devcgroup: fix race against rmdir()

During the use of a dev_cgroup, we should guarantee the corresponding
cgroup won't be deleted (i.e.  via rmdir).  This can be done through
css_get(&dev_cgroup->css), but here we can just get and use the dev_cgroup
under rcu_read_lock.

And also remove checking NULL dev_cgroup, it won't be NULL since a task
always belongs to a cgroup.
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Acked-by: NSerge Hallyn <serue@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

security/device_cgroup.c

@@ -508,12 +508,11 @@ int devcgroup_inode_permission(struct inode *inode, int mask)
 		return 0;
 	if (!S_ISBLK(inode->i_mode) && !S_ISCHR(inode->i_mode))
 		return 0;
-	dev_cgroup = css_to_devcgroup(task_subsys_state(current,
-				devices_subsys_id));
-	if (!dev_cgroup)
-		return 0;
 
 	rcu_read_lock();
+
+	dev_cgroup = task_devcgroup(current);
+
 	list_for_each_entry_rcu(wh, &dev_cgroup->whitelist, list) {
 		if (wh->type & DEV_ALL)
 			goto acc_check;
@@ -533,6 +532,7 @@ int devcgroup_inode_permission(struct inode *inode, int mask)
 		rcu_read_unlock();
 		return 0;
 	}
+
 	rcu_read_unlock();
 
 	return -EPERM;
@@ -543,12 +543,10 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
 	struct dev_cgroup *dev_cgroup;
 	struct dev_whitelist_item *wh;
 
-	dev_cgroup = css_to_devcgroup(task_subsys_state(current,
-				devices_subsys_id));
-	if (!dev_cgroup)
-		return 0;
-
 	rcu_read_lock();
+
+	dev_cgroup = task_devcgroup(current);
+
 	list_for_each_entry(wh, &dev_cgroup->whitelist, list) {
 		if (wh->type & DEV_ALL)
 			goto acc_check;
@@ -566,6 +564,8 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
 		rcu_read_unlock();
 		return 0;
 	}
+
 	rcu_read_unlock();
+
 	return -EPERM;
 }