netdev - Re: [PATCH bpf 8/9] bpf: fix integer overflows

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Date:   Tue, 19 Dec 2017 10:29:13 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        Jann Horn <jannh@...gle.com>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH bpf 8/9] bpf: fix integer overflows

On 19/12/17 04:12, Alexei Starovoitov wrote:
> Also reduce the scope of "scalar op scalar" tracking.
<snip>
> @@ -2046,6 +2088,12 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
>  	src_known = tnum_is_const(src_reg.var_off);
>  	dst_known = tnum_is_const(dst_reg->var_off);
>  
> +	if (!src_known &&
> +	    opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
> +		__mark_reg_unknown(dst_reg);
> +		return 0;
> +	}
> +
>  	switch (opcode) {
>  	case BPF_ADD:
>  		if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
Still not seeing any explanation for why this change is necessary.
It also seems arbitrary, e.g. why is AND allowed but not OR?

Hypothetical use case: combining a series of flags based on data read from
 packet into an array index used for storing into a map value.  That sounds
 to me like something someone may be legitimately doing.

-Ed