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

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Aug 30, 2016 at 1:20 PM, Mickaël Salaün <mic@digikod.net> wrote:
>
> On 30/08/2016 20:55, Andy Lutomirski wrote:
>> On Sun, Aug 28, 2016 at 2:42 AM, Mickaël Salaün <mic@digikod.net> wrote:
>>>
>>>
>>> On 28/08/2016 10:13, Andy Lutomirski wrote:
>>>> On Aug 27, 2016 11:14 PM, "Mickaël Salaün" <mic@digikod.net> wrote:
>>>>>
>>>>>
>>>>> On 27/08/2016 22:43, Alexei Starovoitov wrote:
>>>>>> On Sat, Aug 27, 2016 at 09:35:14PM +0200, Mickaël Salaün wrote:
>>>>>>> On 27/08/2016 20:06, Alexei Starovoitov wrote:
>>>>>>>> On Sat, Aug 27, 2016 at 04:06:38PM +0200, Mickaël Salaün 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 view…
>>>>>
>>>>
>>>> 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.
>>
>
> This is interesting! It should be possible to append the seccomp stack
> of a source process to the seccomp stack of the target process when a
> Landlock program is passed and then activated through seccomp(2).
>
> For the FMODE_EXECUTE/FMODE_WRITE, are you suggesting to manage
> permission of the eBPF program FD in a specific way?
>

This wouldn't be an eBPF program FD -- it would be an FD encapsulating
an entire configuration including seccomp BPF program, whatever
landlock stuff is associated, and eventual seccomp monitor
configuration (once I write that code), etc.

You wouldn't say "attach this process's seccomp stack to me" -- you'd
say "attach this seccomp layer to me".

A decision that we'd have to make would be whether the FD links to the
parent layer or whether it can be attached without regard to what the
parent layer is.

--Andy