Re: [PATCH v5 00/14] ceph: support idmapped mounts

[Gregory Farnum]

On Mon, Jun 12, 2023 at 6:43 PM Xiubo Li <xiubli@xxxxxxxxxx> wrote:
>
>
> On 6/9/23 18:12, Aleksandr Mikhalitsyn wrote:
> > On Fri, Jun 9, 2023 at 12:00 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> >> On Fri, Jun 09, 2023 at 10:59:19AM +0200, Aleksandr Mikhalitsyn wrote:
> >>> On Fri, Jun 9, 2023 at 3:57 AM Xiubo Li <xiubli@xxxxxxxxxx> wrote:
> >>>>
> >>>> On 6/8/23 23:42, Alexander Mikhalitsyn wrote:
> >>>>> Dear friends,
> >>>>>
> >>>>> This patchset was originally developed by Christian Brauner but I'll continue
> >>>>> to push it forward. Christian allowed me to do that :)
> >>>>>
> >>>>> This feature is already actively used/tested with LXD/LXC project.
> >>>>>
> >>>>> Git tree (based on https://github.com/ceph/ceph-client.git master):
> >>> Hi Xiubo!
> >>>
> >>>> Could you rebase these patches to 'testing' branch ?
> >>> Will do in -v6.
> >>>
> >>>> And you still have missed several places, for example the following cases:
> >>>>
> >>>>
> >>>>      1    269  fs/ceph/addr.c <<ceph_netfs_issue_op_inline>>
> >>>>                req = ceph_mdsc_create_request(mdsc, CEPH_MDS_OP_GETATTR,
> >>>> mode);
> >>> +
> >>>
> >>>>      2    389  fs/ceph/dir.c <<ceph_readdir>>
> >>>>                req = ceph_mdsc_create_request(mdsc, op, USE_AUTH_MDS);
> >>> +
> >>>
> >>>>      3    789  fs/ceph/dir.c <<ceph_lookup>>
> >>>>                req = ceph_mdsc_create_request(mdsc, op, USE_ANY_MDS);
> >>> We don't have an idmapping passed to lookup from the VFS layer. As I
> >>> mentioned before, it's just impossible now.
> >> ->lookup() doesn't deal with idmappings and really can't otherwise you
> >> risk ending up with inode aliasing which is really not something you
> >> want. IOW, you can't fill in inode->i_{g,u}id based on a mount's
> >> idmapping as inode->i_{g,u}id absolutely needs to be a filesystem wide
> >> value. So better not even risk exposing the idmapping in there at all.
> > Thanks for adding, Christian!
> >
> > I agree, every time when we use an idmapping we need to be careful with
> > what we map. AFAIU, inode->i_{g,u}id should be based on the filesystem
> > idmapping (not mount),
> > but in this case, Xiubo want's current_fs{u,g}id to be mapped
> > according to an idmapping.
> > Anyway, it's impossible at now and IMHO, until we don't have any
> > practical use case where
> > UID/GID-based path restriction is used in combination with idmapped
> > mounts it's not worth to
> > make such big changes in the VFS layer.
> >
> > May be I'm not right, but it seems like UID/GID-based path restriction
> > is not a widespread
> > feature and I can hardly imagine it to be used with the container
> > workloads (for instance),
> > because it will require to always keep in sync MDS permissions
> > configuration with the
> > possible UID/GID ranges on the client. It looks like a nightmare for sysadmin.
> > It is useful when cephfs is used as an external storage on the host, but if you
> > share cephfs with a few containers with different user namespaces idmapping...
>
> Hmm, while this will break the MDS permission check in cephfs then in
> lookup case. If we really couldn't support it we should make it to
> escape the check anyway or some OPs may fail and won't work as expected.

I don't pretend to know the details of the VFS (or even our linux
client implementation), but I'm confused that this is apparently so
hard. It looks to me like we currently always fill in the "caller_uid"
with "from_kuid(&init_user_ns, req->r_cred->fsuid))". Is this actually
valid to begin with? If it is, why can't the uid mapping be applied on
that?

As both the client and the server share authority over the inode's
state (including things like mode bits and owners), and need to do
permission checking, being able to tell the server the relevant actor
is inherently necessary. We also let admins restrict keys to
particular UID/GID combinations as they wish, and it's not the most
popular feature but it does get deployed. I would really expect a user
of UID mapping to be one of the *most* likely to employ such a
facility...maybe not with containers, but certainly end-user homedirs
and shared spaces.

Disabling the MDS auth checks is really not an option. I guess we
could require any user employing idmapping to not be uid-restricted,
and set the anonymous UID (does that work, Xiubo, or was it the broken
one? In which case we'd have to default to root?). But that seems a
bit janky to me.
-Greg

> @Greg
>
> For the lookup requests the idmapping couldn't get the mapped UID/GID
> just like all the other requests, which is needed by the MDS permission
> check. Is that okay to make it disable the check for this case ? I am
> afraid this will break the MDS permssions logic.
>
> Any idea ?
>
> Thanks
>
> - Xiubo
>
>
> > Kind regards,
> > Alex
> >
>