed in 10.0.1 (but if we do want a
> kernel-side work-around for 10.0.0, I would suggest doing the version
> check in the Kconfig for FORTIFY_SOURCE instead of in the Makefile,
> as that's where these things are supposed to live these days).

Indeed this belongs in the Makefile. I can send a patch adjusting
that, if you want, but I think I'd rather do nothing and have a fix be
rolled out in 10.0.1. Clang users should know what to expect in that
regard.

> (Though as was mentioned, it's likely that FORTIFY_SOURCE isn't working
> _at all_ under Clang, so I may still send a patch to depend on !clang
> just to avoid surprises until it's fixed, but I haven't had time to
> chase down a solution yet.)

That might be the most coherent thing to do, at least so that people
don't get some false sense of mitigation.

Jason