linux-kernel - Re: [lxc-devel] Detecting if you are running in a container

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Date:	Tue, 01 Nov 2011 16:51:22 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Kay Sievers <kay.sievers@...y.org>,
	Lennart Poettering <mzxreary@...inter.de>, greg@...ah.com,
	Paul Menage <paul@...lmenage.org>,
	linux-kernel@...r.kernel.org, david@...ar.dk,
	Linux Containers <lxc-devel@...ts.sourceforge.net>,
	Linux Containers <containers@...ts.osdl.org>,
	"Serge E. Hallyn" <serge@...lyn.com>, harald@...hat.com
Subject: Re: [lxc-devel] Detecting if you are running in a container

Michael Tokarev <mjt@....msk.ru> writes:

> [Replying to an oldish email...]
>
> On 12.10.2011 20:59, Kay Sievers wrote:
>> On Mon, Oct 10, 2011 at 23:41, Lennart Poettering <mzxreary@...inter.de> wrote:
>>> On Mon, 10.10.11 13:59, Eric W. Biederman (ebiederm@...ssion.com) wrote:
>> 
>>>> - udev.  All of the kernel interfaces for udev should be supported in
>>>>   current kernels.  However I believe udev is useless because container
>>>>   start drops CAP_MKNOD so we can't do evil things.  So I would
>>>>   recommend basing the startup of udev on presence of CAP_MKNOD.
>>>
>>> Using CAP_MKNOD as test here is indeed a good idea. I'll make sure udev
>>> in a systemd world makes use of that.
>> 
>> Done.
>> 
>> http://git.kernel.org/?p=linux/hotplug/udev.git;a=commitdiff;h=9371e6f3e04b03692c23e392fdf005a08ccf1edb
>
> Maybe CAP_MKNOD isn't actually a good idea, having in mind devtmpfs?
>
> Without CAP_MKNOD, is devtmpfs still being populated internally by
> the kernel, so that udev only needs to change ownership/permissions
> and maintain symlinks in response to device changes, and perform
> other duties (reacting to other types of events) normally?
>
> In other words, provided devtmpfs works even without CAP_MKNOD,
> I can easily imagine a whole system running without this capability
> from the very early boot, with all functionality in place, including
> udev and what not...

Agreed devtmpfs does pretty much make dropping CAP_MKNOD useless.  I
expect we should verify that whoever mounts devtmpfs has CAP_MKNOD.

> And having CAP_MKNOD in container may not be that bad either, while
> cgroup device.permission is set correctly - some nodes may need to
> be created still, even in an unprivileged containers.  Who filters
> out CAP_MKNOD during container startup (I don't see it in the code,
> which only removes CAP_SYS_BOOT, and even that due to current
> limitation), and which evil things can be done if it is not filtered?

If you don't filter which device nodes you a process can read/write then
that process can access any device on the system.  Steal the keyboard,
the X display, access any filesystem, directly access memory.  Basically
the process can escalate that permission to full control of the system
without needing any kernel bugs to help it.

Eric
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