2 files changed, 199 insertions, 10 deletions

diff --git a/Documentation/cgroups/devices.txt b/Documentation/cgroups/devices.txt
index 16624a7f8222..3c1095ca02ea 100644
--- a/Documentation/cgroups/devices.txt
+++ b/Documentation/cgroups/devices.txt
@@ -13,9 +13,7 @@ either an integer or * for all.  Access is a composition of r
 The root device cgroup starts with rwm to 'all'.  A child device
 cgroup gets a copy of the parent.  Administrators can then remove
 devices from the whitelist or add new entries.  A child cgroup can
-never receive a device access which is denied by its parent.  However
-when a device access is removed from a parent it will not also be
-removed from the child(ren).
+never receive a device access which is denied by its parent.
 
 2. User Interface
 
@@ -50,3 +48,69 @@ task to a new cgroup.  (Again we'll probably want to change that).
 
 A cgroup may not be granted more permissions than the cgroup's
 parent has.
+
+4. Hierarchy
+
+device cgroups maintain hierarchy by making sure a cgroup never has more
+access permissions than its parent.  Every time an entry is written to
+a cgroup's devices.deny file, all its children will have that entry removed
+from their whitelist and all the locally set whitelist entries will be
+re-evaluated.  In case one of the locally set whitelist entries would provide
+more access than the cgroup's parent, it'll be removed from the whitelist.
+
+Example:
+      A
+     / \
+        B
+
+    group        behavior       exceptions
+    A            allow          "b 8:* rwm", "c 116:1 rw"
+    B            deny           "c 1:3 rwm", "c 116:2 rwm", "b 3:* rwm"
+
+If a device is denied in group A:
+        # echo "c 116:* r" > A/devices.deny
+it'll propagate down and after revalidating B's entries, the whitelist entry
+"c 116:2 rwm" will be removed:
+
+    group        whitelist entries                        denied devices
+    A            all                                      "b 8:* rwm", "c 116:* rw"
+    B            "c 1:3 rwm", "b 3:* rwm"                 all the rest
+
+In case parent's exceptions change and local exceptions are not allowed
+anymore, they'll be deleted.
+
+Notice that new whitelist entries will not be propagated:
+      A
+     / \
+        B
+
+    group        whitelist entries                        denied devices
+    A            "c 1:3 rwm", "c 1:5 r"                   all the rest
+    B            "c 1:3 rwm", "c 1:5 r"                   all the rest
+
+when adding "c *:3 rwm":
+        # echo "c *:3 rwm" >A/devices.allow
+
+the result:
+    group        whitelist entries                        denied devices
+    A            "c *:3 rwm", "c 1:5 r"                   all the rest
+    B            "c 1:3 rwm", "c 1:5 r"                   all the rest
+
+but now it'll be possible to add new entries to B:
+        # echo "c 2:3 rwm" >B/devices.allow
+        # echo "c 50:3 r" >B/devices.allow
+or even
+        # echo "c *:3 rwm" >B/devices.allow
+
+Allowing or denying all by writing 'a' to devices.allow or devices.deny will
+not be possible once the device cgroups has children.
+
+4.1 Hierarchy (internal implementation)
+
+device cgroups is implemented internally using a behavior (ALLOW, DENY) and a
+list of exceptions.  The internal state is controlled using the same user
+interface to preserve compatibility with the previous whitelist-only
+implementation.  Removal or addition of exceptions that will reduce the access
+to devices will be propagated down the hierarchy.
+For every propagated exception, the effective rules will be re-evaluated based
+on current parent's access rules.
diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 16c9e1069be6..221967d4690c 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -49,6 +49,8 @@ struct dev_cgroup {
         struct cgroup_subsys_state css;
         struct list_head exceptions;
         enum devcg_behavior behavior;
+        /* temporary list for pending propagation operations */
+        struct list_head propagate_pending;
 };
 
 static inline struct dev_cgroup *css_to_devcgroup(struct cgroup_subsys_state *s)
@@ -185,6 +187,11 @@ static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
         __dev_exception_clean(dev_cgroup);
 }
 
+static inline bool is_devcg_online(const struct dev_cgroup *devcg)
+{
+        return (devcg->behavior != DEVCG_DEFAULT_NONE);
+}
+
 /**
  * devcgroup_online - initializes devcgroup's behavior and exceptions based on
  *                    parent's
@@ -235,6 +242,7 @@ static struct cgroup_subsys_state *devcgroup_css_alloc(struct cgroup *cgroup)
         if (!dev_cgroup)
                 return ERR_PTR(-ENOMEM);
         INIT_LIST_HEAD(&dev_cgroup->exceptions);
+        INIT_LIST_HEAD(&dev_cgroup->propagate_pending);
         dev_cgroup->behavior = DEVCG_DEFAULT_NONE;
         parent_cgroup = cgroup->parent;
 
@@ -413,6 +421,111 @@ static inline int may_allow_all(struct dev_cgroup *parent)
         return parent->behavior == DEVCG_DEFAULT_ALLOW;
 }
 
