confile: forbid absolute paths in config items that modify the cgroup layout

src/lxc/confile.c

@@ -1824,6 +1824,9 @@ static int set_config_cgroup_dir(const char *key, const char *value,
 	if (lxc_config_value_empty(value))
 		return clr_config_cgroup_dir(key, lxc_conf, NULL);
 
+	if (abspath(value))
+		return syserrno_set(-EINVAL, "%s paths may not be absolute", key);
+
 	if (dotdot(value))
 		return syserrno_set(-EINVAL, "%s paths may not walk upwards via \"../\"", key);
 
@@ -1836,6 +1839,9 @@ static int set_config_cgroup_monitor_dir(const char *key, const char *value,
 	if (lxc_config_value_empty(value))
 		return clr_config_cgroup_monitor_dir(key, lxc_conf, NULL);
 
+	if (abspath(value))
+		return syserrno_set(-EINVAL, "%s paths may not be absolute", key);
+
 	if (dotdot(value))
 		return syserrno_set(-EINVAL, "%s paths may not walk upwards via \"../\"", key);
 
@@ -1848,6 +1854,9 @@ static int set_config_cgroup_monitor_pivot_dir(const char *key, const char *valu
 	if (lxc_config_value_empty(value))
 		return clr_config_cgroup_monitor_pivot_dir(key, lxc_conf, NULL);
 
+	if (abspath(value))
+		return syserrno_set(-EINVAL, "%s paths may not be absolute", key);
+
 	if (dotdot(value))
 		return syserrno_set(-EINVAL, "%s paths may not walk upwards via \"../\"", key);
 
@@ -1861,6 +1870,9 @@ static int set_config_cgroup_container_dir(const char *key, const char *value,
 	if (lxc_config_value_empty(value))
 		return clr_config_cgroup_container_dir(key, lxc_conf, NULL);
 
+	if (abspath(value))
+		return syserrno_set(-EINVAL, "%s paths may not be absolute", key);
+
 	if (dotdot(value))
 		return syserrno_set(-EINVAL, "%s paths may not walk upwards via \"../\"", key);
 
@@ -1875,6 +1887,9 @@ static int set_config_cgroup_container_inner_dir(const char *key,
 	if (lxc_config_value_empty(value))
 		return clr_config_cgroup_container_inner_dir(key, lxc_conf, NULL);
 
+	if (abspath(value))
+		return syserrno_set(-EINVAL, "%s paths may not be absolute", key);
+
 	if (strchr(value, '/') || strequal(value, ".") || strequal(value, ".."))
 		return log_error_errno(-EINVAL, EINVAL, "lxc.cgroup.dir.container.inner must be a single directory name");
 

src/lxc/string_utils.h

@@ -145,6 +145,11 @@ static inline bool dotdot(const char *str)
 	return !!strstr(str, "..");
 }
 
+static inline bool abspath(const char *str)
+{
+	return *str == '/';
+}
+
 #define strnprintf(buf, buf_size, ...)                                            \
 	({                                                                        \
 		int __ret_strnprintf;                                             \