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

[Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>]

On Tue, Jul 18, 2023 at 3:45 AM Xiubo Li <xiubli@redhat.com> wrote:
>
>
> On 7/14/23 20:57, Aleksandr Mikhalitsyn wrote:
> > On Tue, Jul 4, 2023 at 3:09 AM Xiubo Li <xiubli@redhat.com> wrote:
> >> Sorry, not sure, why my last reply wasn't sent out.
> >>
> >> Do it again.
> >>
> >>
> >> On 6/26/23 19:23, Aleksandr Mikhalitsyn wrote:
> >>> On Mon, Jun 26, 2023 at 4:12 AM Xiubo Li<xiubli@redhat.com>  wrote:
> >>>> On 6/24/23 15:11, Aleksandr Mikhalitsyn wrote:
> >>>>> On Sat, Jun 24, 2023 at 3:37 AM Xiubo Li<xiubli@redhat.com>  wrote:
> >>>>>> [...]
> >>>>>>
> >>>>>>     > > >
> >>>>>>     > > > I thought about this too and came to the same conclusion, that
> >>>>>> UID/GID
> >>>>>>     > > > based
> >>>>>>     > > > restriction can be applied dynamically, so detecting it on mount-time
> >>>>>>     > > > helps not so much.
> >>>>>>     > > >
> >>>>>>     > > For this you please raise one PR to ceph first to support this, and in
> >>>>>>     > > the PR we can discuss more for the MDS auth caps. And after the PR
> >>>>>>     > > getting merged then in this patch series you need to check the
> >>>>>>     > > corresponding option or flag to determine whether could the idmap
> >>>>>>     > > mounting succeed.
> >>>>>>     >
> >>>>>>     > I'm sorry but I don't understand what we want to support here. Do we
> >>>>>> want to
> >>>>>>     > add some new ceph request that allows to check if UID/GID-based
> >>>>>>     > permissions are applied for
> >>>>>>     > a particular ceph client user?
> >>>>>>
> >>>>>> IMO we should prevent user to set UID/GID-based permisions caps from
> >>>>>> ceph side.
> >>>>>>
> >>>>>> As I know currently there is no way to prevent users to set MDS auth
> >>>>>> caps, IMO in ceph side at least we need one flag or option to disable
> >>>>>> this once users want this fs cluster sever for idmap mounts use case.
> >>>>> How this should be visible from the user side? We introducing a new
> >>>>> kernel client mount option,
> >>>>> like "nomdscaps", then pass flag to the MDS and MDS should check that
> >>>>> MDS auth permissions
> >>>>> are not applied (on the mount time) and prevent them from being
> >>>>> applied later while session is active. Like that?
> >>>>>
> >>>>> At the same time I'm thinking about protocol extension that adds 2
> >>>>> additional fields for UID/GID. This will allow to correctly
> >>>>> handle everything. I wanted to avoid any changes to the protocol or
> >>>>> server-side things. But if we want to change MDS side,
> >>>>> maybe it's better then to go this way?
> >>> Hi Xiubo,
> >>>
> >>>> There is another way:
> >>>>
> >>>> For each client it will have a dedicated client auth caps, something like:
> >>>>
> >>>> client.foo
> >>>>      key: *key*
> >>>>      caps: [mds] allow r, allow rw path=/bar
> >>>>      caps: [mon] allow r
> >>>>      caps: [osd] allow rw tag cephfs data=cephfs_a
> >>> Do we have any infrastructure to get this caps list on the client side
> >>> right now?
> >>> (I've taken a quick look through the code and can't find anything
> >>> related to this.)
> >> I am afraid there is no.
> >>
> >> But just after the following ceph PR gets merged it will be easy to do this:
> >>
> >> https://github.com/ceph/ceph/pull/48027
> >>
> >> This is still under testing.
> >>
> >>>> When mounting this client with idmap enabled, then we can just check the
> >>>> above [mds] caps, if there has any UID/GID based permissions set, then
> >>>> fail the mounting.
> >>> understood
> >>>
> >>>> That means this kind client couldn't be mounted with idmap enabled.
> >>>>
> >>>> Also we need to make sure that once there is a mount with idmap enabled,
> >>>> the corresponding client caps couldn't be append the UID/GID based
> >>>> permissions. This need a patch in ceph anyway IMO.
> >>> So, yeah we will need to effectively block cephx permission changes if
> >>> there is a client mounted with
> >>> an active idmapped mount. Sounds as something that require massive
> >>> changes on the server side.
> >> Maybe no need much, it should be simple IMO. But I am not 100% sure.
> >>
> >>> At the same time this will just block users from using idmapped mounts
> >>> along with UID/GID restrictions.
> >>>
> >>> If you want me to change server-side anyways, isn't it better just to
> >>> extend cephfs protocol to properly
> >>> handle UID/GIDs with idmapped mounts? (It was originally proposed by Christian.)
> >>> What we need to do here is to add a separate UID/GID fields for ceph
> >>> requests those are creating a new inodes
> >>> (like mknod, symlink, etc).
> > Dear Xiubo,
> >
> > I'm sorry for delay with reply, I've missed this message accidentally.
> >
> >> BTW, could you explain it more ? How could this resolve the issue we are
> >> discussing here ?
> > This was briefly mentioned here
> > https://lore.kernel.org/all/20220105141023.vrrbfhti5apdvkz7@wittgenstein/#t
> > by Christian. Let me describe it in detail.
> >
> > In the current approach we apply mount idmapping to
> > head->caller_{uid,gid} fields
> > to make mkdir/mknod/symlink operations set a proper inode owner
> > uid/gid in according with an idmapping.
>
> Sorry for late.
>
> I still couldn't get how this could resolve the lookup case.
>
> For a lookup request the caller_{uid, gid} still will be the mapped
> {uid, gid}, right ?

No, the idea is to stop mapping a caller_{uid, gid}. And to add a new
fields like
inode_owner_{uid, gid} which will be idmapped (this field will be specific only
for those operations that create a new inode).

>
And also the same for other non-create requests. If
> so this will be incorrect for the cephx perm checks IMO.

Thanks,
Alex

>
> Thanks
>
> - Xiubo
>
>
> > This makes a problem with path-based UID/GID restriction mechanism,
> > because it uses head->caller_{uid,gid} fields
> > to check if UID/GID is permitted or not.
> >
> > So, the problem is that we have one field in ceph request for two
> > different needs - to control permissions and to set inode owner.
> > Christian pointed that the most saner way is to modify ceph protocol
> > and add a separate field to store inode owner UID/GID,
> > and only this fields should be idmapped, but head->caller_{uid,gid}
> > will be untouched.
> >
> > With this approach, we will not affect UID/GID-based permission rules
> > with an idmapped mounts at all.
> >
> > Kind regards,
> > Alex
> >
> >> Thanks
> >>
> >> - Xiubo
> >>
> >>
> >>> Kind regards,
> >>> Alex
> >>>
> >>>> Thanks
> >>>>
> >>>> - Xiubo
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>> Thanks,
> >>>>> Alex
> >>>>>
> >>>>>> Thanks
> >>>>>>
> >>>>>> - Xiubo
> >>>>>>
>