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

[Pavel Emelyanov <xemul@openvz.org>]

Oren Laadan wrote:
> 
> 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 :)

Great! Thanks.

>>> 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.

Thank you, Oren and Serge! I will revisit this issue then, but
I have a vacation the next week and, after this, we have a New
Year and Christmas holidays in Russia. So I will be able to go
on with it only after the 7th January :( Hope this is OK for you.

Besides, Andrew told that he would pay little attention to new
features till the 2.6.24 release, so I'm afraid we won't have this 
even in -mm in the nearest months :(

Thanks,
Pavel

> 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
>