netdev - Re: [PATCH bpf-next] bpf: relax verifier restriction on BPF_MOV

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Date:   Wed, 5 Dec 2018 15:32:50 +0000
From:   Jiong Wang <jiong.wang@...ronome.com>
To:     Edward Cree <ecree@...arflare.com>, ast@...nel.org,
        daniel@...earbox.net
Cc:     netdev@...r.kernel.org, oss-drivers@...ronome.com
Subject: Re: [PATCH bpf-next] bpf: relax verifier restriction on BPF_MOV |
 BPF_ALU

On 05/12/2018 14:52, Edward Cree wrote:
> On 05/12/18 09:46, Jiong Wang wrote:
>> There is NO processed instruction number regression, either with or without
>> -mattr=+alu32.
> <snip>
>> Cilium bpf
>> ===
>> bpf_lb-DLB_L3.o         2110/2110    1730/1733
> That looks like a regression of 3 insns in the 32-bit case.
> May be worth investigating why.

Will look into this.

>
>> +			dst_reg = insn->dst_reg;
>> +			regs[dst_reg] = regs[src_reg];
>> +			if (BPF_CLASS(insn->code) == BPF_ALU) {
>> +				/* Update type and range info. */
>> +				regs[dst_reg].type = SCALAR_VALUE;
>> +				coerce_reg_to_size(&regs[dst_reg], 4);
> Won't this break when handed a pointer (as root, so allowed to leak
>   it)?  E.g. (pointer + x) gets turned into scalar x, rather than
>   unknown scalar in range [0, 0xffffffff].

Initially I was gating this to scalar_value only, later was thinking it
could be extended to ptr case if ptr leak is allowed.

But, your comment remind me min/max value doesn't mean real min/max value
for ptr types value, it means the offset only if I am understanding the
issue correctly. So, it will break pointer case.

Regards,
Jiong

> The existing behaviour is correct for pointers: 32 unknown bits,
>   because the value of the pointer base is unknown.
> It's only for scalar_values that you want to copy and truncate the
>   var_off and min/max from the src_reg.
>
> -Ed