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Date: Mon, 22 Jan 2024 20:07:52 -0800
From: Kees Cook <kees@kernel.org>
To: Yonghong Song <yonghong.song@linux.dev>, Kees Cook <keescook@chromium.org>,
 linux-hardening@vger.kernel.org
CC: Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>,
 John Fastabend <john.fastabend@gmail.com>,
 Andrii Nakryiko <andrii@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>,
 Song Liu <song@kernel.org>, KP Singh <kpsingh@kernel.org>,
 Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
 Jiri Olsa <jolsa@kernel.org>, bpf@vger.kernel.org,
 "Gustavo A. R. Silva" <gustavoars@kernel.org>,
 Bill Wendling <morbo@google.com>, Justin Stitt <justinstitt@google.com>,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH 43/82] bpf: Refactor intentional wrap-around test
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On January 22, 2024 8:00:26 PM PST, Yonghong Song <yonghong=2Esong@linux=
=2Edev> 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=2E One of the most common code patterns of this is:
>>=20
>> 	VAR + value < VAR
>>=20
>> 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)=2E Regardless, w=
e
>> 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=2E
>>=20
>> Refactor open-coded wrap-around addition test to use add_would_overflow=
()=2E
>> This paves the way to enabling the wrap-around sanitizers in the future=
=2E
>>=20
>> Link: https://git=2Ekernel=2Eorg/linus/68df3755e383e6fecf2354a67b08f92f=
18536594 [1]
>> Link: https://github=2Ecom/KSPP/linux/issues/26 [2]
>> Link: https://github=2Ecom/KSPP/linux/issues/27 [3]
>> Link: https://github=2Ecom/KSPP/linux/issues/344 [4]
>> Cc: Alexei Starovoitov <ast@kernel=2Eorg>
>> Cc: Daniel Borkmann <daniel@iogearbox=2Enet>
>> Cc: John Fastabend <john=2Efastabend@gmail=2Ecom>
>> Cc: Andrii Nakryiko <andrii@kernel=2Eorg>
>> Cc: Martin KaFai Lau <martin=2Elau@linux=2Edev>
>> Cc: Song Liu <song@kernel=2Eorg>
>> Cc: Yonghong Song <yonghong=2Esong@linux=2Edev>
>> Cc: KP Singh <kpsingh@kernel=2Eorg>
>> Cc: Stanislav Fomichev <sdf@google=2Ecom>
>> Cc: Hao Luo <haoluo@google=2Ecom>
>> Cc: Jiri Olsa <jolsa@kernel=2Eorg>
>> Cc: bpf@vger=2Ekernel=2Eorg
>> Signed-off-by: Kees Cook <keescook@chromium=2Eorg>
>> ---
>>   kernel/bpf/verifier=2Ec | 12 ++++++------
>>   1 file changed, 6 insertions(+), 6 deletions(-)
>>=20
>> diff --git a/kernel/bpf/verifier=2Ec b/kernel/bpf/verifier=2Ec
>> index 65f598694d55=2E=2E21e3f30c8757 100644
>> --- a/kernel/bpf/verifier=2Ec
>> +++ b/kernel/bpf/verifier=2Ec
>> @@ -12901,8 +12901,8 @@ static int adjust_ptr_min_max_vals(struct bpf_v=
erifier_env *env,
>>   			dst_reg->smin_value =3D smin_ptr + smin_val;
>>   			dst_reg->smax_value =3D 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=
=2E

Sure! It was earlier in the series:
https://lore=2Ekernel=2Eorg/linux-hardening/20240123002814=2E1396804-2-kee=
scook@chromium=2Eorg/

The cover letter also has more details:
https://lore=2Ekernel=2Eorg/linux-hardening/20240122235208=2Ework=2E748-ke=
es@kernel=2Eorg/

>The patch itselfs looks good to me=2E

Thanks!

-Kees

--=20
Kees Cook