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

From: Andy Lutomirski <luto@amacapital.net>
To: "Mickaël Salaün" <mic@digikod.net>
Cc: "kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	Alexei Starovoitov <ast@kernel.org>, Tejun Heo <tj@kernel.org>,
	Sargun Dhillon <sargun@sargun.me>,
	Network Development <netdev@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Kees Cook <keescook@chromium.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	Daniel Mack <daniel@zonque.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [RFC v2 09/10] landlock: Handle cgroups (performance)
Date: Sun, 28 Aug 2016 01:13:13 -0700	[thread overview]
Message-ID: <CALCETrXgQ+njtupXiKcOXG8ZRzD=n2LhT45NiJr1Ehob7bHskQ@mail.gmail.com> (raw)
In-Reply-To: <57C202BF.7000207@digikod.net>

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.

--Andy