devcg: propagate local changes down the hierarchy

This patch makes exception changes to propagate down in hierarchy respecting
when possible local exceptions.

New exceptions allowing additional access to devices won't be propagated, but
it'll be possible to add an exception to access all of part of the newly
allowed device(s).

New exceptions disallowing access to devices will be propagated down and the
local group's exceptions will be revalidated for the new situation.
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 new exception is added to group A:
# echo "c 116:* r" > A/devices.deny
it'll propagate down and after revalidating B's local exceptions, the exception
"c 116:2 rwm" will be removed.

In case parent's exceptions change and local exceptions are not allowed anymore,
they'll be deleted.

v7:
- do not allow behavior change when the cgroup has children
- update documentation

v6: fixed issues pointed by Serge Hallyn
- only copy parent's exceptions while propagating behavior if the local
behavior is different
- while propagating exceptions, do not clear and copy parent's: it'd be against
the premise we don't propagate access to more devices

v5: fixed issues pointed by Serge Hallyn
- updated documentation
- not propagating when an exception is written to devices.allow
- when propagating a new behavior, clean the local exceptions list if they're
for a different behavior

v4: fixed issues pointed by Tejun Heo
- separated function to walk the tree and collect valid propagation targets

v3: fixed issues pointed by Tejun Heo
- update documentation
- move css_online/css_offline changes to a new patch
- use cgroup_for_each_descendant_pre() instead of own descendant walk
- move exception_copy rework to a separared patch
- move exception_clean rework to a separated patch

v2: fixed issues pointed by Tejun Heo
- instead of keeping the local settings that won't apply anymore, remove them

Cc: Tejun Heo <[e-mail]>
Cc: Serge Hallyn <[e-mail]>
Signed-off-by: Aristeu Rozanski <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/cgroups/devices.txt

···
 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
 
···
 
 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.

security/device_cgroup.c

···
 	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)
···
 	__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
···
 	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;
 
···
 	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
···
 	case 'a':
 		switch (filetype) {
 		case DEVCG_ALLOW:
+			if (has_children(devcgroup))
+				return -EINVAL;
+
 			if (!may_allow_all(parent))
 				return -EPERM;
 			dev_exception_clean(devcgroup);
···
 				return rc;
 			break;
 		case DEVCG_DENY:
+			if (has_children(devcgroup))
+				return -EINVAL;
+
 			dev_exception_clean(devcgroup);
 			devcgroup->behavior = DEVCG_DEFAULT_DENY;
 			break;
···
 			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,