[PATCH 5/5] device_cgroup: propagate local changes down the hierarchy

[Aristeu Rozanski]

This patch makes all changes propagate down in hierarchy respecting when
possible local configurations.

Behavior changes will clean up exceptions in all the children except when the
parent changes the behavior from allow to deny and the child's behavior was
already deny, in which case the local exceptions will be reused. The inverse
is not possible: you can't have a parent with behavior deny and a child with
behavior accept.

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.

The local preferences will be kept and applied when allowed by its parent.

Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
Signed-off-by: Aristeu Rozanski <aris@xxxxxxxxxx>

---
 security/device_cgroup.c |  215 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 187 insertions(+), 28 deletions(-)

Index: github/security/device_cgroup.c
===================================================================
--- github.orig/security/device_cgroup.c	2012-11-26 17:28:12.415367313 -0500
+++ github/security/device_cgroup.c	2012-11-26 17:28:17.023491874 -0500
@@ -89,28 +89,38 @@
 	return 0;
 }
 
+static int dev_exception_copy(struct list_head *dest,
+			      struct dev_exception_item *ex)
+{
+	struct dev_exception_item *new;
+
+	new = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
+	if (!new)
+		return -ENOMEM;
+	list_add_tail_rcu(&new->list, dest);
+	return 0;
+}
+
 /*
  * called under devcgroup_mutex
  */
 static int dev_exceptions_copy(struct list_head *dest, struct list_head *orig)
 {
-	struct dev_exception_item *ex, *tmp, *new;
+	struct dev_exception_item *ex, *tmp;
 
 	lockdep_assert_held(&devcgroup_mutex);
 
 	list_for_each_entry(ex, orig, list) {
-		new = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
-		if (!new)
+		if (dev_exception_copy(dest, ex))
 			goto free_and_exit;
-		list_add_tail(&new->list, dest);
 	}
 
 	return 0;
 
 free_and_exit:
 	list_for_each_entry_safe(ex, tmp, dest, list) {
-		list_del(&ex->list);
-		kfree(ex);
+		list_del_rcu(&ex->list);
+		kfree_rcu(ex, rcu);
 	}
 	return -ENOMEM;
 }
@@ -202,29 +212,30 @@
 	return rc;
 }
 
-/**
- * dev_exception_clean - frees all entries of the exception list
- * @dev_cgroup: dev_cgroup with the exception list to be cleaned
- *
- * called under devcgroup_mutex
- */
-static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
+static void __dev_exception_clean(struct list_head *exceptions)
 {
 	struct dev_exception_item *ex, *tmp;
 
 	lockdep_assert_held(&devcgroup_mutex);
 
-	list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
-		list_del_rcu(&ex->list);
-		kfree_rcu(ex, rcu);
-	}
-	list_for_each_entry_safe(ex, tmp, &dev_cgroup->local.exceptions,
-				 list) {
+	list_for_each_entry_safe(ex, tmp, exceptions, list) {
 		list_del_rcu(&ex->list);
 		kfree_rcu(ex, rcu);
 	}
 }
 
+/**
+ * dev_exception_clean - frees all entries of the exception list
+ * @dev_cgroup: dev_cgroup with the exception list to be cleaned
+ *
+ * called under devcgroup_mutex
+ */
+static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
+{
+	__dev_exception_clean(&dev_cgroup->exceptions);
+	__dev_exception_clean(&dev_cgroup->local.exceptions);
+}
+
 /*
  * called from kernel/cgroup.c with cgroup_lock() held.
  */
@@ -435,6 +446,138 @@
 	return parent->behavior == DEVCG_DEFAULT_ALLOW;
 }
 
