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Date:   Sat, 27 Aug 2016 23:18:23 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Mack <daniel@...que.org>,
        "David S . Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        Sargun Dhillon <sargun@...gun.me>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-security-module@...r.kernel.org, netdev@...r.kernel.org,
        Tejun Heo <tj@...nel.org>
Subject: Re: [RFC v2 09/10] landlock: Handle cgroups (program types)


On 27/08/2016 22:56, Alexei Starovoitov wrote:
> On Sat, Aug 27, 2016 at 09:55:01PM +0200, Mickaël Salaün wrote:
>>
>> On 27/08/2016 20:19, Alexei Starovoitov wrote:
>>> On Sat, Aug 27, 2016 at 04:34:55PM +0200, Mickaël Salaün wrote:
>>>>
>>>> On 27/08/2016 01:05, Alexei Starovoitov wrote:
>>>>> On Fri, Aug 26, 2016 at 05:10:40PM +0200, Mickaël Salaün wrote:
>>>>>
>>>>>>> As far as safety and type checking that bpf programs has to do,
>>>>>>> I like the approach of patch 06/10:
>>>>>>> +LANDLOCK_HOOK2(file_open, FILE_OPEN,
>>>>>>> +       PTR_TO_STRUCT_FILE, struct file *, file,
>>>>>>> +       PTR_TO_STRUCT_CRED, const struct cred *, cred
>>>>>>> +)
>>>>>>> teaching verifier to recognize struct file, cred, sockaddr
>>>>>>> will let bpf program access them naturally without any overhead.
>>>>>>> Though:
>>>>>>> @@ -102,6 +102,9 @@ enum bpf_prog_type {
>>>>>>>         BPF_PROG_TYPE_SCHED_CLS,
>>>>>>>         BPF_PROG_TYPE_SCHED_ACT,
>>>>>>>         BPF_PROG_TYPE_TRACEPOINT,
>>>>>>> +       BPF_PROG_TYPE_LANDLOCK_FILE_OPEN,
>>>>>>> +       BPF_PROG_TYPE_LANDLOCK_FILE_PERMISSION,
>>>>>>> +       BPF_PROG_TYPE_LANDLOCK_MMAP_FILE,
>>>>>>>  };
>>>>>>> is a bit of overkill.
>>>>>>> I think it would be cleaner to have single
>>>>>>> BPF_PROG_TYPE_LSM and at program load time pass
>>>>>>> lsm_hook_id as well, so that verifier can do safety checks
>>>>>>> based on type info provided in LANDLOCK_HOOKs
>>>>>>
>>>>>> I first started with a unique BPF_PROG_TYPE but, the thing is, the BPF
>>>>>> verifier check programs according to their types. If we need to check
>>>>>> specific context value types (e.g. PTR_TO_STRUCT_FILE), we need a
>>>>>> dedicated program types. I don't see any other way to do it with the
>>>>>> current verifier code. Moreover it's the purpose of program types, right?
>>>>>
>>>>> Adding new bpf program type for every lsm hook is not acceptable.
>>>>> Either do one new program type + pass lsm_hook_id as suggested
>>>>> or please come up with an alternative approach.
>>>>
>>>> OK, so we have to modify the verifier to not only rely on the program
>>>> type but on another value to check the context accesses. Do you have a
>>>> hint from where this value could come from? Do we need to add a new bpf
>>>> command to associate a program to a subtype?
>>>
>>> It's another field prog_subtype (or prog_hook_id) in union bpf_attr.
>>> Both prog_type and prog_hook_id are used during verification.
>>> prog_type distinguishes the main aspects whereas prog_hook_id selects
>>> which lsm_hook's argument definition to apply.
>>> At the time of attaching to a hook, the prog_hook_id passed at the
>>> load time should match lsm's hook_id.
>>
>> OK, so this new prog_subtype field should be use/set by a new bpf_cmd,
>> right? Something like BPF_PROG_SUBTYPE or BPF_PROG_METADATA?
> 
> No new command. It will be an optional field to existing BPF_PROG_LOAD.
> In other words instead of replicating everything that different
> bpf_prog_type-s need, we can pass this hook_id field to fine tune
> the purpose (and applicability) of the program.
> And one BPF_PROG_TYPE_LANDLOCK type will cover all lsm hooks.
> For example existing BPF_PROG_TYPE_TRACEPOINT checks the safety
> for all tracepoints while they all have different arguments.
> For tracepoints it's easier, since the only difference between them
> is the range of ctx access. Here we need strong type safety
> of arguments therefore need extra hook_id at load time.

OK, I will do it.



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