Re: [PATCH V2 0/1] ubd: add io_uring based userspace block driver

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, May 18, 2022 at 03:09:46PM +0800, Ming Lei wrote:
> On Tue, May 17, 2022 at 03:06:34PM +0100, Stefan Hajnoczi wrote:
> > Here are some more thoughts on the ubd-control device:
> > 
> > The current patch provides a ubd-control device for processes with
> > suitable permissions (i.e. root) to create, start, stop, and fetch
> > information about devices.
> > 
> > There is no isolation between devices created by one process and those
> 
> I understand linux hasn't device namespace yet, so can you share the
> rational behind the idea of device isolation, is it because ubd device
> is served by ubd daemon which belongs to one pid NS? Or the user creating
> /dev/ubdbN belongs to one user NS?

With the current model a process with access to ubd-control has control
over all ubd devices. This is not desirable for most container use cases
because ubd-control usage within a container means that container could
stop any ubd device on the system.

Even for non-container use cases it's problematic that two applications
that use ubd can interfere with each other. If an application passes the
wrong device ID they can stop the other application's device, for
example.

I think it's worth supporting a model where there are multiple ubd
daemons that are not cooperating/aware of each other. They should be
isolated from each other.

> IMO, ubd device is one file in VFS, and FS permission should be applied,
> then here the closest model should be user NS, and process privilege &
> file ownership.

Yes, /dev/ubdbN can has file ownership/permissions and the cgroup device
controller can restrict access too. That works fine when the device was
created previously.

But what about ubd device creation via ubd-control?

The problem is a global control interface like ubd-control gives access
to all ubd devices. There is no way to let an application/container
control (create/start/stop/etc) some ubd devices but not all. I think
ubd-control must be more fine-grained so multiple
applications/containers can use it without the possibility of
interference.

/dev/ubdcN is a separate problem. The cgroup device controller can limit
the device nodes that are accessible from a process. However, this
requires reserving device minor number ranges for each
application/container so they can only mknod/open their own ubd devices
and not devices that don't belong to them. Maybe there is a better
solution?

/dev/ubdbN has similar requirements to /dev/ubdcN. It should be possible
to create a new /dev/ubdbN but not access an existing device that belong

So if we want to let containers create ubd devices without granting them
access to all devices on the system, then the ubd-control interface
needs to be changed (see below) and the container needs a reserved range
of ubdcN minor numbers. Any container using ubdbN needs the cgroup
device controller and file ownership/permissions to open the block
device.

> > created by another. Therefore two processes that do not trust each other
> > cannot both use UBD without potential interference. There is also no
> 
> Can you share what the expectation is for this situation?

Two users should be able to run ubd daemons on the same system without
being able to stop each other's devices.

> It is the created UBD which can only be used in this NS, or can only be
> visible inside this NS? I guess the latter isn't possible since we don't
> have this kind of isolation framework yet.

It should be possible to access the ubd device according to file
ownership/permissions. No new isolation framework is needed for that.

But ubd-control should not grant global access to all ubd devices, at
least not in the typical case of a ubd daemon that just wishes to
create/start/stop its own devices.

> > isolation for containers.
> > 
> > I think it would be a mistake to keep the ubd-control interface in its
> > current form since the current global/root model is limited. Instead I
> > suggest:
> > - Creating a device returns a new file descriptor instead of a global
> >   dev_id. The device can be started/stopped/configured through this (and
> >   only through this) per-device file descriptor. The device is not
> >   visible to other processes through ubd-control so interference is not
> >   possible. In order to give another process control over the device the
> >   fd can be passed (e.g. SCM_RIGHTS). 
> > 
> 
> /dev/ubdcN can only be opened by the process which is the descendant of
> the process which creates the device by sending ADD_DEV.
> 
> But the device can be deleted/queried by other processes, however, I
> think it is reasonable if all these processes has permission to do that,
> such as all processes owns the device with same uid.

I don't think it's a good idea to require all ubd daemons to have
CAP_SYS_ADMIN/same uid. That's the main point I'm trying to make and the
discussion is based on that.

> So can we apply process privilege & file ownership for isolating ubd device?
> 
> If per-process FD is used, it may confuse people, because process can
> not delete/query ubd device even though its uid shows it has the
> privilege.

Is it better to stop the device via ubd-control instead of a
daemon-specific command (or just killing the daemon process)?

Regarding querying the device, the daemon has more information
associated with the device (e.g. if it's an iSCSI initiator it will have
the iSCSI URI). The ubd driver can only tell you the daemon pid and the
block device attributes that should already be available via sysfs.
Quering the daemon will yield more useful information than using
ubd-control.

> > Now multiple applications/containers/etc can use ubd-control without
> > interfering with each other. The security model still requires root
> > though since devices can be malicious.
> > 
> > FUSE allows unprivileged mounts (see fuse_allow_current_process()). Only
> > processes with the same uid as the FUSE daemon can access such mounts
> > (in the default configuration). This prevents security issues while
> > still allowing unprivileged use cases.
> 
> OK, looks FUSE applies process privilege & file ownership for dealing
> with unprivileged mounts.
> 
> > 
> > I suggest adapting the FUSE security model to block devices:
> > - Devices can be created without CAP_SYS_ADMIN but they have an
> >   'unprivileged' flag set to true.
> > - Unprivileged devices are not probed for partitions and LVM doesn't
> >   touch them. This means the kernel doesn't access these devices via
> >   code paths that might be exploitable.
> 
> The above two makes sense.
> 
> > - When another process with a different uid from ubdsrv opens an
> >   unprivileged device, -EACCES is returned. This protects other
> >   uids from the unprivileged device.
> 
> OK, only the user who owns the device can access unprivileged device.
> 
> > - When another process with a different uid from ubdsrv opens a
> >   _privileged_ device there is no special access check because ubdsrv is
> >   privileged.
> 
> IMO, it depends if uid of this process has permission to access the
> ubd device, and we can set ubd device's owership by the process
> credentials.

Yes, file ownership/permissions are still relevant.

> 
> > 
> > With these changes UBD can be used by unprivileged processes and
> > containers. I think it's worth discussing the details and having this
> > model from the start so UBD can be used in a wide range of use cases.
> 
> I am pretty happy to discuss & figure out the details, but not sure
> it is one blocker for ubd:
> 
> 1) kernel driver of loop/nbd or others haven't support the isolation

It may be better to compare it with FUSE where unprivileged users can
run their own servers. Imagine FUSE required a global root control
interface like ubd-control, then it wouldn't be possible to have
unprivileged FUSE mounts.

> 2) still don't know exact ubd use case for containers

There are two common use cases for block devices:
1. File systems or volume managers
2. Direct access for databases, backup tools, disk image tools, etc

The file system use case involved kernel code and probably needs to be
restricted to untrusted containers cannot exploit the kernel file system
implementations. I'll ignore this use case and containers probably
shouldn't do this.

The second use case is when you have any program that can operate on a
block device. It could be an application that imports/exports a block
device from network storage. This kind of application should be able to
do its job without CAP_SYS_ADMIN and it should be able to run in a
container. It might be part of KubeVirt's Containerized Data Importer,
for example, and is deployed as a container.

If ubd supports unprivileged operation then this container use case is
straightforward. If not, then it's problematic because it either
requires a privileged container or some kind of privileged helper
outside the container. At that point people may avoid ubd because it's
too hard to deploy with privilege requirements.

Stefan

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