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

  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(-)
  

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.
The patch itselfs looks good to me.

>   			dst_reg->umin_value = 0;
>   			dst_reg->umax_value = U64_MAX;
>   		} else {
> @@ -13023,8 +13023,8 @@ static void scalar32_min_max_add(struct bpf_reg_state *dst_reg,
>   		dst_reg->s32_min_value += smin_val;
>   		dst_reg->s32_max_value += smax_val;
>   	}
> -	if (dst_reg->u32_min_value + umin_val < umin_val ||
> -	    dst_reg->u32_max_value + umax_val < umax_val) {
> +	if (add_would_overflow(umin_val, dst_reg->u32_min_value) ||
> +	    add_would_overflow(umax_val, dst_reg->u32_max_value)) {
>   		dst_reg->u32_min_value = 0;
>   		dst_reg->u32_max_value = U32_MAX;
>   	} else {
> @@ -13049,8 +13049,8 @@ static void scalar_min_max_add(struct bpf_reg_state *dst_reg,
>   		dst_reg->smin_value += smin_val;
>   		dst_reg->smax_value += smax_val;
>   	}
> -	if (dst_reg->umin_value + umin_val < umin_val ||
> -	    dst_reg->umax_value + umax_val < umax_val) {
> +	if (add_would_overflow(umin_val, dst_reg->umin_value) ||
> +	    add_would_overflow(umax_val, dst_reg->umax_value)) {
>   		dst_reg->umin_value = 0;
>   		dst_reg->umax_value = U64_MAX;
>   	} else {
  

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/

>The patch itselfs looks good to me.

Thanks!

-Kees
  

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
>