netdev - Re: [PATCHv6 net-next 04/15] bpf: don't (ab)use instructions to store state

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Date:   Mon, 19 Sep 2016 23:55:49 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
CC:     netdev@...r.kernel.org, ast@...nel.org, kubakici@...pl
Subject: Re: [PATCHv6 net-next 04/15] bpf: don't (ab)use instructions to store
 state

On 09/19/2016 11:48 PM, Jakub Kicinski wrote:
> On Mon, 19 Sep 2016 23:44:50 +0200, Daniel Borkmann wrote:
>> On 09/19/2016 11:36 PM, Jakub Kicinski wrote:
>>> On Mon, 19 Sep 2016 23:03:17 +0200, Daniel Borkmann wrote:
>>>> On 09/18/2016 05:09 PM, Jakub Kicinski wrote:
>>>>> Storing state in reserved fields of instructions makes
>>>>> it impossible to run verifier on programs already
>>>>> marked as read-only. Allocate and use an array of
>>>>> per-instruction state instead.
>>>>>
>>>>> While touching the error path rename and move existing
>>>>> jump target.
>>>>>
>>>>> Suggested-by: Alexei Starovoitov <ast@...nel.org>
>>>>> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
>>>>> Acked-by: Alexei Starovoitov <ast@...nel.org>
>>>>> Acked-by: Daniel Borkmann <daniel@...earbox.net>
>>>>
>>>> I believe there's still an issue here. Could you please double check
>>>> and confirm?
>>>>
>>>> I rebased my locally pending stuff on top of your set and suddenly my
>>>> test case breaks. So I did a bisect and it pointed me to this commit
>>>> eventually.
>>>>
>>>> [...]
>>>>> @@ -2697,11 +2706,8 @@ static int convert_ctx_accesses(struct verifier_env *env)
>>>>>     		else
>>>>>     			continue;
>>>>>
>>>>> -		if (insn->imm != PTR_TO_CTX) {
>>>>> -			/* clear internal mark */
>>>>> -			insn->imm = 0;
>>>>> +		if (env->insn_aux_data[i].ptr_type != PTR_TO_CTX)
>>>>>     			continue;
>>>>> -		}
>>>>>
>>>>>     		cnt = env->prog->aux->ops->
>>>>>     			convert_ctx_access(type, insn->dst_reg, insn->src_reg,
>>>>
>>>> Looking at the code, I believe the issue is in above snippet. In the
>>>> convert_ctx_accesses() rewrite loop, each time we bpf_patch_insn_single()
>>>> a program, the program can grow in size (due to __sk_buff access rewrite,
>>>> for example). After rewrite, we do 'i += insn_delta' for adjustment to
>>>> process next insn.
>>>>
>>>> However, env->insn_aux_data is alloced under the assumption that the
>>>> very initial, pre-verification prog->len doesn't change, right? So in
>>>> the above conversion access to env->insn_aux_data[i].ptr_type is off,
>>>> since after rewrites, corresponding mappings to ptr_type might not be
>>>> related anymore.
>>>>
>>>> I noticed this with direct packet access where suddenly the data vs
>>>> data_end test failed and contained some "semi-random" value always
>>>> bailing out for me.
>>>
>>> You are correct.  Should I respin or would you like to post your set? :)
>>
>> Heh, if you don't mind I would go ahead tonight, the conflict at two spots
>> when exposing verifier is really minor turns out. Are you okay with this?
>
> Yes, please go ahead :)

Ok, thanks!

>> What's the plan wrt env->insn_aux_data? Realloc plus rewrite of the array,
>> or do you see a more straight forward solution?
>
> I was thinking about something like this: (untested)

Yep, much better. :)