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

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Message-ID: <4EB0FA73.1020600@msgid.tls.msk.ru>
Date:	Wed, 02 Nov 2011 12:08:19 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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

On 02.11.2011 03:51, Eric W. Biederman wrote:
[]
>> 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.

There's cap_mknod, and cgroup/devices.{allow,deny}.  Even with CAP_MKNOD,
container can not _use_ devices not allowed in the latter.  That's what
I'm talking about - there's more fine control exist than CAP_MKNOD.  And
my question was about this context - with proper cgroup-level device
control in place, what bad CAP_MKNOD have?

Thanks,

/mjt
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