Re: cgroups(7): documenting cgroups v2 delegation

Michael Kerrisk (man-pages)
2018-01-08T23:27:58Z

Hello Tejun,

On 01/08/2018 03:14 PM, Tejun Heo wrote:
> Hello, Michael.
>
> On Tue, Jan 02, 2018 at 07:22:05PM +0100, Michael Kerrisk (man-pages) wrote:
>> To perform delegation, the delegater makes certain directories
>> and files writable by the delegatee, typically by changing the
>> ownership of the objects to be the user ID of the delegatee.
>> Assuming that we want to delegate the hierarchy rooted at (say)
>> /dlgt_grp and that there are not yet any child cgroups under
>> that cgroup, the ownership of the following is changed to the
>> user ID of the delegatee:
>>
>> /dlgt_grp
>> Changing the ownership of the root of the subtree means
>> that any new cgroups created under the subtree (and the
>> files they contain) will also be owned by the delegatee.
>>
>> /dlgt_grp/cgroup.procs
>> Changing the ownership of this file means that the dele‐
>> gatee can move processes into the root of the delegated
>> subtree.
>>
>> /dlgt_grp/cgroup.subtree_control
>> Making this file owned by the delegatee is optional.
>> Doing so means that that the delegatee can enable con‐
>> trollers (that are present in /dlgt_grp/cgroup.con‐
>> trollers) in order to further redistribute resources at
>> lower levels in the subtree. As an alternative to
>> changing the ownership of this file, the delegater might
>> instead add selected controllers to this file.
>
> I'm not sure how useful it is to describe this to be optional. In the
> same sense, cgroup.procs would be optional too as the delegatee can
> take control from its first children. Users of course can choose to
> do mix and match as they see fit but outside of the defined model,
> there can be surprises - e.g. nsdelegate or some future delegation
> aware feature can behave differently. I think it'd be better to keep
> it simple - either a subtree is delegated or not.

So, I changed that piece to:

/dlgt_grp/cgroup.subtree_control
Changing the ownership of this file means that that the
delegatee can enable controllers (that are present in
/dlgt_grp/cgroup.controllers) in order to further redis‐
tribute resources at lower levels in the subtree. (As an
alternative to changing the ownership of this file, the
delegater might instead add selected controllers to this
file.)

Okay?

>> The delegater should not change the ownership of any of the
>> controller interfaces files (e.g., pids.max, memory.high) in
>> dlgt_grp. Those files are used from the next level above the
>> delegated subtree in order to distribute resources into the
>> subtree, and the delegatee should not have permission to change
>> the resources that are distributed into the delegated subtree.
>
> Roman recently added /sys/kernel/cgroup/delegate and
> /sys/kernel/cgroup/features. The former contains newline separated
> list of cgroup files which should be chowned on delegation (in
> addition to the directory itself) and the latter contains optional
> features (currently only nsdelegate).

Oh -- and this reminds that I've been meaning to ask you for a while
now: could you please (please please please) CC all cgroup interface
changes to linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org (and prod others to do so
please). There have been many of these changes in recent times
(addition of new v2 controllers, thread mode, nsdelegate, the
changes that Roman made that you refer to above), and they really
all should have been CCed to linux-api@. It's often the only (easy)
way that I have to discover changes that should be documented in
the manual pages. And there are many other groups that are also
interested in tracking kernel-user-space interface changes; see
https://www.kernel.org/doc/man-pages/linux-api-ml.html

> Roman, can you please update
> cgroup-v2.txt accordingly?

And I've already sent a separate mail re the cgroups(7) changes
relating to /sys/kernel/cgroup/*

> The file list was added because the cgroup OOM support added a knob
> which belongs to the cgroup itself rather than the parent and we might
> have more of those files in the future (not too likely and there won't
> be many).
>
> It could be also worthwhile to describe nsdelegate, which prevents
> cgroup-namespaced delegatee, which may have the same UID as the
> delegator, from writing to interface files in its cgroup root which
> belong to the parent.

Yes. See my separate mail.

>> Some delegation containment rules ensure that the delegatee can
>> move processes between cgroups within the delegated subtree,
>> but can't move processes from outside the delegated subtree
>> into the subtree or vice versa. A nonprivileged process (i.e.,
>> the delegatee) can write the PID of a "target" process into a
>> cgroup.procs file only if all of the following are true:
>>
>> * The effective UID of the writer (i.e., the delegatee)
>> matches the real user ID or the saved set-user-ID of the
>> target process.
>
> cgroup2 doesn't check the above anymore.

Please please see my request regarding linux-api@ :-)

I see the change was in 4.11, commit 576dd464505fc53d501bb94569db76f220104d28

I rewrote that rule as follows, and moved it to the end of the list (i.e.,
the point XXX below):

* Before Linux 4.11: the effective UID of the writer (i.e., the
delegatee) matches the real user ID or the saved set-user-ID of
the target process. (This was a historical requirement inher‐
ited from cgroups v1 that was later deemed unnecessary, since
the other rules suffice for containment in cgroups v2.)

>> * The writer has write permission on the cgroup.procs file in
>> the destination cgroup.
>>
>> * The writer has write permission on the cgroup.procs file in
>> the common ancestor of the source and destination cgroups.
>> (In some cases, the common ancestor may be the source or
>> destination cgroup itself.)
>

> Also, if nsdelegate is enabled, both the source and destination
> cgroups must be visible (cgroup namespace-wise) to the writer.

I added the following:

* If the cgroup v2 filesystem was mounted with the nsdelegate
option, the writer must be able to see the source and destina‐
tion cgroup from its cgroup namespace.

Okay?

XXX

>
>> ┌─────────────────────────────────────────────────────┐
>> │FIXME │
>> ├─────────────────────────────────────────────────────┤
>> │Please confirm that the following is correct: │
>> └─────────────────────────────────────────────────────┘
>>
>> Note: one consequence of these delegation containment rules is
>> that the unprivileged delegatee can't place the first process
>> into the delegated subtree; instead, the delegater must place
>> the first process (a process owned by the delegatee) into the
>> delegated subtree.
>
> Yeah, that'd be the case. Seeding of a delegated subtree should be
> done by the delegator or before the priviledges are dropped.

Thanks for the confirmation.

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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