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Bug#501359: mount devpts with the newinstance option

md@Linux.IT (Marco d'Itri) writes:

> On Jun 14, Ferenc Wagner <wferi@niif.hu> wrote:
>> Michael Prokop <mika@debian.org> writes:
>>> * Ferenc Wagner <wferi@niif.hu> [Mon Jun 14, 2010 at 12:47:42PM +0200]:
>>>> Michael Prokop <mika@debian.org> writes:
>>>>> * Ferenc Wagner <wferi@niif.hu> [Wed Jun 09, 2010 at 04:14:22PM +0200]:
>>>>>> A side note: a couple of lines later devpts is still mounted in "legacy"
>>>>>> mode.  This makes full devpts isolation impossible, which is a problem
>>>>>> if the running system wants to use Linux containers.  I didn't track,
>>>>>> maybe this devpts mount does not survive the initramfs phase, but if it
>>>>>> does, this issue is something to think about (until the devpts default
>>>>>> changes, which won't be soon, and surely not for 2.6.32).
>>>>> Sorry, I'm not sure I can follow you. Can you please elaborate a bit?
>>>> You may want to refer to Documentation/devpts.txt in the Linux kernel
>>>> tree for more details.  Above I mixed up the terminology somewhat.  The
>>>> Squeeze kernel has CONFIG_DEVPTS_MULTIPLE_INSTANCES=y, so it isn't
>>>> running in legacy mode, but the devpts mounts (both in the initramfs and
>>>> in the startup scripts) don't use the 'newinstance' option, so the
>>>> system ends up using the "initial kernel mount of devpts".  Now even if
>>>> the container setup scripts take care to mount their own devpts
>>>> instances using the 'newinstance' option, this does not forbit the
>>>> contained system to also mount the "initial kernel mount of devpts" and
>>>> possibly interfere with the host system through it.  The solution is to
>>>> mount devpts in the host system with the 'newinstance' option, too, and
>>>> thus insulate it from the containers possibly running on the host.
>>>> I'm still disturbed by the fact that several containers could mount the
>>>> "initial kernel mount of devpts" and cooperate through it, but that's
>>>> their choice to use the inherently insecure common instance.
>>>> Hope I managed to make it clearer now.  I'm no expert of this, but this
>>>> concern seems somewhat founded, cf.
>>>> http://article.gmane.org/gmane.linux.kernel.containers.lxc.devel/298
>>> Ok thanks.
>>> Would be a "mount -o remount,newinstance /dev/pts" right after
>>> mounting /dev/pts be an option for us? The command fails without
>>> causing any problems if the required kernel option isn't set, so it
>>> should be safe.
>> Probably yes.
>>> Though AFAICS udev unmounts /dev/pts via /etc/init.d/udev anyway
>>> (stating "we need to unmount /dev/pts/ and remount it later over the
>>> tmpfs"). Should we ask the udev maintainer about it this issue as
>>> well?
> It's not really hard to check the init script. /dev/pts/ and /dev/shm/
> are mounted by /etc/init.d/mountkernfs.sh .

You mean /etc/init.d/mountdevsubfs.sh .

> Send patches to the initscripts maintainer.

That's why didn't find this stuff: because I mentioned it instead to the
initscripts maintainer in Bug#584742.  That bug was meanwhile closed,
let's ask Petter about it again!

Besides that, I still think fixing this in the initramfs would be a good
idea, just in case something goes wrong or changes later.

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