device_cgroup: Roll back to original exceptions after copy failure

When add the 'a *:* rwm' entry to devcgroup A's whitelist, at first A's
exceptions will be cleaned and A's behavior is changed to
DEVCG_DEFAULT_ALLOW. Then parent's exceptions will be copyed to A's
whitelist. If copy failure occurs, just return leaving A to grant
permissions to all devices. And A may grant more permissions than
parent.

Backup A's whitelist and recover original exceptions after copy
failure.

Cc: stable@vger.kernel.org
Fixes: 4cef7299 ("device_cgroup: add proper checking when changing default behavior")
Signed-off-by: Wang Weiyang <wangweiyang2@huawei.com>
Reviewed-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/device_cgroup.c

@@ -82,6 +82,17 @@ static int dev_exceptions_copy(struct list_head *dest, struct list_head *orig)
 	return -ENOMEM;
 }
 
+static void dev_exceptions_move(struct list_head *dest, struct list_head *orig)
+{
+	struct dev_exception_item *ex, *tmp;
+
+	lockdep_assert_held(&devcgroup_mutex);
+
+	list_for_each_entry_safe(ex, tmp, orig, list) {
+		list_move_tail(&ex->list, dest);
+	}
+}
+
 /*
  * called under devcgroup_mutex
  */
@@ -604,11 +615,13 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
 	int count, rc = 0;
 	struct dev_exception_item ex;
 	struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
+	struct dev_cgroup tmp_devcgrp;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	memset(&ex, 0, sizeof(ex));
+	memset(&tmp_devcgrp, 0, sizeof(tmp_devcgrp));
 	b = buffer;
 
 	switch (*b) {
@@ -620,15 +633,27 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
 
 			if (!may_allow_all(parent))
 				return -EPERM;
-			dev_exception_clean(devcgroup);
+			if (!parent) {
 				devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
-			if (!parent)
+				dev_exception_clean(devcgroup);
 				break;
+			}
 
+			INIT_LIST_HEAD(&tmp_devcgrp.exceptions);
+			rc = dev_exceptions_copy(&tmp_devcgrp.exceptions,
+						 &devcgroup->exceptions);
+			if (rc)
+				return rc;
+			dev_exception_clean(devcgroup);
 			rc = dev_exceptions_copy(&devcgroup->exceptions,
 						 &parent->exceptions);
-			if (rc)
+			if (rc) {
+				dev_exceptions_move(&devcgroup->exceptions,
+						    &tmp_devcgrp.exceptions);
 				return rc;
+			}
+			devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
+			dev_exception_clean(&tmp_devcgrp);
 			break;
 		case DEVCG_DENY:
 			if (css_has_online_children(&devcgroup->css))