Re: [PATCH v2] cgroup: allow management of subtrees by new cgroup namespaces

[Aleksa Sarai]

> > Change the mode of the cgroup directory for each cgroup association,
> > allowing the process to create subtrees and modify the limits of the
> > subtrees *without* allowing the process to modify its own limits. Due
> > to the cgroup core restrictions and unix permission model, this
> > allows for processes to create new subtrees without breaking the
> > cgroup limits for the process.
>
> Actually, that's not really what this patch does.  If you unshare
> without having created any cgroups, it sets the other permission of the
> entire top level hierarchy to o+rwx:

While that is odd, it makes sense (because that's the "current cgroup" 
you are in). But I agree with your point that this patch is less than ideal.

> ironically, this now makes the root group a permission denier (at least
> for my distribution), because if I were in the root group (and not
> root), the r-x on the group would rule the rwx on other ... I really
> don't think that sounds correct.

You're right, that's odd. I'm confused why your root cgroups have u-w 
though.

> Perhaps what you should to be arguing then that the default permissions
> of the cgroup directories need to be all rwx for everyone and then your
> patch becomes unnecessary?

I don't think that would be the nicest way of dealing with this (then a 
process can make very large numbers of cgroups all over the tree, which 
might not cause huge issues but would still be a pain for administrators 
and systemds alike).

> Alternatively, if the desire is fully to virtualize /sys/fs/cgroups,
> then I think we have to decide how that would happen.  I think the
> default requirements would be that a pid namespace be established (so
> only the tasks in that pid namespace would be able to be controlled by
> the cgroup namespace.  That, I think requires that any given cgroup
> namespace "own" a pid namespace (being the one present when it was
> created) but that it only gets a new virtual set of directories owned
> by the userns owner if there's a pid namespace established for the
> cgroup and cgroup->user_ns == pid_ns->user_ns (meaning we established a
> user ns then a pid one then a cgroup one, so it's now safe to treat
> root in the user_ns as owning the virtualized cgroup directories).

I know this is probably a stupid question, but why couldn't we just 
compare the user_ns with the tcred->user_ns? Or are you worried about a 
process in a cgroup namespace moving processes to a subtree that isn't 
in the same pid namespace (even though they're in the same user 
namespace)? I don't mind implementing that this way (although we'd have 
to change a bunch of the checks with pid_ns to use the 
cgroup_ns->pid_ns), I'm just wondering if it's necessary.

> We could do this in the same way that proc gets virtualized after
> remounting (in a new mount namespace) on fork into a pid namespace.

I actually really like this idea. I'll get to work on it.

--
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/