On Tue, Jan 04, 2022 at 12:40:51PM -0500, Jeff Layton wrote:
> On Tue, 2022-01-04 at 15:04 +0100, Christian Brauner wrote:
> > From: Christian Brauner <christian.brauner@xxxxxxxxxx>
> >
> > Inode operations that create a new filesystem object such as ->mknod,
> > ->create, ->mkdir() and others don't take a {g,u}id argument explicitly.
> > Instead the caller's fs{g,u}id is used for the {g,u}id of the new
> > filesystem object.
> >
> > Cephfs mds creation request argument structures mirror this filesystem
> > behavior. They don't encode a {g,u}id explicitly. Instead the caller's
> > fs{g,u}id that is always sent as part of any mds request is used by the
> > servers to set the {g,u}id of the new filesystem object.
> >
> > In order to ensure that the correct {g,u}id is used map the caller's
> > fs{g,u}id for creation requests. This doesn't require complex changes.
> > It suffices to pass in the relevant idmapping recorded in the request
> > message. If this request message was triggered from an inode operation
> > that creates filesystem objects it will have passed down the relevant
> > idmaping. If this is a request message that was triggered from an inode
> > operation that doens't need to take idmappings into account the initial
> > idmapping is passed down which is an identity mapping and thus is
> > guaranteed to leave the caller's fs{g,u}id unchanged.,u}id is sent.
> >
> > The last few weeks before Christmas 2021 I have spent time not just
> > reading and poking the cephfs kernel code but also took a look at the
> > ceph mds server userspace to ensure I didn't miss some subtlety.
> >
> > This made me aware of one complication to solve. All requests send the
> > caller's fs{g,u}id over the wire. The caller's fs{g,u}id matters for the
> > server in exactly two cases:
> >
> > 1. to set the ownership for creation requests
> > 2. to determine whether this client is allowed access on this server
> >
> > Case 1. we already covered and explained. Case 2. is only relevant for
> > servers where an explicit uid access restriction has been set. That is
> > to say the mds server restricts access to requests coming from a
> > specific uid. Servers without uid restrictions will grant access to
> > requests from any uid by setting MDS_AUTH_UID_ANY.
> >
> > Case 2. introduces the complication because the caller's fs{g,u}id is
> > not just used to record ownership but also serves as the {g,u}id used
> > when checking access to the server.
> >
> > Consider a user mounting a cephfs client and creating an idmapped mount
> > from it that maps files owned by uid 1000 to be owned uid 0:
> >
> > mount -t cephfs -o [...] /unmapped
> > mount-idmapped --map-mount 1000:0:1 /idmapped
> >
> > That is to say if the mounted cephfs filesystem contains a file "file1"
> > which is owned by uid 1000:
> >
> > - looking at it via /unmapped/file1 will report it as owned by uid 1000
> > (One can think of this as the on-disk value.)
> > - looking at it via /idmapped/file1 will report it as owned by uid 0
> >
> > Now, consider creating new files via the idmapped mount at /idmapped.
> > When a caller with fs{g,u}id 1000 creates a file "file2" by going
> > through the idmapped mount mounted at /idmapped it will create a file
> > that is owned by uid 1000 on-disk, i.e.:
> >
> > - looking at it via /unmapped/file2 will report it as owned by uid 1000
> > - looking at it via /idmapped/file2 will report it as owned by uid 0
> >
> > Now consider an mds server that has a uid access restriction set and
> > only grants access to requests from uid 0.
> >
> > If the client sends a creation request for a file e.g. /idmapped/file2
> > it will send the caller's fs{g,u}id idmapped according to the idmapped
> > mount. So if the caller has fs{g,u}id 1000 it will be mapped to {g,u}id
> > 0 in the idmapped mount and will be sent over the wire allowing the
> > caller access to the mds server.
> >
> > However, if the caller is not issuing a creation request the caller's
> > fs{g,u}id will be send without the mount's idmapping applied. So if the
> > caller that just successfully created a new file on the restricted mds
> > server sends a request as fs{g,u}id 1000 access will be refused. This
> > however is inconsistent.
> >
>
> IDGI, why would you send the fs{g,u}id without the mount's idmapping
> applied in this case? ISTM that idmapping is wholly a client-side
> feature, and that you should always map id's regardless of whether
> you're creating or not.
Since the idmapping is a property of the mount and not a property of the
caller the caller's fs{g,u}id aren't mapped. What is mapped are the
inode's i{g,u}id when accessed from a particular mount.
The fs{g,u}id are only ever mapped when a new filesystem object is
created. So if I have an idmapped mount that makes it so that files
owned by 1000 on-disk appear to be owned by uid 0 then a user with uid 0
creating a new file will create files with uid 1000 on-disk when going
through that mount. For cephfs that'd be the uid we would be sending
with creation requests as I've currently written it.
So then when the user looks at the file it created it will see it as
being owned by uid 0 from that idmapped mount (whereas on-disk it's
1000). But the user's fs{g,u}id isn't per se changed when going through
that mount. So in my opinion I was thinking that the server with access
permissions set would want to always check permissions on the users
"raw" fs{g,u}id. That would mean I'd have to change the patch obviously.
My suggestion was to send the {g,u}id the file will be created with
separately. The alternative would be to not just pass the idmapping into
the creation iop's but into all iops so that we can always map it for
cephfs. But this would mean a lot of vfs changes for one filesystem. So
if we could first explore alternatives approaches I'd be grateful.
(I'll be traveling for the latter half of this week starting today at
CET afternoon so apologies but I'll probably take some time to respond.)
