gcc-plugins: Add structleak for more stack initialization

This plugin detects any structures that contain __user attributes and
makes sure it is being fully initialized so that a specific class of
information exposure is eliminated. (This plugin was originally designed
to block the exposure of siginfo in CVE-2013-2141.)

Ported from grsecurity/PaX. This version adds a verbose option to the
plugin and the Kconfig.

Signed-off-by: Kees Cook <keescook@chromium.org>

arch/Kconfig

@@ -410,6 +410,28 @@ config GCC_PLUGIN_LATENT_ENTROPY
 	   * https://grsecurity.net/
 	   * https://pax.grsecurity.net/
 
+config GCC_PLUGIN_STRUCTLEAK
+	bool "Force initialization of variables containing userspace addresses"
+	depends on GCC_PLUGINS
+	help
+	  This plugin zero-initializes any structures that containing a
+	  __user attribute. This can prevent some classes of information
+	  exposures.
+
+	  This plugin was ported from grsecurity/PaX. More information at:
+	   * https://grsecurity.net/
+	   * https://pax.grsecurity.net/
+
+config GCC_PLUGIN_STRUCTLEAK_VERBOSE
+	bool "Report forcefully initialized variables"
+	depends on GCC_PLUGIN_STRUCTLEAK
+	depends on !COMPILE_TEST
+	help
+	  This option will cause a warning to be printed each time the
+	  structleak plugin finds a variable it thinks needs to be
+	  initialized. Since not all existing initializers are detected
+	  by the plugin, this can produce false positive warnings.
+
 config HAVE_CC_STACKPROTECTOR
 	bool
 	help

include/linux/compiler.h

@@ -27,7 +27,11 @@ extern void __chk_user_ptr(const volatile void __user *);
 extern void __chk_io_ptr(const volatile void __iomem *);
 # define ACCESS_PRIVATE(p, member) (*((typeof((p)->member) __force *) &(p)->member))
 #else /* __CHECKER__ */
-# define __user
+# ifdef STRUCTLEAK_PLUGIN
+#  define __user __attribute__((user))
+# else
+#  define __user
+# endif
 # define __kernel
 # define __safe
 # define __force

scripts/Makefile.gcc-plugins

@@ -25,6 +25,10 @@ ifdef CONFIG_GCC_PLUGINS
     endif
   endif
 
+  gcc-plugin-$(CONFIG_GCC_PLUGIN_STRUCTLEAK)	+= structleak_plugin.so
+  gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STRUCTLEAK_VERBOSE)	+= -fplugin-arg-structleak_plugin-verbose
+  gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STRUCTLEAK)	+= -DSTRUCTLEAK_PLUGIN
+
   GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
 
