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

Kees Cook
2021-12-03T16:21:56Z

On December 3, 2021 2:49:53 AM PST, Marco Elver <elver@google.com> wrote:
>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.

I will double check, but I think it's only the object-size test, which seems to confirm my suspicion that -Warray-bounds provides sufficient coverage and object-size can be removed.

I have another patch I intend to send today for the sk_buff/sk_buff_head issue, as -Warray-bounds warns for that as well.

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

Are there any cases where object-size does a run-time check that couldn't be done at compile time? That's the only reason I could see to keep it at this point, as -Warray-bounds can do the compile time checks.

>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

Thanks, I'll refer to that in my sk_buff patch. It seems -Warray-bounds suffers from the same conservativism about object casts, which is frustrating on the one hand since the warning can be a false positive (cast vs access), but on the other, it does call attention to fragile arrangements which maybe could do with adjustment.

>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.

And maybe:
5. -Warray-bounds provides the same coverage and is about to be enabled globally.

--
Kees Cook