[PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns

[Andy Lutomirski]

On Mon, Nov 3, 2014 at 3:12 PM, Aditya Kali <adityakali at google.com> wrote:
> On Fri, Oct 31, 2014 at 5:07 PM, Andy Lutomirski <luto at amacapital.net> wrote:
>> On Fri, Oct 31, 2014 at 12:19 PM, Aditya Kali <adityakali at google.com> wrote:
>>> This patch enables cgroup mounting inside userns when a process
>>> as appropriate privileges. The cgroup filesystem mounted is
>>> rooted at the cgroupns-root. Thus, in a container-setup, only
>>> the hierarchy under the cgroupns-root is exposed inside the container.
>>> This allows container management tools to run inside the containers
>>> without depending on any global state.
>>> In order to support this, a new kernfs api is added to lookup the
>>> dentry for the cgroupns-root.
>>>
>>> Signed-off-by: Aditya Kali <adityakali at google.com>
>>> ---
>>>  fs/kernfs/mount.c      | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>  include/linux/kernfs.h |  2 ++
>>>  kernel/cgroup.c        | 47 +++++++++++++++++++++++++++++++++++++++++++++--
>>>  3 files changed, 95 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
>>> index f973ae9..e334f45 100644
>>> --- a/fs/kernfs/mount.c
>>> +++ b/fs/kernfs/mount.c
>>> @@ -62,6 +62,54 @@ struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
>>>         return NULL;
>>>  }
>>>
>>> +/**
>>> + * kernfs_make_root - create new root dentry for the given kernfs_node.
>>> + * @sb: the kernfs super_block
>>> + * @kn: kernfs_node for which a dentry is needed
>>> + *
>>> + * This can used used by callers which want to mount only a part of the kernfs
>>> + * as root of the filesystem.
>>> + */
>>> +struct dentry *kernfs_obtain_root(struct super_block *sb,
>>> +                                 struct kernfs_node *kn)
>>> +{
>>
>> I can't usefully review this, but kernfs_make_root and
>> kernfs_obtain_root aren't the same string...
>>
>>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>>> index 7e5d597..250aaec 100644
>>> --- a/kernel/cgroup.c
>>> +++ b/kernel/cgroup.c
>>> @@ -1302,6 +1302,13 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
>>>
>>>         memset(opts, 0, sizeof(*opts));
>>>
>>> +       /* Implicitly add CGRP_ROOT_SANE_BEHAVIOR if inside a non-init cgroup
>>> +        * namespace.
>>> +        */
>>> +       if (current->nsproxy->cgroup_ns != &init_cgroup_ns) {
>>> +               opts->flags |= CGRP_ROOT_SANE_BEHAVIOR;
>>> +       }
>>> +
>>
>> I don't like this implicit stuff.  Can you just return -EINVAL if sane
>> behavior isn't requested?
>>
>
> I think the sane-behavior flag is only temporary and will be removed
> anyways, right? So I didn't bother asking user to supply it. But I can
> make the change as you suggested. We just have to make sure that tasks
> inside cgroupns cannot mount non-default hierarchies as it would be a
> regression.
>
>>>         while ((token = strsep(&o, ",")) != NULL) {
>>>                 nr_opts++;
>>>
>>> @@ -1391,7 +1398,7 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
>>>
>>>         if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
>>>                 pr_warn("sane_behavior: this is still under development and its behaviors will change, proceed at your own risk\n");
>>> -               if (nr_opts != 1) {
>>> +               if (nr_opts > 1) {
>>>                         pr_err("sane_behavior: no other mount options allowed\n");
>>>                         return -EINVAL;
>>
>> This looks wrong.  But, if you make the change above, then it'll be right.
>>
>
> It would have been nice if simple 'mount -t cgroup cgroup <mnt>' from
> cgroupns does the right thing automatically.
>

This is a debatable point, but it's not what I meant.  Won't your code
let 'mount -t cgroup -o one_evil_flag cgroup mountpoint' through?

>
>>> @@ -1685,6 +1701,14 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
>>>         int ret;
>>>         int i;
>>>         bool new_sb;
>>> +       struct cgroup_namespace *ns =
>>> +               get_cgroup_ns(current->nsproxy->cgroup_ns);
>>> +
>>> +       /* Check if the caller has permission to mount. */
>>> +       if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) {
>>> +               put_cgroup_ns(ns);
>>> +               return ERR_PTR(-EPERM);
>>> +       }
>>
>> Why is this necessary?
>>
>
> Without this, if I unshare userns and mntns (but no cgroupns), I will
> be able to mount my parent's cgroupfs hierarchy. This is deviation
> from whats allowed today (i.e., today I can't mount cgroupfs even
> after unsharing userns & mntns). This check is there to prevent the
> unintended effect of cgroupns feature.

Oh, I get it.  I misunderstood the code.

I guess this is reasonable.  If it annoys anyone, it can be reverted
or weakened.

--Andy