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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Aug 30, 2016 at 10:33:31PM +0200, Mickaël Salaün wrote:
> 
> 
> On 30/08/2016 22:23, Andy Lutomirski wrote:
> > 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.
> 
> OK, I like that, but I think it could be done on a second time. :)

I don't. Single FD that is a collection of objects seems an odd abstraction
to me. I also don't see what it actually solves.
I think lsm and seccomp should be orthogonal and not tied into each other.