linux-kernel - Re: [PATCH v2 0/6] device_cgroup: replace internally whitelist with exception list

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Date:	Wed, 5 Sep 2012 03:30:02 +0000
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Aristeu Rozanski <aris@...hat.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	James Morris <jmorris@...ei.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 0/6] device_cgroup: replace internally whitelist
 with exception list

Quoting Aristeu Rozanski (aris@...hat.com):
> The original model of device_cgroup is having a whitelist where all the
> allowed devices are listed. The problem with this approach is that is
> impossible to have the case of allowing everything but few devices.
> 
> The reason for that lies in the way the whitelist is handled internally:
> since there's only a whitelist, the "all devices" entry would have to be
> removed and replaced by the entire list of possible devices but the ones
> that are being denied.  Since dev_t is 32 bits long, representing the allowed
> devices as a bitfield is not memory efficient.
> 
> This patch replaces the "whitelist" by a "exceptions" list and the default
> policy is kept as "deny_all" variable in dev_cgroup structure.
> 
> The current interface determines that whenever "a" is written to devices.allow
> or devices.deny, the entry masking all devices will be added or removed,
> respectively. This behavior is kept and it's what will determine the default
> policy:
> 
> 	# cat devices.list 
> 	a *:* rwm
> 	# echo a >devices.deny
> 	# cat devices.list 
> 	# echo a >devices.allow
> 	# cat devices.list 
> 	a *:* rwm
> 
> The interface is also preserved. For example, if one wants to block only access
> to /dev/null:
> 	# ls -l /dev/null
> 	crw-rw-rw- 1 root root 1, 3 Jul 24 16:17 /dev/null
> 	# echo a >devices.allow
> 	# echo "c 1:3 rwm" >devices.deny
> 	# cat /dev/null
> 	cat: /dev/null: Operation not permitted
> 	# echo >/dev/null
> 	bash: /dev/null: Operation not permitted
> 	# mknod /tmp/null c 1 3
> 	mknod: /tmp/null: Operation not permitted
> 	# echo "c 1:3 r" >devices.allow
> 	# cat /dev/null
> 	# echo >/dev/null
> 	bash: /dev/null: Operation not permitted
> 	# mknod /tmp/null c 1 3
> 	mknod: /tmp/null: Operation not permitted
> 	# echo "c 1:3 rw" >devices.allow
> 	# echo >/dev/null
> 	# cat /dev/null
> 	# mknod /tmp/null c 1 3
> 	mknod: /tmp/null: Operation not permitted
> 	# echo "c 1:3 rwm" >devices.allow
> 	# echo >/dev/null
> 	# cat /dev/null
> 	# mknod /tmp/null c 1 3
> 	#
> 
> v2:
> - stop using simple_strtoul()
> - fix checkpatch warnings
> - rename deny_all to behavior
> - updated documentation
> - added new files to cgroupfs to better reflect the internal state
> 
>  Documentation/cgroups/devices.txt |   73 ++++--
>  security/device_cgroup.c          |  443 +++++++++++++++++++++++---------------
>  2 files changed, 333 insertions(+), 183 deletions(-)
> 
> -- 
> Aristeu

Thanks, Aristeu, very nice.

-serge
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