Re: [RFC v2 09/10] landlock: Handle cgroups (performance)

[Andy Lutomirski]

On Sun, Aug 28, 2016 at 2:42 AM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>
>
> On 28/08/2016 10:13, Andy Lutomirski wrote:
>> On Aug 27, 2016 11:14 PM, "MickaÃl SalaÃn" <mic@xxxxxxxxxxx> wrote:
>>>
>>>
>>> On 27/08/2016 22:43, Alexei Starovoitov wrote:
>>>> On Sat, Aug 27, 2016 at 09:35:14PM +0200, MickaÃl SalaÃn wrote:
>>>>> On 27/08/2016 20:06, Alexei Starovoitov wrote:
>>>>>> On Sat, Aug 27, 2016 at 04:06:38PM +0200, MickaÃl SalaÃn wrote:
>>>>>>> As said above, Landlock will not run an eBPF programs when not strictly
>>>>>>> needed. Attaching to a cgroup will have the same performance impact as
>>>>>>> attaching to a process hierarchy.
>>>>>>
>>>>>> Having a prog per cgroup per lsm_hook is the only scalable way I
>>>>>> could come up with. If you see another way, please propose.
>>>>>> current->seccomp.landlock_prog is not the answer.
>>>>>
>>>>> Hum, I don't see the difference from a performance point of view between
>>>>> a cgroup-based or a process hierarchy-based system.
>>>>>
>>>>> Maybe a better option should be to use an array of pointers with N
>>>>> entries, one for each supported hook, instead of a unique pointer list?
>>>>
>>>> yes, clearly array dereference is faster than link list walk.
>>>> Now the question is where to keep this prog_array[num_lsm_hooks] ?
>>>> Since we cannot keep it inside task_struct, we have to allocate it.
>>>> Every time the task is creted then. What to do on the fork? That
>>>> will require changes all over. Then the obvious optimization would be
>>>> to share this allocated array of prog pointers across multiple tasks...
>>>> and little by little this new facility will look like cgroup.
>>>> Hence the suggestion to put this array into cgroup from the start.
>>>
>>> I see your point :)
>>>
>>>>
>>>>> Anyway, being able to attach an LSM hook program to a cgroup thanks to
>>>>> the new BPF_PROG_ATTACH seems a good idea (while keeping the possibility
>>>>> to use a process hierarchy). The downside will be to handle an LSM hook
>>>>> program which is not triggered by a seccomp-filter, but this should be
>>>>> needed anyway to handle interruptions.
>>>>
>>>> what do you mean 'not triggered by seccomp' ?
>>>> You're not suggesting that this lsm has to enable seccomp to be functional?
>>>> imo that's non starter due to overhead.
>>>
>>> Yes, for now, it is triggered by a new seccomp filter return value
>>> RET_LANDLOCK, which can take a 16-bit value called cookie. This must not
>>> be needed but could be useful to bind a seccomp filter security policy
>>> with a Landlock one. Waiting for Kees's point of viewâ
>>>
>>
>> I'm not Kees, but I'd be okay with that.  I still think that doing
>> this by process hierarchy a la seccomp will be easier to use and to
>> understand (which is quite important for this kind of work) than doing
>> it by cgroup.
>>
>> A feature I've wanted to add for a while is to have an fd that
>> represents a seccomp layer, the idea being that you would set up your
>> seccomp layer (with syscall filter, landlock hooks, etc) and then you
>> would have a syscall to install that layer.  Then an unprivileged
>> sandbox manager could set up its layer and still be able to inject new
>> processes into it later on, no cgroups needed.
>
> A nice thing I didn't highlight about Landlock is that a process can
> prepare a layer of rules (arraymap of handles + Landlock programs) and
> pass the file descriptors of the Landlock programs to another process.
> This process could then apply this programs to get sandboxed. However,
> for now, because a Landlock program is only triggered by a seccomp
> filter (which do not follow the Landlock programs as a FD), they will be
> useless.
>
> The FD referring to an arraymap of handles can also be used to update a
> map and change the behavior of a Landlock program. A master process can
> then add or remove restrictions to another process hierarchy on the fly.

Maybe this could be extended a little bit.  The fd could hold the
seccomp filter *and* the LSM hook filters.  FMODE_EXECUTE could give
the ability to install it and FMODE_WRITE could give the ability to
modify it.