[Xen-devel] [PATCH 2/3] gcc-plugins: Introduce stackinit plugin

[Kees Cook <keescook@xxxxxxxxxxxx>]

This attempts to duplicate the proposed gcc option -finit-local-vars[1]
in an effort to implement the "always initialize local variables" kernel
development goal[2].

Enabling CONFIG_GCC_PLUGIN_STACKINIT should stop all "uninitialized
stack variable" flaws as long as they don't depend on being zero. :)

[1] https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00615.html
[2] 
https://lkml.kernel.org/r/CA+55aFykZL+cSBJjBBts7ebEFfyGPdMzTmLSxKnT_29=j942dA@xxxxxxxxxxxxxx

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 scripts/Makefile.gcc-plugins           |  6 ++
 scripts/gcc-plugins/Kconfig            |  9 +++
 scripts/gcc-plugins/gcc-common.h       | 11 +++-
 scripts/gcc-plugins/stackinit_plugin.c | 79 ++++++++++++++++++++++++++
 4 files changed, 102 insertions(+), 3 deletions(-)
 create mode 100644 scripts/gcc-plugins/stackinit_plugin.c

diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index 35042d96cf5d..2483121d781c 100644
--- a/scripts/Makefile.gcc-plugins
+++ b/scripts/Makefile.gcc-plugins
@@ -12,6 +12,12 @@ export DISABLE_LATENT_ENTROPY_PLUGIN
 
 gcc-plugin-$(CONFIG_GCC_PLUGIN_SANCOV)         += sancov_plugin.so
 
+gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKINIT)      += stackinit_plugin.so
+ifdef CONFIG_GCC_PLUGIN_STACKINIT
+    DISABLE_STACKINIT_PLUGIN += -fplugin-arg-stackinit_plugin-disable
+endif
+export DISABLE_STACKINIT_PLUGIN
+
 gcc-plugin-$(CONFIG_GCC_PLUGIN_STRUCTLEAK)     += structleak_plugin.so
 gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STRUCTLEAK_VERBOSE)      \
                += -fplugin-arg-structleak_plugin-verbose
diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
index d45f7f36b859..b117fe83f1d3 100644
--- a/scripts/gcc-plugins/Kconfig
+++ b/scripts/gcc-plugins/Kconfig
@@ -66,6 +66,15 @@ config GCC_PLUGIN_LATENT_ENTROPY
           * https://grsecurity.net/
           * https://pax.grsecurity.net/
 
+config GCC_PLUGIN_STACKINIT
+       bool "Initialize all stack variables to zero by default"
+       depends on GCC_PLUGINS
+       depends on !GCC_PLUGIN_STRUCTLEAK
+       help
+         This plugin zero-initializes all stack variables. This is more
+         comprehensive than GCC_PLUGIN_STRUCTLEAK, and attempts to
+         duplicate the proposed -finit-local-vars gcc build flag.
+
 config GCC_PLUGIN_STRUCTLEAK
        bool "Force initialization of variables containing userspace addresses"
        # Currently STRUCTLEAK inserts initialization out of live scope of
diff --git a/scripts/gcc-plugins/gcc-common.h b/scripts/gcc-plugins/gcc-common.h
index 552d5efd7cb7..f690b4deeabd 100644
--- a/scripts/gcc-plugins/gcc-common.h
+++ b/scripts/gcc-plugins/gcc-common.h
@@ -76,6 +76,14 @@
 #include "c-common.h"
 #endif
 
+#if BUILDING_GCC_VERSION > 4005
+#include "c-tree.h"
+#else
+/* should come from c-tree.h if only it were installed for gcc 4.5... */
+#define C_TYPE_FIELDS_READONLY(TYPE) TREE_LANG_FLAG_1(TYPE)
+extern bool global_bindings_p (void);
+#endif
+
 #if BUILDING_GCC_VERSION <= 4008
 #include "tree-flow.h"
 #else
@@ -158,9 +166,6 @@ void dump_gimple_stmt(pretty_printer *, gimple, int, int);
 #define TYPE_NAME_POINTER(node) IDENTIFIER_POINTER(TYPE_NAME(node))
 #define TYPE_NAME_LENGTH(node) IDENTIFIER_LENGTH(TYPE_NAME(node))
 
-/* should come from c-tree.h if only it were installed for gcc 4.5... */
-#define C_TYPE_FIELDS_READONLY(TYPE) TREE_LANG_FLAG_1(TYPE)
-
 static inline tree build_const_char_string(int len, const char *str)
 {
        tree cstr, elem, index, type;
diff --git a/scripts/gcc-plugins/stackinit_plugin.c 
b/scripts/gcc-plugins/stackinit_plugin.c
new file mode 100644
index 000000000000..41055cd7098e
--- /dev/null
+++ b/scripts/gcc-plugins/stackinit_plugin.c
@@ -0,0 +1,79 @@
+/* SPDX-License: GPLv2 */
+/*
+ * This will zero-initialize local stack variables. (Though structure
+ * padding may remain uninitialized in certain cases.)
+ *
+ * Implements Florian Weimer's "-finit-local-vars" gcc patch as a plugin:
+ * https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00615.html
+ *
+ * Plugin skeleton code thanks to PaX Team.
+ *
+ * Options:
+ * -fplugin-arg-stackinit_plugin-disable
+ */
+
+#include "gcc-common.h"
+
+__visible int plugin_is_GPL_compatible;
+
+static struct plugin_info stackinit_plugin_info = {
+       .version        = "20190122",
+       .help           = "disable\tdo not activate plugin\n",
+};
+
+static void finish_decl(void *event_data, void *data)
+{
+       tree decl = (tree)event_data;
+       tree type;
+
+       if (TREE_CODE (decl) != VAR_DECL)
+               return;
+
+       if (DECL_EXTERNAL (decl))
+               return;
+
+       if (DECL_INITIAL (decl) != NULL_TREE)
+               return;
+
+       if (global_bindings_p ())
+               return;
+
+       type = TREE_TYPE (decl);
+       if (AGGREGATE_TYPE_P (type))
+               DECL_INITIAL (decl) = build_constructor (type, NULL);
+       else
+               DECL_INITIAL (decl) = fold_convert (type, integer_zero_node);
+}
+
+__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;
+
+       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;
+               }
+               error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, 
argv[i].key);
+       }
+
+       register_callback(plugin_name, PLUGIN_INFO, NULL, 
&stackinit_plugin_info);
+       if (enable)
+               register_callback(plugin_name, PLUGIN_FINISH_DECL, finish_decl, 
NULL);
+
+       return 0;
+}
-- 
2.17.1


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