Re: [PATCH] gcc-plugins: Add structleak for more stack initialization

[Mark Rutland]

Hi,

[adding Dave, so retaining full context below]

On Fri, Jan 13, 2017 at 02:02:56PM -0800, Kees Cook wrote:
> This plugin detects any structures that contain __user attributes and
> makes sure it is being fulling initialized so that a specific class of

Nit: s/fulling/fully/

> information exposure is eliminated. (For example, the exposure of siginfo
> in CVE-2013-2141 would have been blocked by this plugin.)
> 
> Ported from grsecurity/PaX. This version adds a verbose option to the
> plugin and the Kconfig.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/Kconfig                            |  22 +++
>  include/linux/compiler.h                |   6 +-
>  scripts/Makefile.gcc-plugins            |   4 +
>  scripts/gcc-plugins/structleak_plugin.c | 246 ++++++++++++++++++++++++++++++++
>  4 files changed, 277 insertions(+), 1 deletion(-)
>  create mode 100644 scripts/gcc-plugins/structleak_plugin.c

I tried giving this a go, but I got the build failure below:

----
[mark@leverpostej:~/src/linux]% uselinaro 15.08 make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-
arch/arm64/Makefile:43: Detected assembler with broken .inst; disassembly will be unreliable
  CHK     include/config/kernel.release
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
  UPD     include/generated/utsrelease.h
  HOSTCXX -fPIC scripts/gcc-plugins/structleak_plugin.o
scripts/gcc-plugins/structleak_plugin.c: In function âint plugin_init(plugin_name_args*, plugin_gcc_version*)â:
scripts/gcc-plugins/structleak_plugin.c:214:12: error: âstructleakâ was not declared in this scope
  PASS_INFO(structleak, "early_optimizations", 1, PASS_POS_INSERT_BEFORE);
            ^
scripts/gcc-plugins/structleak_plugin.c:214:72: error: âPASS_INFOâ was not declared in this scope
  PASS_INFO(structleak, "early_optimizations", 1, PASS_POS_INSERT_BEFORE);
                                                                        ^
scripts/gcc-plugins/structleak_plugin.c:240:68: error: âstructleak_pass_infoâ was not declared in this scope
   register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &structleak_pass_info);
                                                                    ^
make[1]: *** [scripts/gcc-plugins/structleak_plugin.o] Error 1
make: *** [gcc-plugins] Error 2
----

I'm using the Linaro 15.08 x86_64 aarch64-linux-gnu- cross toolchain. I believe
that can be found at:

https://releases.linaro.org/components/toolchain/binaries/5.1-2015.08/aarch64-linux-gnu/gcc-linaro-5.1-2015.08-x86_64_aarch64-linux-gnu.tar.xz

Unfortunately, due to that (and lack of a good testcase), I can't give
much substantial feedback on this.

> diff --git a/arch/Kconfig b/arch/Kconfig
> index 99839c23d453..f1250ba0b06f 100644
> --- a/arch/Kconfig
> +++ b/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 initialized variables"

It might be better to say "Report forcefully initialized variables", to make it
clear that this is only reporting initialization performed by the plugin.

> +	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
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index cf0fa5d86059..91c30cba984e 100644
> --- a/include/linux/compiler.h
> +++ b/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
> diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
> index 060d2cb373db..a084f7a511d8 100644
> --- a/scripts/Makefile.gcc-plugins
> +++ b/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
> diff --git a/scripts/gcc-plugins/structleak_plugin.c b/scripts/gcc-plugins/structleak_plugin.c
> new file mode 100644
> index 000000000000..deddb7291a26
> --- /dev/null
> +++ b/scripts/gcc-plugins/structleak_plugin.c
> @@ -0,0 +1,246 @@
> +/*
> + * Copyright 2013-2017 by PaX Team <pageexec@xxxxxxxxxxx>
> + * 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

It's my understanding that some architectures do link against libgcc, so we
might want some kind of guard to avoid mishaps. e.g.
ARCH_CAN_USE_NON_GPLV3_GCC_PLUGINS.

> + *
> + * 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",

Has this been modified at all in the porting process? Maybe it would be
good to include KERNELVERSION here?

> +	.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);

I assume that this is only guaranteed to initialise fields in a struct,
and not padding, is that correct? I ask due to the issue described in:

https://lwn.net/Articles/417989/

As a further step, it would be interesting to see if we could fix
padding for __user variables, but that certainly shouldn't hold this
back.

> +}
> +
> +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)));
> +	}
> +
> +	/* enumarate all local variables and forcibly initialize our targets */

Nit: s/enumarate/enumerate/

> +	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;
> +}
> -- 
> 2.7.4

Thanks,
Mark.