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

From: Mickaël Salaün <[email-masked]>
Date: Thu, 25 Aug 2016 16:44:13 +0200

On 25/08/2016 13:09, Andy Lutomirski wrote:
> On Thu, Aug 25, 2016 at 3:32 AM, Mickaël Salaün <[email-masked]> wrote:
>> Add an eBPF function bpf_landlock_cmp_cgroup_beneath(opt, map, map_op)
>> to compare the current process cgroup with a cgroup handle, The handle
>> can match the current cgroup if it is the same or a child. This allows
>> to make conditional rules according to the current cgroup.
>>
>> A cgroup handle is a map entry created from a file descriptor referring
>> a cgroup directory (e.g. by opening /sys/fs/cgroup/X). In this case, the
>> map entry is of type BPF_MAP_HANDLE_TYPE_LANDLOCK_CGROUP_FD and the
>> inferred array map is of type BPF_MAP_ARRAY_TYPE_LANDLOCK_CGROUP.
> 
> Can you elaborate on why this is useful?  I.e. why not just supply
> different policies to different subtrees.

The main use case I see is to load the security policies at the start of
a user session for all processes but not enforce them right away. The
user can then keep a shell for Landlock administration tasks and lock
the other processes with a dedicated cgroup on the fly. This allows the
user to make unremovable Landlock security policies but only activate
them when needed for specific processes.

> 
> Also, how does this interact with the current cgroup v1 vs v2 mess?
> As far as I can tell, no one can even really agree on what "what
> cgroup am I in" means right now.

I tested with cgroup-v2 but indeed, it seems a bit different with
cgroup-v1 :)
Does anyone know how to handle both cases?

> 
>>
>> An unprivileged process can create and manipulate cgroups thanks to
>> cgroup delegation.
> 
> What is cgroup delegation?

This is simply the action of changing the owner of cgroup sysfs files to
allow an unprivileged user to handle them (cf. Documentation/cgroup-v2.txt)

 Mickaël