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

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