linux-kernel - Re: [PATCH] capabilities: add capability cgroup controller

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Date:	Thu, 23 Jun 2016 14:03:46 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Topi Miettinen <toiwoton@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...nel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Jonathan Corbet <corbet@....net>, Tejun Heo <tj@...nel.org>,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	James Morris <james.l.morris@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Petr Mladek <pmladek@...e.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
	"open list:CAPABILITIES" <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH] capabilities: add capability cgroup controller

On Thu, Jun 23, 2016 at 8:07 AM, Topi Miettinen <toiwoton@...il.com> wrote:
> There are many basic ways to control processes, including capabilities,
> cgroups and resource limits. However, there are far fewer ways to find
> out useful values for the limits, except blind trial and error.
>
> Currently, there is no way to know which capabilities are actually used.
> Even the source code is only implicit, in-depth knowledge of each
> capability must be used when analyzing a program to judge which
> capabilities the program will exercise.
>
> Add a new cgroup controller for monitoring of capabilities
> in the cgroup.
>
> Test case demonstrating basic capability monitoring and how the
> capabilities are combined at next level (boot to rdshell):
>
> (initramfs) cd /sys/fs
> (initramfs) mount -t cgroup2 cgroup cgroup
> (initramfs) cd cgroup
> (initramfs) echo +capability > cgroup.subtree_control
> (initramfs) mkdir test; cd test
> (initramfs) echo +capability > cgroup.subtree_control
> (initramfs) ls
> capability.used         cgroup.events           cgroup.subtree_control
> cgroup.controllers      cgroup.procs
> (initramfs) mkdir first second
> (initramfs) sh
>
> BusyBox v1.22.1 (Debian 1:1.22.0-19) built-in shell (ash)
> Enter 'help' for a list of built-in commands.
>
> (initramfs) cd first
> (initramfs) echo $$ >cgroup.procs
> (initramfs) cat capability.used
> 0000000000000000 # nothing so far
> (initramfs) mknod /dev/z_$$ c 1 2
> (initramfs) cat capability.used
> 0000000008000000 # CAP_MKNOD
> (initramfs) cat ../capability.used
> 0000000008000000 # also seen at next higher level
> (initramfs) exit
> (initramfs) sh
>
> BusyBox v1.22.1 (Debian 1:1.22.0-19) built-in shell (ash)
> Enter 'help' for a list of built-in commands.
>
> (initramfs) cd second
> (initramfs) echo $$ >cgroup.procs
> (initramfs) cat capability.used
> 0000000000000000 # nothing so far
> (initramfs) chown 1234 /dev/z_*
> (initramfs) cat capability.used
> 0000000000000001 # CAP_CHROOT

nitpick: this is CAP_CHOWN, not CAP_CHROOT

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security