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Commit 81a56f6d authored by Kees Cook's avatar Kees Cook
Browse files

gcc-plugins: structleak: Generalize to all variable types 



This adjusts structleak to also work with non-struct types when they
are passed by reference, since those variables may leak just like
anything else. This is exposed via an improved set of Kconfig options.
(This does mean structleak is slightly misnamed now.)

Building with CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL should give the
kernel complete initialization coverage of all stack variables passed
by reference, including padding (see lib/test_stackinit.c).

Using CONFIG_GCC_PLUGIN_STRUCTLEAK_VERBOSE to count added initializations
under defconfig:

	..._BYREF:      5945 added initializations
	..._BYREF_ALL: 16606 added initializations

There is virtually no change to text+data size (both have less than 0.05%
growth):

   text    data     bss     dec     hex filename
19502103        5051456 1917000 26470559        193e89f vmlinux.stock
19513412        5051456 1908808 26473676        193f4cc vmlinux.byref
19516974        5047360 1900616 26464950        193d2b6 vmlinux.byref_all

The measured performance difference is in the noise for hackbench and
kernel build benchmarks:

Stock:

	5x hackbench -g 20 -l 1000
	Mean:   10.649s
	Std Dev: 0.339

	5x kernel build (4-way parallel)
	Mean:  261.98s
	Std Dev: 1.53

CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF:

	5x hackbench -g 20 -l 1000
	Mean:   10.540s
	Std Dev: 0.233

	5x kernel build (4-way parallel)
	Mean:  260.52s
	Std Dev: 1.31

CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL:

	5x hackbench -g 20 -l 1000
	Mean:   10.320
	Std Dev: 0.413

	5x kernel build (4-way parallel)
	Mean:  260.10
	Std Dev: 0.86

This does not yet solve missing padding initialization for structures
on the stack that are never passed by reference (which should be a tiny
minority). Hopefully this will be more easily addressed by upstream
compiler fixes after clarifying the C11 padding initialization
specification.

Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Reviewed-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
parent 49a57857

scripts/Makefile.gcc-plugins

+2 −0
Original line number Diff line number Diff line
@@ -15,6 +15,8 @@ gcc-plugin-$(CONFIG_GCC_PLUGIN_SANCOV) += sancov_plugin.so 
gcc-plugin-$(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_BYREF)		\

		+= -fplugin-arg-structleak_plugin-byref

gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL)	\

		+= -fplugin-arg-structleak_plugin-byref-all

gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STRUCTLEAK)		\

scripts/gcc-plugins/Kconfig

+47 −11
Original line number Diff line number Diff line
@@ -67,27 +67,63 @@ config GCC_PLUGIN_LATENT_ENTROPY 
	   * https://pax.grsecurity.net/



config GCC_PLUGIN_STRUCTLEAK

	bool "Force initialization of variables containing userspace addresses"

	bool "Zero initialize stack variables"

	# Currently STRUCTLEAK inserts initialization out of live scope of

	# variables from KASAN point of view. This leads to KASAN false

	# positive reports. Prohibit this combination for now.

	depends on !KASAN_EXTRA

	help

	  This plugin zero-initializes any structures containing a

	  __user attribute. This can prevent some classes of information

	  exposures.



	  This plugin was ported from grsecurity/PaX. More information at:

	  While the kernel is built with warnings enabled for any missed

	  stack variable initializations, this warning is silenced for

	  anything passed by reference to another function, under the

	  occasionally misguided assumption that the function will do

	  the initialization. As this regularly leads to exploitable

	  flaws, this plugin is available to identify and zero-initialize

	  such variables, depending on the chosen level of coverage.



	  This plugin was originally ported from grsecurity/PaX. More

	  information at:

	   * https://grsecurity.net/

	   * https://pax.grsecurity.net/



config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL

	bool "Force initialize all struct type variables passed by reference"

choice

	prompt "Coverage"

	depends on GCC_PLUGIN_STRUCTLEAK

	depends on !COMPILE_TEST

	default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL

	help

	  This chooses the level of coverage over classes of potentially

	  uninitialized variables. The selected class will be

	  zero-initialized before use.



	config GCC_PLUGIN_STRUCTLEAK_USER

		bool "structs marked for userspace"

		help

	  Zero initialize any struct type local variable that may be passed by

	  reference without having been initialized.