+/**
+ * __revalidate_exceptions - walks through the exception list and revalidates
+ *			     the exceptions based on parents' behavior and
+ *			     exceptions. Called with devcgroup_mutex held.
+ * @devcg: cgroup which exceptions will be checked
+ * returns: 0 in success, -ENOMEM in case of out of memory
+ */
+static int __revalidate_exceptions(struct dev_cgroup *devcg)
+{
+	struct dev_exception_item *ex;
+	struct list_head *this, *tmp;
+
+	list_for_each_safe(this, tmp, &devcg->local.exceptions) {
+		ex = container_of(this, struct dev_exception_item, list);
+		if (parent_has_perm(devcg, ex)) {
+			if (dev_exception_copy(&devcg->exceptions, ex))
+				goto error;
+		}
+	}
+	return 0;
+
+error:
+	__dev_exception_clean(&devcg->exceptions);
+	return -ENOMEM;
+}
+
+/**
+ * propagate_behavior - propagates a change in the behavior to the children
+ * @devcg: device cgroup that changed behavior
+ *
+ * returns: 0 in case of success, != 0 in case of error
+ */
+static int propagate_behavior(struct dev_cgroup *devcg)
+{
+	struct cgroup *root = devcg->css.cgroup, *pos, *saved = NULL,
+			*prev = NULL;
+	struct dev_cgroup *parent;
+	int rc = 0;
+
+	while (1) {
+		rcu_read_lock();
+		cgroup_for_each_descendant_pre(pos, root) {
+			if (saved && prev != saved) {
+				prev = pos;
+				continue;
+			}
+			break;
+		}
+		rcu_read_unlock();
+
+		if (!pos)
+			break;
+
+		devcg = cgroup_to_devcgroup(pos);
+		parent = cgroup_to_devcgroup(pos->parent);
+
+		/* first copy parent's state */
+		devcg->behavior = parent->behavior;
+		__dev_exception_clean(&devcg->exceptions);
+		rc = dev_exceptions_copy(&devcg->exceptions, &parent->exceptions);
+		if (rc) {
+			devcg->behavior = DEVCG_DEFAULT_DENY;
+			break;
+		}
+
+		if (devcg->local.behavior == DEVCG_DEFAULT_DENY &&
+		    devcg->behavior == DEVCG_DEFAULT_ALLOW) {
+			devcg->behavior = DEVCG_DEFAULT_DENY;
+		}
+		if (devcg->local.behavior == devcg->behavior)
+			rc = __revalidate_exceptions(devcg);
+		if (rc)
+			break;
+		saved = pos;
+	}
+
+	return rc;
+}
+
+/**
+ * propagate_exception - propagates a new exception to the children
+ * @devcg: device cgroup that added a new exception
+ *
+ * returns: 0 in case of success, != 0 in case of error
+ */
+static int propagate_exception(struct dev_cgroup *devcg)
+{
+	struct cgroup *root = devcg->css.cgroup, *pos, *saved = NULL,
+			*prev = NULL;
+	struct dev_cgroup *parent;
+	int rc = 0;
+
+	while(1) {
+		rcu_read_lock();
+		cgroup_for_each_descendant_pre(pos, root) {
+			if (saved && prev != saved) {
+				prev = pos;
+				continue;
+			}
+			break;
+		}
+		rcu_read_unlock();
+
+		if (!pos)
+			break;
+
+		devcg = cgroup_to_devcgroup(pos);
+		parent = cgroup_to_devcgroup(pos->parent);
+
+		__dev_exception_clean(&devcg->exceptions);
+		if (devcg->behavior == parent->behavior) {
+			rc = dev_exceptions_copy(&devcg->exceptions, &parent->exceptions);
+			if (rc) {
+				devcg->behavior = DEVCG_DEFAULT_DENY;
+				break;
+			}
+			rc = __revalidate_exceptions(devcg);
+			if (rc)
+				break;
+		} else {
+			/* we never give more permissions to the child */
+			WARN_ONCE(devcg->behavior == DEVCG_DEFAULT_ALLOW,
+				  "devcg: parent/child behavior is inconsistent");
+			rc = __revalidate_exceptions(devcg);
+			if (rc)
+				break;
+		}
+		saved = pos;
+	}
+	return rc;
+}
+
 /*
  * Modify the exception list using allow/deny rules.
  * CAP_SYS_ADMIN is needed for this.  It's at least separate from CAP_MKNOD
@@ -453,7 +596,7 @@
 {
 	const char *b;
 	char temp[12];		/* 11 + 1 characters needed for a u32 */
-	int count, rc;
+	int count, rc = 0;
 	struct dev_exception_item ex;
 	struct cgroup *p = devcgroup->css.cgroup;
 	struct dev_cgroup *parent = NULL;
@@ -483,16 +626,18 @@
 						 &parent->exceptions);
 			if (rc)
 				return rc;
+			rc = propagate_behavior(devcgroup);
 			break;
 		case DEVCG_DENY:
 			dev_exception_clean(devcgroup);
 			devcgroup->behavior = DEVCG_DEFAULT_DENY;
 			devcgroup->local.behavior = DEVCG_DEFAULT_DENY;
+			rc = propagate_behavior(devcgroup);
 			break;
 		default:
-			return -EINVAL;
+			rc = -EINVAL;
 		}
-		return 0;
+		return rc;
 	case 'b':
 		ex.type = DEV_BLOCK;
 		break;
@@ -578,9 +723,14 @@
 		 */
 		if (devcgroup->behavior == DEVCG_DEFAULT_ALLOW) {
 			dev_exception_rm(devcgroup, &ex);
-			return 0;
+			rc = propagate_exception(devcgroup);
+			return rc;
 		}
-		return dev_exception_add(devcgroup, &ex);
+		rc = dev_exception_add(devcgroup, &ex);
+		if (!rc)
+			/* if a local behavior wasn't explicitely choosen, pick it */
+			devcgroup->local.behavior = devcgroup->behavior;
+		break;
 	case DEVCG_DENY:
 		/*
 		 * If the default policy is to deny by default, try to remove
@@ -589,13 +739,22 @@
 		 */
 		if (devcgroup->behavior == DEVCG_DEFAULT_DENY) {
 			dev_exception_rm(devcgroup, &ex);
-			return 0;
+			rc = propagate_exception(devcgroup);
+			return rc;
 		}
-		return dev_exception_add(devcgroup, &ex);
+		rc = dev_exception_add(devcgroup, &ex);
+		if (rc)
+			return rc;
+		/* we only propagate new restrictions */
+		rc = propagate_exception(devcgroup);
+		if (!rc)
+			/* if a local behavior wasn't explicitely choosen, pick it */
+			devcgroup->local.behavior = devcgroup->behavior;
+		break;
 	default:
-		return -EINVAL;
+		rc = -EINVAL;
 	}
-	return 0;
+	return rc;
 }
 
 static int devcgroup_access_write(struct cgroup *cgrp, struct cftype *cft,

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