[PATCH 1/1] device_cgroup: remove can_attach

[Serge Hallyn]

Quoting Tim Gardner (tim.gardner at canonical.com):
> On 10/24/2013 12:17 PM, Serge Hallyn wrote:
> >[ Hi - the following patch has been applied to Tejun's
> >cgroup/for-3.13 branch.  It's needed for non-root users to be
> >able to create containers ]
> >
> >It is really only wanting to duplicate a check which is already done by the
> >cgroup subsystem.
> >
> >With this patch, user jdoe still cannot move pid 1 into a devices cgroup
> >he owns, but now he can move his own other tasks into devices cgroups.
> >
> >Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>
> >Signed-off-by: Tejun Heo <tj at kernel.org>
> >Cc: Aristeu Rozanski <aris at redhat.com>
> >---
> >  security/device_cgroup.c |   11 -----------
> >  1 file changed, 11 deletions(-)
> >
> >diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> >index c123628..7c2a0a7 100644
> >--- a/security/device_cgroup.c
> >+++ b/security/device_cgroup.c
> >@@ -63,16 +63,6 @@ static inline struct dev_cgroup *task_devcgroup(struct task_struct *task)
> >
> >  struct cgroup_subsys devices_subsys;
> >
> >-static int devcgroup_can_attach(struct cgroup_subsys_state *new_css,
> >-				struct cgroup_taskset *set)
> >-{
> >-	struct task_struct *task = cgroup_taskset_first(set);
> >-
> >-	if (current != task && !capable(CAP_SYS_ADMIN))
> >-		return -EPERM;
> >-	return 0;
> >-}
> >-
> >  /*
> >   * called under devcgroup_mutex
> >   */
> >@@ -697,7 +687,6 @@ static struct cftype dev_cgroup_files[] = {
> >
> >  struct cgroup_subsys devices_subsys = {
> >  	.name = "devices",
> >-	.can_attach = devcgroup_can_attach,
> >  	.css_alloc = devcgroup_css_alloc,
> >  	.css_free = devcgroup_css_free,
> >  	.css_online = devcgroup_online,
> >
> 
> This is kind of a fundamental change. Can you write a test case ?

This test case checks whether the patch is present (and makes sure it
doesn't let non-root users move root owned tasks).

#!/bin/bash

die() {
	echo $1
	exit 1
}

create_chown_cgroup() {
	echo "Creating a cgroup to use"
	sudo mkdir -p /sys/fs/cgroup/devices/xxx
	sudo chown -R $1 /sys/fs/cgroup/devices/xxx || return 1
	return 0
}

test_move_init() {
	echo "Testing whether we can change init's cgroup as non-root (should fail)"
	echo 1 | /sys/fs/cgroup/devices/xxx/tasks && return 1
	return 0
}

test_move_child() {
	echo "Testing whether we can change our child's cgroup as non-root (should succeed)"
	sleep 100 &
	echo $! > /sys/fs/cgroup/devices/xxx/tasks || return 1
	kill $!
	return 0
}

myid=$(id -u)
if [ $myid -eq 0 ]; then
	die "don't run this as root"
	exit 1
fi

create_chown_cgroup $myid || die "failed to create test cgroup"

test_move_init || die "was able to move init to new cgroup"

test_move_child || die "was unable to move my child to new cgroup"

echo "Success"
exit 0