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

[Kees Cook]

On January 22, 2024 8:00:26 PM PST, Yonghong Song <yonghong.song@xxxxxxxxx> 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@xxxxxxxxxx>
>> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
>> Cc: John Fastabend <john.fastabend@xxxxxxxxx>
>> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
>> Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
>> Cc: Song Liu <song@xxxxxxxxxx>
>> Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
>> Cc: KP Singh <kpsingh@xxxxxxxxxx>
>> Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
>> Cc: Hao Luo <haoluo@xxxxxxxxxx>
>> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
>> Cc: bpf@xxxxxxxxxxxxxxx
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>   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@xxxxxxxxxxxx/

The cover letter also has more details:
https://lore.kernel.org/linux-hardening/20240122235208.work.748-kees@xxxxxxxxxx/

>The patch itselfs looks good to me.

Thanks!

-Kees

-- 
Kees Cook