Re: [patch 1/2] [RFC] Simple tamper-proof device filesystem.

[Oren Laadan <orenl@cs.columbia.edu>]

Serge E. Hallyn wrote:
> Quoting Pavel Emelyanov (xemul@openvz.org):
>> Oren Laadan wrote:
>>> Serge E. Hallyn wrote:
>>>> Quoting Oren Laadan (orenl@cs.columbia.edu):
>>>>> I hate to bring this again, but what if the admin in the container
>>>>> mounts an external file system (eg. nfs, usb, loop mount from a file,
>>>>> or via fuse), and that file system already has a device that we would
>>>>> like to ban inside that container ?
>>>> Miklos' user mount patches enforced that if !capable(CAP_MKNOD),
>>>> then mnt->mnt_flags |= MNT_NODEV.  So that's no problem.
>>> Yes, that works to disallow all device files from a mounted file system.
>>>
>>> But it's a black and white thing: either they are all banned or allowed;
>>> you can't have some devices allowed and others not, depending on type
>>> A scenario where this may be useful is, for instance, if we some apps in
>>> the container to execute withing a pre-made chroot (sub)tree within that
>>> container.
>>>
>>>> But that's been pulled out of -mm! ?  Crap.
>>>>
>>>>> Since anyway we will have to keep a white- (or black-) list of devices
>>>>> that are permitted in a container, and that list may change even change
>>>>> per container -- why not enforce the access control at the VFS layer ?
>>>>> It's safer in the long run.
>>>> By that you mean more along the lines of Pavel's patch than my whitelist
>>>> LSM, or you actually mean Tetsuo's filesystem (i assume you don't mean that
>>>> by 'vfs layer' :), or something different entirely?
>>> :)
>>>
>>> By 'vfs' I mean at open() time, and not at mount(), or mknod() time.
>>> Either yours or Pavel's; I tend to prefer not to use LSM as it may
>>> collide with future security modules.
>> Oren, AFAIS you've seen my patches for device access controller, right?

If you mean this one:
http://openvz.org/pipermail/devel/2007-September/007647.html
then ack :)

>>
>> Maybe we can revisit the issue then and try to come to agreement on what
>> kind of model and implementation we all want?
> 
> That would be great, Pavel.  I do prefer your solution over my LSM, so
> if we can get an elegant block device control right in the vfs code that
> would be my preference.

I concur.

So it seems to me that we are all in favor of the model where open()
of a device will consult a black/white-list. Also, we are all in favor
of a non-LSM implementation, Pavel's code being a good example.

Oren.

> The only thing that makes me keep wanting to go back to an LSM is the
> fact that the code defining the whitelist seems out of place in the vfs.
> But I guess that's actually separated into a modular cgroup, with the
> actual enforcement built in at the vfs.  So that's really the best
> solution.
> 
> -serge