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Date: Mon, 3 Nov 2014 15:23:47 -0800
From: Aditya Kali <adityakali@...gle.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
Serge Hallyn <serge.hallyn@...ntu.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
cgroups@...r.kernel.org,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
Ingo Molnar <mingo@...hat.com>,
Linux Containers <containers@...ts.linux-foundation.org>,
Rohit Jnagal <jnagal@...gle.com>
Subject: Re: [PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns
On Mon, Nov 3, 2014 at 3:15 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Nov 3, 2014 at 3:12 PM, Aditya Kali <adityakali@...gle.com> wrote:
>> On Fri, Oct 31, 2014 at 5:07 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Fri, Oct 31, 2014 at 12:19 PM, Aditya Kali <adityakali@...gle.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@...gle.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
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