Re: [PATCH] lib/test_ubsan: Silence compile-time array bounds warnings

From: Marco Elver <elver@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Jarkko Sakkinen <jarkko@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Jiri Olsa <jolsa@kernel.org>,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] lib/test_ubsan: Silence compile-time array bounds warnings
Date: Fri, 3 Dec 2021 11:49:53 +0100	[thread overview]
Message-ID: <CANpmjNPCjXp_0iQjdznpvS2vF-VvDkxJrtWnBAoj6TAU488VAQ@mail.gmail.com> (raw)
In-Reply-To: <20211203093000.3714620-1-keescook@chromium.org>

On Fri, 3 Dec 2021 at 10:30, Kees Cook <keescook@chromium.org> wrote:
> The UBSAN tests intentionally operate beyond array bounds, so silence
> the warning visible with a -Warray-bounds build:
>
> lib/test_ubsan.c: In function 'test_ubsan_object_size_mismatch':
> lib/test_ubsan.c:109:16: error: array subscript 'long long int[0]' is partly outside array bounds of 'volatile int[1]' [-Werror=array-bounds]
>   109 |         val2 = *ptr;
>       |                ^~~~
> lib/test_ubsan.c:104:22: note: while referencing 'val'
>   104 |         volatile int val __aligned(8) = 4;
>       |                      ^~~
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  lib/Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/lib/Makefile b/lib/Makefile
> index 08959b10bac9..2742a54a4275 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -70,6 +70,7 @@ obj-$(CONFIG_KASAN_MODULE_TEST) += test_kasan_module.o
>  CFLAGS_test_kasan_module.o += -fno-builtin
>  obj-$(CONFIG_TEST_UBSAN) += test_ubsan.o
>  CFLAGS_test_ubsan.o += $(call cc-disable-warning, vla)
> +CFLAGS_test_ubsan.o += $(call cc-disable-warning, array-bounds)
>  UBSAN_SANITIZE_test_ubsan.o := y
>  obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
>  obj-$(CONFIG_TEST_LIST_SORT) += test_list_sort.o

Are there other warnings or only the one for the fsanitize=object-size
test? I think this is fine if there are other warnings.

But, if it's only about the fsanitize=object-size test, I'm going to
propose something more drastic. :-)

I had wanted to wait a bit and dig a little deeper, but I just posted
part of my analysis here:
https://bugzilla.kernel.org/show_bug.cgi?id=214861#c4

My proposal is to remove UBSAN_OBJECT_SIZE and its related tests. The
bugzilla bug goes into the details, but the TLDR is:
1. fsanitize=object-size is incomplete,
2. it should have been a compiler warning,
3. for everything else there is KASAN which detects real OOB,
4. for GCC we already disable UBSAN_OBJECT_SIZE.

Thanks,
-- Marco