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

From: Mickaël Salaün <[email-masked]>
Date: Tue, 30 Aug 2016 22:33:31 +0200

On 30/08/2016 22:23, Andy Lutomirski wrote:
> On Tue, Aug 30, 2016 at 1:20 PM, Mickaël Salaün <[email-masked]> wrote:
>>
>> On 30/08/2016 20:55, Andy Lutomirski wrote:
>>> On Sun, Aug 28, 2016 at 2:42 AM, Mickaël Salaün <[email-masked]> wrote:
>>>>
>>>>
>>>> On 28/08/2016 10:13, Andy Lutomirski wrote:
>>>>> On Aug 27, 2016 11:14 PM, "Mickaël Salaün" <[email-masked]> 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.

OK, I like that, but I think it could be done on a second time. :)