Re: [PATCH 43/82] bpf: Refactor intentional wrap-around test

[Yonghong Song <yonghong.song@linux.dev>]

On 1/22/24 8:07 PM, Kees Cook wrote:
>
> On January 22, 2024 8:00:26 PM PST, Yonghong Song <yonghong.song@linux.dev> wrote:
>> On 1/22/24 4:27 PM, Kees Cook wrote:
>>> In an effort to separate intentional arithmetic wrap-around from
>>> unexpected wrap-around, we need to refactor places that depend on this
>>> kind of math. One of the most common code patterns of this is:
>>>
>>> 	VAR + value < VAR
>>>
>>> Notably, this is considered "undefined behavior" for signed and pointer
>>> types, which the kernel works around by using the -fno-strict-overflow
>>> option in the build[1] (which used to just be -fwrapv). Regardless, we
>>> want to get the kernel source to the position where we can meaningfully
>>> instrument arithmetic wrap-around conditions and catch them when they
>>> are unexpected, regardless of whether they are signed[2], unsigned[3],
>>> or pointer[4] types.
>>>
>>> Refactor open-coded wrap-around addition test to use add_would_overflow().
>>> This paves the way to enabling the wrap-around sanitizers in the future.
>>>
>>> Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
>>> Link: https://github.com/KSPP/linux/issues/26 [2]
>>> Link: https://github.com/KSPP/linux/issues/27 [3]
>>> Link: https://github.com/KSPP/linux/issues/344 [4]
>>> Cc: Alexei Starovoitov <ast@kernel.org>
>>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>>> Cc: John Fastabend <john.fastabend@gmail.com>
>>> Cc: Andrii Nakryiko <andrii@kernel.org>
>>> Cc: Martin KaFai Lau <martin.lau@linux.dev>
>>> Cc: Song Liu <song@kernel.org>
>>> Cc: Yonghong Song <yonghong.song@linux.dev>
>>> Cc: KP Singh <kpsingh@kernel.org>
>>> Cc: Stanislav Fomichev <sdf@google.com>
>>> Cc: Hao Luo <haoluo@google.com>
>>> Cc: Jiri Olsa <jolsa@kernel.org>
>>> Cc: bpf@vger.kernel.org
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>> ---
>>>    kernel/bpf/verifier.c | 12 ++++++------
>>>    1 file changed, 6 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index 65f598694d55..21e3f30c8757 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -12901,8 +12901,8 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
>>>    			dst_reg->smin_value = smin_ptr + smin_val;
>>>    			dst_reg->smax_value = smax_ptr + smax_val;
>>>    		}
>>> -		if (umin_ptr + umin_val < umin_ptr ||
>>> -		    umax_ptr + umax_val < umax_ptr) {
>>> +		if (add_would_overflow(umin_ptr, umin_val) ||
>>> +		    add_would_overflow(umax_ptr, umax_val)) {
>> Maybe you could give a reference to the definition of add_would_overflow()?
>> A link or a patch with add_would_overflow() defined cc'ed to bpf program.
> Sure! It was earlier in the series:
> https://lore.kernel.org/linux-hardening/20240123002814.1396804-2-keescook@chromium.org/
>
> The cover letter also has more details:
> https://lore.kernel.org/linux-hardening/20240122235208.work.748-kees@kernel.org/

Thanks for the pointer.

Acked-by: Yonghong Song <yonghong.song@linux.dev>

>
>> The patch itselfs looks good to me.
> Thanks!
>
> -Kees
>