device_cgroup: fix unchecked cgroup parent usage

In 4cef7299b478687 ("device_cgroup: add proper checking when changing
default behavior") the cgroup parent usage is unchecked.  root will not
have a parent and trying to use device.{allow,deny} will cause problems.
For some reason my stressing scripts didn't test the root directory so I
didn't catch it on my regular tests.

Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: James Morris <jmorris@namei.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 842c254396db..96d87eab1660 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -352,6 +352,8 @@ static int parent_has_perm(struct dev_cgroup *childcg,
  */
 static inline int may_allow_all(struct dev_cgroup *parent)
 {
+        if (!parent)
+                return 1;
         return parent->behavior == DEVCG_DEFAULT_ALLOW;
 }
 
@@ -376,11 +378,14 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
         int count, rc;
         struct dev_exception_item ex;
         struct cgroup *p = devcgroup->css.cgroup;
-        struct dev_cgroup *parent = cgroup_to_devcgroup(p->parent);
+        struct dev_cgroup *parent = NULL;
 
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;
 
+        if (p->parent)
+                parent = cgroup_to_devcgroup(p->parent);
+
         memset(&ex, 0, sizeof(ex));
         b = buffer;
 
@@ -391,11 +396,14 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
                         if (!may_allow_all(parent))
                                 return -EPERM;
                         dev_exception_clean(devcgroup);
+                        devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
+                        if (!parent)
+                                break;
+
                         rc = dev_exceptions_copy(&devcgroup->exceptions,
                                                  &parent->exceptions);
                         if (rc)
                                 return rc;
-                        devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
                         break;
                 case DEVCG_DENY:
                         dev_exception_clean(devcgroup);