device_cgroup: fix RCU usage

commit 201e72acb2d3821e2de9ce6091e98859c316b29a upstream.

dev_cgroup->exceptions is protected with devcgroup_mutex for writes
and RCU for reads; however, RCU usage isn't correct.

* dev_exception_clean() doesn't use RCU variant of list_del() and
  kfree().  The function can race with may_access() and may_access()
  may end up dereferencing already freed memory.  Use list_del_rcu()
  and kfree_rcu() instead.

* may_access() may be called only with RCU read locked but doesn't use
  RCU safe traversal over ->exceptions.  Use list_for_each_entry_rcu().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Cc: Aristeu Rozanski <aris@redhat.com>
Cc: Li Zefan <lizefan@huawei.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Exception list is called whitelist]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 4450fbe..92e24bb 100644 (file)
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -202,8 +202,8 @@ static void devcgroup_destroy(struct cgroup_subsys *ss,
 
        dev_cgroup = cgroup_to_devcgroup(cgroup);
        list_for_each_entry_safe(wh, tmp, &dev_cgroup->whitelist, list) {
-               list_del(&wh->list);
-               kfree(wh);
+               list_del_rcu(&wh->list);
+               kfree_rcu(wh, rcu);
        }
        kfree(dev_cgroup);
 }
@@ -278,7 +278,7 @@ static int may_access_whitelist(struct dev_cgroup *c,
 {
        struct dev_whitelist_item *whitem;
 
-       list_for_each_entry(whitem, &c->whitelist, list) {
+       list_for_each_entry_rcu(whitem, &c->whitelist, list) {
                if (whitem->type & DEV_ALL)
                        return 1;
                if ((refwh->type & DEV_BLOCK) && !(whitem->type & DEV_BLOCK))