Re: Controlling devices and device namespaces

Thinking about this a bit more I think we have been asking the wrong
question.

I think the correct question should be: How do we safely allow for
unprivileged creation of device nodes and devices?

One piece of the puzzle is that we should be able to allow unprivileged
device node creation and access for any device on any filesystem
for which it unprivileged access is safe.

Something like the current device control group hooks but
with the whitelist implemented like:

static bool unpriv_mknod_ok(struct device *dev)
{
	char *tmp, *name;
	umode_t mode = 0;

	name = device_get_devnode(dev, &mode, &tmp);
	if (!name)
        	return false;
	kfree(tmp);
        return mode == 0666;
}

Are there current use cases where people actually want arbitrary
access to hardware devices?  I really want to say no and get
udev and sysfs out of the picture as much as possible.

"Serge E. Hallyn" [off-list ref] writes:

Quoting Eric W. Biederman (ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org):

quoted

"Serge E. Hallyn" [off-list ref] writes:

quoted

Quoting Aristeu Rozanski (aris-moeOTchvdi7YtjvyW6yDsg@public.gmane.org):

quoted

Tejun,
On Thu, Sep 13, 2012 at 01:58:27PM -0700, Tejun Heo wrote:

quoted

  memcg can be handled by memcg people and I can handle cgroup_freezer
  and others with help from the authors.  The problematic one is
  blkio.  If anyone is interested in working on blkio, please be my
  guest.  Vivek?  Glauber?

if Serge is not planning to do it already, I can take a look in device_cgroup.

That's fine with me, thanks.

quoted

also, heard about the desire of having a device namespace instead with
support for translation ("sda" -> "sdf"). If anyone see immediate use for
this please let me know.

Before going down this road, I'd like to discuss this with at least you,
me, and Eric Biederman (cc:d) as to how it relates to a device
namespace.

The problem with devices.

- An unrestricted mknod gives you access to effectively any device in
  the system.

- During process migration if the device number changes using
  stat to file descriptors can fail on the same file descriptor.

- Devices coming from prexisting filesystems that we mount
  as unprivileged users are as dangerous as mknod but show
  that the problem is not limited to mknod.

- udev thinks mknod is a system call we can remove from the kernel.

Also,

 - udevadm trigger --action=add

causes all the devices known on the host to be re-sent to
everyone (all namespaces).  Which floods everyone and causes the
host to reset some devices.

I think this is all userspace activity, and should be largely
fixed by not begin root in a container.

quoted

---

The use cases seem comparitively simple to enumerate.

- Giving unfiltered access to a device to someone not root.

- Virtual devices that everyone uses and have no real privilege
  requirements: /dev/null /dev/tty /dev/zero etc.

- Dynamically created devices /dev/loopN /dev/tun /dev/macvtapN,
  nbd, iscsi, /dev/ptsN, etc

and

 - per-namespace uevent filtering.

One possible solution there is to just send the kernel uevents (except
for the network ones) into the initial network namespace.

quoted

---

There are a couple of solution to these problems.

- The classic solution of creating a /dev for a container
  before starting it.

- The devpts filesystem.  This works well for unprivileged access
  to ptys.  Except for the /dev/ptmx sillines I very like how
  things are handled today with devpts.

- Device control groups.  I am not quite certain what to make
  of them.  The only case I see where they are better than
  a prebuilt static dev is if there is a hotppluged device
  that I want to push into my container.

  I think the only problem with device control groups and
  hierarchies is that removing a device from a whitelist
  does not recurse down the hierarchy.

That's going to be fixed soon thanks to Aristeu  :)

quoted

  Can a process inside of a device control group create
  a child group that has access to a subset of it's
  devices?  The actually checks don't need to be hierarchical
  but the presence of device nodes should be.

If I understand your question right, yes.

I should also have asked can we do this without any capabilities
and without our uid being 0?

quoted

---

I see a couple of holes in the device control picture.

- How do we handle hotplug events?

  I think we can do this by relaying events trough userspace,
  upating the device control groups etc.

- Unprivileged processess interacting with all of this.
  (possibly with privilege in their user namespace)
  What I don't know how to do is how to create a couple of different
  subhierarchies each for different child processes.

- Dynamically created devices.

  My gut feel is that we should replicate the success of devpts
  and give each type of dynamically created device it's own
  filesystem and mount point under /dev, and just bend
  the handful of userspace users into that model.

Phew.  Maybe.  Had not considered that.  But seems daunting.

I think the list of device types that we care about here is pretty
small.  Please correct me if I am wrong.

loop nbd iscsi macvtap

And if we want it to be safe to use these devices in a user namespace
without global root privileges we need to go through the code anyway.

So I think it is the gradual safe and sane approach assume we don't
run into something like the devpts /dev/ptmx silliness that stalled
devpts.

quoted

- Sysfs

  My gut says for the container use case we should aim to
  simply not have dynamically created devices in sysfs
  and then we can simply not care.

I guess what I keep thinking for sysfs is that it should be for real
hardware backed devices.  If we can get away with that like we do with
ptys it just makes everyone's life simpler.

Primarily sysfs and uevents are for allowing the system to take
automatic action when a new device is created.  Do we have an actual
need for hotplug support in containers?

quoted

- Migration

  Either we need block device numbers that can migrate with us,
  (possibly a subset of the entire range ala devpts) or we need to send
  hotplug events to userspace right after a migration so userspace
  processes that care can invalidate their caches of stat data.

---

With the code in my userns development tree I can create a user
namespace, create a new mount namespace, and then if I have
access to any block devices mount filesystems, all without
needing to have any special privileges.  What I haven't
figured out is what it would take to get the the device
control group into the middle that.

I'm really not sure that's a question we want to ask.  The
device control group, like the ns cgroup, was meant as a
temporary workaround to not having user and device namespaces.

If we can come up with a device cgroup model that works to
fill all the requirements we would have for a devices ns, then
great.  But I don't want us to be constrained by that.

quoted

It feels like it should be possible to get the checks straight
and use the device control group hooks to control which devices
are usable in a user namespace.  Unfortunately when I try and work
it out the independence of the user namespace and the device
control group seem to make that impossible.

Shrug there is most definitely something missing from our
model on how to handle devices well.  I am hoping we can
sprinkling some devpts derived pixie dust at the problem
migrate userspace to some new interfaces and have life
be good.

Eric

Me too!

I'm torn between suggesting that we have a session at UDS to
discuss this, and not wanting to so that we can focus on the
remaining questions with the user namespace.

Eric