		  Zero-initialize any structures on the stack containing

		  a __user attribute. This can prevent some classes of

		  uninitialized stack variable exploits and information

		  exposures, like CVE-2013-2141:

		  https://git.kernel.org/linus/b9e146d8eb3b9eca



	config GCC_PLUGIN_STRUCTLEAK_BYREF

		bool "structs passed by reference"

		help

		  Zero-initialize any structures on the stack that may

		  be passed by reference and had not already been

		  explicitly initialized. This can prevent most classes

		  of uninitialized stack variable exploits and information

		  exposures, like CVE-2017-1000410:

		  https://git.kernel.org/linus/06e7e776ca4d3654



	config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL

		bool "anything passed by reference"

		help

		  Zero-initialize any stack variables that may be passed

		  by reference and had not already been explicitly

		  initialized. This is intended to eliminate all classes

		  of uninitialized stack variable exploits and information

		  exposures.



endchoice



config GCC_PLUGIN_STRUCTLEAK_VERBOSE

	bool "Report forcefully initialized variables"

scripts/gcc-plugins/structleak_plugin.c

+25 −11
Original line number Diff line number Diff line
@@ -16,6 +16,7 @@ 
 * Options:

 * -fplugin-arg-structleak_plugin-disable

 * -fplugin-arg-structleak_plugin-verbose

 * -fplugin-arg-structleak_plugin-byref

 * -fplugin-arg-structleak_plugin-byref-all

 *

 * Usage:
@@ -26,7 +27,6 @@ 
 * $ gcc -fplugin=./structleak_plugin.so test.c -O2

 *

 * TODO: eliminate redundant initializers

 *       increase type coverage

 */



#include "gcc-common.h"
@@ -37,13 +37,18 @@ 
__visible int plugin_is_GPL_compatible;



static struct plugin_info structleak_plugin_info = {

	.version	= "201607271510vanilla",

	.version	= "20190125vanilla",

	.help		= "disable\tdo not activate plugin\n"

			  "byref\tinit structs passed by reference\n"

			  "byref-all\tinit anything passed by reference\n"

			  "verbose\tprint all initialized variables\n",

};



#define BYREF_STRUCT	1

#define BYREF_ALL	2



static bool verbose;

static bool byref_all;

static int byref;



static tree handle_user_attribute(tree *node, tree name, tree args, int flags, bool *no_add_attrs)

{
@@ -118,6 +123,7 @@ static void initialize(tree var) 
	gimple_stmt_iterator gsi;

	tree initializer;

	gimple init_stmt;

	tree type;



	/* this is the original entry bb before the forced split */

	bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
@@ -148,11 +154,15 @@ static void initialize(tree var) 
	if (verbose)

		inform(DECL_SOURCE_LOCATION(var),

			"%s variable will be forcibly initialized",

			(byref_all && TREE_ADDRESSABLE(var)) ? "byref"

			(byref && TREE_ADDRESSABLE(var)) ? "byref"

							 : "userspace");



	/* build the initializer expression */

	initializer = build_constructor(TREE_TYPE(var), NULL);

	type = TREE_TYPE(var);

	if (AGGREGATE_TYPE_P(type))

		initializer = build_constructor(type, NULL);

	else

		initializer = fold_convert(type, integer_zero_node);



	/* build the initializer stmt */

	init_stmt = gimple_build_assign(var, initializer);
@@ -184,13 +194,13 @@ static unsigned int structleak_execute(void) 
		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)

		/* only care about structure types unless byref-all */

		if (byref != BYREF_ALL && TREE_CODE(type) != RECORD_TYPE && TREE_CODE(type) != UNION_TYPE)

			continue;



		/* if the type is of interest, examine the variable */

		if (TYPE_USERSPACE(type) ||

		    (byref_all && TREE_ADDRESSABLE(var)))

		    (byref && TREE_ADDRESSABLE(var)))

			initialize(var);

	}
@@ -232,8 +242,12 @@ __visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gc 
			verbose = true;

			continue;

		}

		if (!strcmp(argv[i].key, "byref")) {

			byref = BYREF_STRUCT;

			continue;

		}

		if (!strcmp(argv[i].key, "byref-all")) {

			byref_all = true;

			byref = BYREF_ALL;

			continue;

		}

		error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);