[PATCH v2 0/6] device_cgroup: replace internally whitelist with exception list

[Aristeu Rozanski <aris@redhat.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