[PATCH v4 03/21] fs: Allow sysfs and cgroupfs to share super blocks between user namespaces

[Eric W. Biederman]

Seth Forshee <seth.forshee at canonical.com> writes:

> On Tue, May 17, 2016 at 05:39:33PM -0500, Eric W. Biederman wrote:
>> Seth Forshee <seth.forshee at canonical.com> writes:
>> 
>> > Both of these filesystems already have use cases for mounting the
>> > same super block from multiple user namespaces. For sysfs this
>> > happens when using criu for snapshotting a container, where sysfs
>> > is mnounted in the containers network ns but the hosts user ns.
>> > The cgroup filesystem shares the same super block for all mounts
>> > of the same hierarchy regardless of the namespace.
>> >
>> > As a result, the restriction on mounting a super block from a
>> > single user namespace creates regressions for existing uses of
>> > these filesystems. For these specific filesystems this
>> > restriction isn't really necessary since the backing store is
>> > objects in kernel memory and thus the ids assigned from inodes
>> > is not subject to translation relative to s_user_ns.
>> >
>> > Add a new filesystem flag, FS_USERNS_SHARE_SB, which when set
>> > causes sget_userns() to skip the check of s_user_ns. Set this
>> > flag for the sysfs and cgroup filesystems to fix the
>> > regressions.
>> 
>> So this one needs to be sget_userns(..., &init_user_ns, ...).
>> And not a new special case.
>
> This is actually what I wanted to do, but based on a previous discussion
> where I had suggested doing this (for a different reason) I came away
> thinking you did not want it that way. So I'm happy with that change.

Yeah.  Somedays it seems like there are a lot of pieces in play here.
The security labels on sysfs seems to be a very compelling case.

> But if we do that it violates some of the assumptions of the patch to
> rework MNT_NODEV on your testing branch (and also those behind patch 2
> in this series). Something will need to be changed there to prevent a
> regression in mount behavior when a user ns tries to mount without
> MNT_NODEV when the mount inherited from its parent has it set.

Thank you for pointing that out.  I will look into that.

I believe I know exactly what you are talking about.  Of the choices I
think it is better to a minor localized change in the fs_fully_visible
logic than it is to cause problems elsewhere.

>> Apologies for not catching this earlier.
>
> Actually this is a more recent patch, so you possibly hadn't seen it
> before.
>
>> I am looking at folding all of this into the patch that introduces
>> sget_userns so that even bisects won't have regresssions.
>
> That's fine with me.

And thank you for keeping everything as separate patches.  That is at
least helping me catch up.  Even if I don't agree that these things
should be separate come merge time.

Eric