Re: [PATCHv1 7/8] cgroup: cgroup namespace setns support

[Aditya Kali]

On Thu, Oct 16, 2014 at 2:12 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Aditya Kali (adityakali@xxxxxxxxxx):
>> setns on a cgroup namespace is allowed only if
>> * task has CAP_SYS_ADMIN in its current user-namespace and
>>   over the user-namespace associated with target cgroupns.
>> * task's current cgroup is descendent of the target cgroupns-root
>>   cgroup.
>
> What is the point of this?
>
> If I'm a user logged into
> /lxc/c1/user.slice/user-1000.slice/session-c12.scope and I start
> a container which is in
> /lxc/c1/user.slice/user-1000.slice/session-c12.scope/x1
> then I will want to be able to enter the container's cgroup.
> The container's cgroup root is under my own (satisfying the
> below condition0 but my cgroup is not a descendent of the
> container's cgroup.
>
This condition is there because we don't want to do implicit cgroup
changes when a process attaches to another cgroupns. cgroupns tries to
preserve the invariant that at any point, your current cgroup is
always under the cgroupns-root of your cgroup namespace. But in your
example, if we allow a process in "session-c12.scope" container to
attach to cgroupns root'ed at "session-c12.scope/x1" container
(without implicitly moving its cgroup), then this invariant won't
hold.

>
>> * target cgroupns-root is same as or deeper than task's current
>>   cgroupns-root. This is so that the task cannot escape out of its
>>   cgroupns-root. This also ensures that setns() only makes the task
>>   get restricted to a deeper cgroup hierarchy.
>>
>> Signed-off-by: Aditya Kali <adityakali@xxxxxxxxxx>
>> ---
>>  kernel/cgroup_namespace.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 42 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/cgroup_namespace.c b/kernel/cgroup_namespace.c
>> index c16604f..c612946 100644
>> --- a/kernel/cgroup_namespace.c
>> +++ b/kernel/cgroup_namespace.c
>> @@ -80,8 +80,48 @@ err_out:
>>
>>  static int cgroupns_install(struct nsproxy *nsproxy, void *ns)
>>  {
>> -     pr_info("setns not supported for cgroup namespace");
>> -     return -EINVAL;
>> +     struct cgroup_namespace *cgroup_ns = ns;
>> +     struct task_struct *task = current;
>> +     struct cgroup *cgrp = NULL;
>> +     int err = 0;
>> +
>> +     if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN) ||
>> +         !ns_capable(cgroup_ns->user_ns, CAP_SYS_ADMIN))
>> +             return -EPERM;
>> +
>> +     /* Prevent cgroup changes for this task. */
>> +     threadgroup_lock(task);
>> +
>> +     cgrp = get_task_cgroup(task);
>> +
>> +     err = -EINVAL;
>> +     if (!cgroup_on_dfl(cgrp))
>> +             goto out_unlock;
>> +
>> +     /* Allow switch only if the task's current cgroup is descendant of the
>> +      * target cgroup_ns->root_cgrp.
>> +      */
>> +     if (!cgroup_is_descendant(cgrp, cgroup_ns->root_cgrp))
>> +             goto out_unlock;
>> +
>> +     /* Only allow setns to a cgroupns root-ed deeper than task's current
>> +      * cgroupns-root. This will make sure that tasks cannot escape their
>> +      * cgroupns by attaching to parent cgroupns.
>> +      */
>> +     if (!cgroup_is_descendant(cgroup_ns->root_cgrp,
>> +                               task_cgroupns_root(task)))
>> +             goto out_unlock;
>> +
>> +     err = 0;
>> +     get_cgroup_ns(cgroup_ns);
>> +     put_cgroup_ns(nsproxy->cgroup_ns);
>> +     nsproxy->cgroup_ns = cgroup_ns;
>> +
>> +out_unlock:
>> +     threadgroup_unlock(current);
>> +     if (cgrp)
>> +             cgroup_put(cgrp);
>> +     return err;
>>  }
>>
>>  static void *cgroupns_get(struct task_struct *task)
>> --
>> 2.1.0.rc2.206.gedb03e5
>>
>> --
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-- 
Aditya
--
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