+/**
+ * revalidate_active_exceptions - walks through the active exception list and
+ *                                revalidates the exceptions based on parent's
+ *                                behavior and exceptions. The exceptions that
+ *                                are no longer valid will be removed.
+ *                                Called with devcgroup_mutex held.
+ * @devcg: cgroup which exceptions will be checked
+ *
+ * This is one of the three key functions for hierarchy implementation.
+ * This function is responsible for re-evaluating all the cgroup's active
+ * exceptions due to a parent's exception change.
+ * Refer to Documentation/cgroups/devices.txt for more details.
+ */
+static void revalidate_active_exceptions(struct dev_cgroup *devcg)
+{
+        struct dev_exception_item *ex;
+        struct list_head *this, *tmp;
+
+        list_for_each_safe(this, tmp, &devcg->exceptions) {
+                ex = container_of(this, struct dev_exception_item, list);
+                if (!parent_has_perm(devcg, ex))
+                        dev_exception_rm(devcg, ex);
+        }
+}
+
+/**
+ * get_online_devcg - walks the cgroup tree and fills a list with the online
+ *                    groups
+ * @root: cgroup used as starting point
+ * @online: list that will be filled with online groups
+ *
+ * Must be called with devcgroup_mutex held. Grabs RCU lock.
+ * Because devcgroup_mutex is held, no devcg will become online or offline
+ * during the tree walk (see devcgroup_online, devcgroup_offline)
+ * A separated list is needed because propagate_behavior() and
+ * propagate_exception() need to allocate memory and can block.
+ */
+static void get_online_devcg(struct cgroup *root, struct list_head *online)
+{
+        struct cgroup *pos;
+        struct dev_cgroup *devcg;
+
+        lockdep_assert_held(&devcgroup_mutex);
+
+        rcu_read_lock();
+        cgroup_for_each_descendant_pre(pos, root) {
+                devcg = cgroup_to_devcgroup(pos);
+                if (is_devcg_online(devcg))
+                        list_add_tail(&devcg->propagate_pending, online);
+        }
+        rcu_read_unlock();
+}
+
+/**
+ * propagate_exception - propagates a new exception to the children
+ * @devcg_root: device cgroup that added a new exception
+ * @ex: new exception to be propagated
+ *
+ * returns: 0 in case of success, != 0 in case of error
+ */
+static int propagate_exception(struct dev_cgroup *devcg_root,
+                               struct dev_exception_item *ex)
+{
+        struct cgroup *root = devcg_root->css.cgroup;
+        struct dev_cgroup *devcg, *parent, *tmp;
+        int rc = 0;
+        LIST_HEAD(pending);
+
+        get_online_devcg(root, &pending);
+
+        list_for_each_entry_safe(devcg, tmp, &pending, propagate_pending) {
+                parent = cgroup_to_devcgroup(devcg->css.cgroup->parent);
+
+                /*
+                 * in case both root's behavior and devcg is allow, a new
+                 * restriction means adding to the exception list
+                 */
+                if (devcg_root->behavior == DEVCG_DEFAULT_ALLOW &&
+                    devcg->behavior == DEVCG_DEFAULT_ALLOW) {
+                        rc = dev_exception_add(devcg, ex);
+                        if (rc)
+                                break;
+                } else {
+                        /*
+                         * in the other possible cases:
+                         * root's behavior: allow, devcg's: deny
+                         * root's behavior: deny, devcg's: deny
+                         * the exception will be removed
+                         */
+                        dev_exception_rm(devcg, ex);
+                }
+                revalidate_active_exceptions(devcg);
+
+                list_del_init(&devcg->propagate_pending);
+        }
+        return rc;
+}
+
+static inline bool has_children(struct dev_cgroup *devcgroup)
+{
+        struct cgroup *cgrp = devcgroup->css.cgroup;
+
+        return !list_empty(&cgrp->children);
+}
+
 /*
  * Modify the exception list using allow/deny rules.
  * CAP_SYS_ADMIN is needed for this.  It's at least separate from CAP_MKNOD
@@ -449,6 +562,9 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
         case 'a':
                 switch (filetype) {
                 case DEVCG_ALLOW:
+                        if (has_children(devcgroup))
+                                return -EINVAL;
+
                         if (!may_allow_all(parent))
                                 return -EPERM;
                         dev_exception_clean(devcgroup);
@@ -462,6 +578,9 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
                                 return rc;
                         break;
                 case DEVCG_DENY:
+                        if (has_children(devcgroup))
+                                return -EINVAL;
+
                         dev_exception_clean(devcgroup);
                         devcgroup->behavior = DEVCG_DEFAULT_DENY;
                         break;
@@ -556,22 +675,28 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
                         dev_exception_rm(devcgroup, &ex);
                         return 0;
                 }
-                return dev_exception_add(devcgroup, &ex);
+                rc = dev_exception_add(devcgroup, &ex);
+                break;
         case DEVCG_DENY:
                 /*
                  * If the default policy is to deny by default, try to remove
                  * an matching exception instead. And be silent about it: we
                  * don't want to break compatibility
                  */
-                if (devcgroup->behavior == DEVCG_DEFAULT_DENY) {
+                if (devcgroup->behavior == DEVCG_DEFAULT_DENY)
                         dev_exception_rm(devcgroup, &ex);
-                        return 0;
-                }
-                return dev_exception_add(devcgroup, &ex);
+                else
+                        rc = dev_exception_add(devcgroup, &ex);
+
+                if (rc)
+                        break;
+                /* we only propagate new restrictions */
+                rc = propagate_exception(devcgroup, &ex);
+                break;
         default:
-                return -EINVAL;
+                rc = -EINVAL;
         }
-        return 0;
+        return rc;
 }
 
 static int devcgroup_access_write(struct cgroup *cgrp, struct cftype *cft,