   export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN GCC_PLUGIN_SUBDIR

scripts/gcc-plugins/structleak_plugin.c

@@ -0,0 +1,246 @@
+/*
+ * Copyright 2013-2017 by PaX Team <pageexec@freemail.hu>
+ * Licensed under the GPL v2
+ *
+ * Note: the choice of the license means that the compilation process is
+ *       NOT 'eligible' as defined by gcc's library exception to the GPL v3,
+ *       but for the kernel it doesn't matter since it doesn't link against
+ *       any of the gcc libraries
+ *
+ * gcc plugin to forcibly initialize certain local variables that could
+ * otherwise leak kernel stack to userland if they aren't properly initialized
+ * by later code
+ *
+ * Homepage: http://pax.grsecurity.net/
+ *
+ * Options:
+ * -fplugin-arg-structleak_plugin-disable
+ * -fplugin-arg-structleak_plugin-verbose
+ *
+ * Usage:
+ * $ # for 4.5/4.6/C based 4.7
+ * $ gcc -I`gcc -print-file-name=plugin`/include -I`gcc -print-file-name=plugin`/include/c-family -fPIC -shared -O2 -o structleak_plugin.so structleak_plugin.c
+ * $ # for C++ based 4.7/4.8+
+ * $ g++ -I`g++ -print-file-name=plugin`/include -I`g++ -print-file-name=plugin`/include/c-family -fPIC -shared -O2 -o structleak_plugin.so structleak_plugin.c
+ * $ gcc -fplugin=./structleak_plugin.so test.c -O2
+ *
+ * TODO: eliminate redundant initializers
+ *       increase type coverage
+ */
+
+#include "gcc-common.h"
+
+/* unused C type flag in all versions 4.5-6 */
+#define TYPE_USERSPACE(TYPE) TYPE_LANG_FLAG_5(TYPE)
+
+__visible int plugin_is_GPL_compatible;
+
+static struct plugin_info structleak_plugin_info = {
+	.version	= "201607271510vanilla",
+	.help		= "disable\tdo not activate plugin\n"
+			   "verbose\tprint all initialized variables\n",
+};
+
+static bool verbose;
+
+static tree handle_user_attribute(tree *node, tree name, tree args, int flags, bool *no_add_attrs)
+{
+	*no_add_attrs = true;
+
+	/* check for types? for now accept everything linux has to offer */
+	if (TREE_CODE(*node) != FIELD_DECL)
+		return NULL_TREE;
+
+	*no_add_attrs = false;
+	return NULL_TREE;
+}
+
+static struct attribute_spec user_attr = {
+	.name			= "user",
+	.min_length		= 0,
+	.max_length		= 0,
+	.decl_required		= false,
+	.type_required		= false,
+	.function_type_required	= false,
+	.handler		= handle_user_attribute,
+#if BUILDING_GCC_VERSION >= 4007
+	.affects_type_identity	= true
+#endif
+};
+
+static void register_attributes(void *event_data, void *data)
+{
+	register_attribute(&user_attr);
+}
+
+static tree get_field_type(tree field)
+{
+	return strip_array_types(TREE_TYPE(field));
+}
+
+static bool is_userspace_type(tree type)
+{
+	tree field;
+
+	for (field = TYPE_FIELDS(type); field; field = TREE_CHAIN(field)) {
+		tree fieldtype = get_field_type(field);
+		enum tree_code code = TREE_CODE(fieldtype);
+
+		if (code == RECORD_TYPE || code == UNION_TYPE)
+			if (is_userspace_type(fieldtype))
+				return true;
+
+		if (lookup_attribute("user", DECL_ATTRIBUTES(field)))
+			return true;
+	}
+	return false;
+}
+
+static void finish_type(void *event_data, void *data)
+{
+	tree type = (tree)event_data;
+
+	if (type == NULL_TREE || type == error_mark_node)
+		return;
+
+#if BUILDING_GCC_VERSION >= 5000
+	if (TREE_CODE(type) == ENUMERAL_TYPE)
+		return;
+#endif
+
+	if (TYPE_USERSPACE(type))
+		return;
+
+	if (is_userspace_type(type))
+		TYPE_USERSPACE(type) = 1;
+}
+
+static void initialize(tree var)
+{
+	basic_block bb;
+	gimple_stmt_iterator gsi;
+	tree initializer;
+	gimple init_stmt;
+
+	/* this is the original entry bb before the forced split */
+	bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
+
+	/* first check if variable is already initialized, warn otherwise */
+	for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi)) {
+		gimple stmt = gsi_stmt(gsi);
+		tree rhs1;
+
+		/* we're looking for an assignment of a single rhs... */
+		if (!gimple_assign_single_p(stmt))
+			continue;
+		rhs1 = gimple_assign_rhs1(stmt);
+#if BUILDING_GCC_VERSION >= 4007
+		/* ... of a non-clobbering expression... */
+		if (TREE_CLOBBER_P(rhs1))
+			continue;
+#endif
+		/* ... to our variable... */
+		if (gimple_get_lhs(stmt) != var)
+			continue;
+		/* if it's an initializer then we're good */
+		if (TREE_CODE(rhs1) == CONSTRUCTOR)
+			return;
+	}
+
+	/* these aren't the 0days you're looking for */
+	if (verbose)
+		inform(DECL_SOURCE_LOCATION(var),
+			"userspace variable will be forcibly initialized");
+
+	/* build the initializer expression */
+	initializer = build_constructor(TREE_TYPE(var), NULL);
+
+	/* build the initializer stmt */
+	init_stmt = gimple_build_assign(var, initializer);
+	gsi = gsi_after_labels(single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
+	gsi_insert_before(&gsi, init_stmt, GSI_NEW_STMT);
+	update_stmt(init_stmt);
+}
+
+static unsigned int structleak_execute(void)
+{
+	basic_block bb;
+	unsigned int ret = 0;
+	tree var;
+	unsigned int i;
+
+	/* split the first bb where we can put the forced initializers */
+	gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
+	bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
+	if (!single_pred_p(bb)) {
+		split_edge(single_succ_edge(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
+		gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
+	}
+
+	/* enumerate all local variables and forcibly initialize our targets */
+	FOR_EACH_LOCAL_DECL(cfun, i, var) {
+		tree type = TREE_TYPE(var);
+
+		gcc_assert(DECL_P(var));
+		if (!auto_var_in_fn_p(var, current_function_decl))
+			continue;
+
+		/* only care about structure types */
+		if (TREE_CODE(type) != RECORD_TYPE && TREE_CODE(type) != UNION_TYPE)
+			continue;
+
+		/* if the type is of interest, examine the variable */
+		if (TYPE_USERSPACE(type))
+			initialize(var);
+	}
+
+	return ret;
+}
+
+#define PASS_NAME structleak
+#define NO_GATE
+#define PROPERTIES_REQUIRED PROP_cfg
+#define TODO_FLAGS_FINISH TODO_verify_il | TODO_verify_ssa | TODO_verify_stmts | TODO_dump_func | TODO_remove_unused_locals | TODO_update_ssa | TODO_ggc_collect | TODO_verify_flow
+#include "gcc-generate-gimple-pass.h"
+
+__visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
+{
+	int i;
+	const char * const plugin_name = plugin_info->base_name;
+	const int argc = plugin_info->argc;
+	const struct plugin_argument * const argv = plugin_info->argv;
+	bool enable = true;
+
+	PASS_INFO(structleak, "early_optimizations", 1, PASS_POS_INSERT_BEFORE);
+
+	if (!plugin_default_version_check(version, &gcc_version)) {
+		error(G_("incompatible gcc/plugin versions"));
+		return 1;
+	}
+
+	if (strncmp(lang_hooks.name, "GNU C", 5) && !strncmp(lang_hooks.name, "GNU C+", 6)) {
+		inform(UNKNOWN_LOCATION, G_("%s supports C only, not %s"), plugin_name, lang_hooks.name);
+		enable = false;
+	}
+
+	for (i = 0; i < argc; ++i) {
+		if (!strcmp(argv[i].key, "disable")) {
+			enable = false;
+			continue;
+		}
+		if (!strcmp(argv[i].key, "verbose")) {
+			verbose = true;
+			continue;
+		}
+		error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
+	}
+
+	register_callback(plugin_name, PLUGIN_INFO, NULL, &structleak_plugin_info);
+	if (enable) {
+		register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &structleak_pass_info);
+		register_callback(plugin_name, PLUGIN_FINISH_TYPE, finish_type, NULL);
+	}
+	register_callback(plugin_name, PLUGIN_ATTRIBUTES, register_attributes, NULL);
+
+	return 0;
+}