Subject: Re: [PATCH net 2/2] bpf/verifier: fix min/max handling in BPF_SUB

From: Daniel Borkmann via iovisor-dev <iovisor-dev-9jONkmmOlFHEE9lA1F8Ukti2O/[email-masked]>
Date: Fri, 21 Jul 2017 16:30:17 +0200

On 07/21/2017 03:37 PM, Edward Cree wrote:
> We have to subtract the src max from the dst min, and vice-versa, since
>   (e.g.) the smallest result comes from the largest subtrahend.
>
> Fixes: 484611357c19 ("bpf: allow access into map value arrays")
> Signed-off-by: Edward Cree <ecree-s/n/[email-masked]>

LGTM, thanks for the fix!

Acked-by: Daniel Borkmann <[email-masked]>

> ---
>   kernel/bpf/verifier.c | 21 +++++++++++++++------
>   1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index af9e84a..664d939 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1865,10 +1865,12 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
>   	 * do our normal operations to the register, we need to set the values
>   	 * to the min/max since they are undefined.
>   	 */
> -	if (min_val == BPF_REGISTER_MIN_RANGE)
> -		dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
> -	if (max_val == BPF_REGISTER_MAX_RANGE)
> -		dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> +	if (opcode != BPF_SUB) {
> +		if (min_val == BPF_REGISTER_MIN_RANGE)
> +			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
> +		if (max_val == BPF_REGISTER_MAX_RANGE)
> +			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> +	}
>
>   	switch (opcode) {
>   	case BPF_ADD:
> @@ -1879,10 +1881,17 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
>   		dst_reg->min_align = min(src_align, dst_align);
>   		break;
>   	case BPF_SUB:
> +		/* If one of our values was at the end of our ranges, then the
> +		 * _opposite_ value in the dst_reg goes to the end of our range.
> +		 */
> +		if (min_val == BPF_REGISTER_MIN_RANGE)
> +			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> +		if (max_val == BPF_REGISTER_MAX_RANGE)
> +			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
>   		if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
> -			dst_reg->min_value -= min_val;
> +			dst_reg->min_value -= max_val;
>   		if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
> -			dst_reg->max_value -= max_val;
> +			dst_reg->max_value -= min_val;
>   		dst_reg->min_align = min(src_align, dst_align);
>   		break;
>   	case BPF_MUL:
>