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: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 7bd296cca041aaa7e62a3f923182ff7ac2adf5ef..46f23971f7e4208d9487cbb9cf477790a8fb378a 100644 (file)
--- a/security/device_cgroup.c
+++ b/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 @@ acc_check:
                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 @@ acc_check:
                rcu_read_unlock();
                return 0;
        }
+
        rcu_read_unlock();
+
        return -EPERM;
 }