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

[Aditya Kali <adityakali@google.com>]

On Mon, Nov 3, 2014 at 3:15 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Mon, Nov 3, 2014 at 3:12 PM, Aditya Kali <adityakali@google.com> wrote:
>> On Fri, Oct 31, 2014 at 5:07 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> On Fri, Oct 31, 2014 at 12:19 PM, Aditya Kali <adityakali@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@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?
>

I don't think so. This check "if (nr_opts > 1)" is nested under "if
(opts->flags & CGRP_ROOT_SANE_BEHAVIOR)". So we know that there is
atleast 1 option ('__DEVEL__sane_behavior') present (implicit or not).
Addition of 'one_evil_flag' will make nr_opts = 2 and result in EINVAL
here.

>>
>>>> @@ -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



-- 
Aditya