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

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Message-ID: <476724E3.6060901@cs.columbia.edu>
Date:	Mon, 17 Dec 2007 20:39:47 -0500
From:	Oren Laadan <orenl@...columbia.edu>
To:	"Serge E. Hallyn" <serue@...ibm.com>
CC:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	linux-fsdevel@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] [RFC] Simple tamper-proof device filesystem.

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 ?

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.

Oren.

Serge E. Hallyn wrote:
> Quoting Tetsuo Handa (penguin-kernel@...ove.sakura.ne.jp):
>> Hello.
>>
>> Serge E. Hallyn wrote:
>>> CAP_MKNOD will be removed from its capability
>> I think it is not enough because the root can rename/unlink device files
>> (mv /dev/sda1 /dev/tmp; mv /dev/sda2 /dev/sda1; mv /dev/tmp /dev/sda2).
> 
> Sure but that doesn't bother us :)
> 
> The admin in the container has his own /dev directory and can do what he
> likes with the devices he's allowed to have.  He just shouldn't have
> access to others.  If he wants to rename /dev/sda1 to /dev/sda5 that's
> his choice.
> 
>>> To use your approach, i guess we would have to use selinux (or tomoyo)
>>> to enforce that devices may only be created under /dev?
>> Everyone can use this filesystem alone.
> 
> Sure but it is worthless alone.
> 
> No?
> 
> What will keep the container admin from doing 'mknod /root/hda1 b 3 1'?
> 
>> But use with MAC (or whatever access control mechanisms that prevent
>> attackers from unmounting/overlaying this filesystem) is recomennded.
> 
> -serge
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> Containers@...ts.linux-foundation.org
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