linux-hardening - Re: [PATCH 0/3] kbuild: remove many tool coverage variables

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Date: Mon, 13 May 2024 21:54:38 +0200
From: Marco Elver <elver@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, linux-kbuild@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Peter Oberparleiter <oberpar@...ux.ibm.com>, 
	Roberto Sassu <roberto.sassu@...weicloud.com>, Johannes Berg <johannes@...solutions.net>, 
	kasan-dev@...glegroups.com, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/3] kbuild: remove many tool coverage variables

On Mon, 13 May 2024 at 20:48, Kees Cook <keescook@...omium.org> wrote:
>
> In the future can you CC the various maintainers of the affected
> tooling? :)
>
> On Mon, May 06, 2024 at 10:35:41PM +0900, Masahiro Yamada wrote:
> >
> > This patch set removes many instances of the following variables:
> >
> >   - OBJECT_FILES_NON_STANDARD
> >   - KASAN_SANITIZE
> >   - UBSAN_SANITIZE
> >   - KCSAN_SANITIZE
> >   - KMSAN_SANITIZE
> >   - GCOV_PROFILE
> >   - KCOV_INSTRUMENT
> >
> > Such tools are intended only for kernel space objects, most of which
> > are listed in obj-y, lib-y, or obj-m.

I welcome the simplification, but see below.

> This is a reasonable assertion, and the changes really simplify things
> now and into the future. Thanks for finding such a clean solution! I
> note that it also immediately fixes the issue noticed and fixed here:
> https://lore.kernel.org/all/20240513122754.1282833-1-roberto.sassu@huaweicloud.com/
>
> > The best guess is, objects in $(obj-y), $(lib-y), $(obj-m) can opt in
> > such tools. Otherwise, not.
> >
> > This works in most places.
>
> I am worried about the use of "guess" and "most", though. :) Before, we
> had some clear opt-out situations, and now it's more of a side-effect. I
> think this is okay, but I'd really like to know more about your testing.
>
> It seems like you did build testing comparing build flags, since you
> call out some of the explicit changes in patch 2, quoting:
>
> >  - include arch/mips/vdso/vdso-image.o into UBSAN, GCOV, KCOV
> >  - include arch/sparc/vdso/vdso-image-*.o into UBSAN
> >  - include arch/sparc/vdso/vma.o into UBSAN
> >  - include arch/x86/entry/vdso/extable.o into KASAN, KCSAN, UBSAN, GCOV, KCOV
> >  - include arch/x86/entry/vdso/vdso-image-*.o into KASAN, KCSAN, UBSAN, GCOV, KCOV
> >  - include arch/x86/entry/vdso/vdso32-setup.o into KASAN, KCSAN, UBSAN, GCOV, KCOV
> >  - include arch/x86/entry/vdso/vma.o into GCOV, KCOV
> >  - include arch/x86/um/vdso/vma.o into KASAN, GCOV, KCOV
>
> I would agree that these cases are all likely desirable.
>
> Did you find any cases where you found that instrumentation was _removed_
> where not expected?

In addition, did you boot test these kernels? While I currently don't
recall if the vdso code caused us problems (besides the linking
problem for non-kernel objects), anything that is opted out from
instrumentation in arch/ code needs to be carefully tested if it
should be opted back into instrumentation. We had many fun hours
debugging boot hangs or other recursion issues due to instrumented
arch code.

Thanks,
-- Marco