[v3,2/4] Add the latent_entropy gcc plugin
diff mbox

Message ID 20160615002033.a318fa0dd807751a596185da@gmail.com
State New
Headers show

Commit Message

Emese Revfy June 14, 2016, 10:20 p.m. UTC
This plugin mitigates the problem of the kernel having too little entropy during
and after boot for generating crypto keys.

It creates a local variable in every marked function. The value of this variable is
modified by randomly chosen operations (add, xor and rol) and
random values (gcc generates them at compile time and the stack pointer at runtime).
It depends on the control flow (e.g., loops, conditions).

Before the function returns the plugin writes this local variable
into the latent_entropy global variable. The value of this global variable is
added to the kernel entropy pool in do_one_initcall() and _do_fork().

Signed-off-by: Emese Revfy <re.emese@gmail.com>
---
 arch/Kconfig                                |  18 +
 arch/powerpc/kernel/Makefile                |   8 +-
 include/linux/random.h                      |  10 +
 init/main.c                                 |   1 +
 kernel/fork.c                               |   1 +
 mm/page_alloc.c                             |   5 +
 scripts/Makefile.gcc-plugins                |   8 +-
 scripts/gcc-plugins/Makefile                |   1 +
 scripts/gcc-plugins/latent_entropy_plugin.c | 597 ++++++++++++++++++++++++++++
 9 files changed, 644 insertions(+), 5 deletions(-)
 create mode 100644 scripts/gcc-plugins/latent_entropy_plugin.c

Comments

Kees Cook June 15, 2016, 6:07 p.m. UTC | #1
On Tue, Jun 14, 2016 at 3:20 PM, Emese Revfy <re.emese@gmail.com> wrote:
> This plugin mitigates the problem of the kernel having too little entropy during
> and after boot for generating crypto keys.
>
> It creates a local variable in every marked function. The value of this variable is
> modified by randomly chosen operations (add, xor and rol) and
> random values (gcc generates them at compile time and the stack pointer at runtime).
> It depends on the control flow (e.g., loops, conditions).
>
> Before the function returns the plugin writes this local variable
> into the latent_entropy global variable. The value of this global variable is
> added to the kernel entropy pool in do_one_initcall() and _do_fork().
>
> Signed-off-by: Emese Revfy <re.emese@gmail.com>
> [...]
> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
> index 2da380f..6c7e448 100644
> --- a/arch/powerpc/kernel/Makefile
> +++ b/arch/powerpc/kernel/Makefile
> @@ -16,10 +16,10 @@ endif
>
>  ifdef CONFIG_FUNCTION_TRACER
>  # Do not trace early boot code
> -CFLAGS_REMOVE_cputable.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
> -CFLAGS_REMOVE_prom_init.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
> -CFLAGS_REMOVE_btext.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
> -CFLAGS_REMOVE_prom.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_cputable.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)
> +CFLAGS_REMOVE_prom_init.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)
> +CFLAGS_REMOVE_btext.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)
> +CFLAGS_REMOVE_prom.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)

This doesn't look right to me: these are CFLAGS_REMOVE_* entries, and
I think you want to _add_ the DISABLE_LATENT_ENTROPY_PLUGIN to the
CFLAGS here.

from scripts/Makefile.lib:
_c_flags       = $(filter-out $(CFLAGS_REMOVE_$(basetarget).o), $(orig_c_flags))

-Kees
Emese Revfy June 15, 2016, 8:49 p.m. UTC | #2
On Wed, 15 Jun 2016 11:07:08 -0700
Kees Cook <keescook@chromium.org> wrote:

> On Tue, Jun 14, 2016 at 3:20 PM, Emese Revfy <re.emese@gmail.com> wrote:
 
> This doesn't look right to me: these are CFLAGS_REMOVE_* entries, and
> I think you want to _add_ the DISABLE_LATENT_ENTROPY_PLUGIN to the
> CFLAGS here.

Thanks for the report. I think this patch fixes it:
https://github.com/ephox-gcc-plugins/gcc-plugins_linux-next/commit/e7601ca00a0aeb5f6b96dc79a51a5089c4d32791
Kees Cook June 15, 2016, 10:38 p.m. UTC | #3
On Wed, Jun 15, 2016 at 1:49 PM, Emese Revfy <re.emese@gmail.com> wrote:
> On Wed, 15 Jun 2016 11:07:08 -0700
> Kees Cook <keescook@chromium.org> wrote:
>
>> On Tue, Jun 14, 2016 at 3:20 PM, Emese Revfy <re.emese@gmail.com> wrote:
>
>> This doesn't look right to me: these are CFLAGS_REMOVE_* entries, and
>> I think you want to _add_ the DISABLE_LATENT_ENTROPY_PLUGIN to the
>> CFLAGS here.
>
> Thanks for the report. I think this patch fixes it:
> https://github.com/ephox-gcc-plugins/gcc-plugins_linux-next/commit/e7601ca00a0aeb5f6b96dc79a51a5089c4d32791

Yup, that looks correct, thanks!

-Kees

Patch
diff mbox

diff --git a/arch/Kconfig b/arch/Kconfig
index 566f922..c099ebd 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -394,6 +394,24 @@  config GCC_PLUGIN_SANCOV
 	  gcc-4.5 on). It is based on the commit "Add fuzzing coverage support"
 	  by Dmitry Vyukov <dvyukov@google.com>.
 
+config GCC_PLUGIN_LATENT_ENTROPY
+	bool "Generate some entropy during boot and runtime"
+	depends on GCC_PLUGINS
+	help
+	  By saying Y here the kernel will instrument some kernel code to
+	  extract some entropy from both original and artificially created
+	  program state.  This will help especially embedded systems where
+	  there is little 'natural' source of entropy normally.  The cost
+	  is some slowdown of the boot process (about 0.5%) and fork and
+	  irq processing.
+
+	  Note that entropy extracted this way is not known to be cryptographically
+	  secure!
+
+	  This plugin was ported from grsecurity/PaX. More information at:
+	   * https://grsecurity.net/
+	   * https://pax.grsecurity.net/
+
 config HAVE_CC_STACKPROTECTOR
 	bool
 	help
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 2da380f..6c7e448 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -16,10 +16,10 @@  endif
 
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace early boot code
-CFLAGS_REMOVE_cputable.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
-CFLAGS_REMOVE_prom_init.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
-CFLAGS_REMOVE_btext.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
-CFLAGS_REMOVE_prom.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_cputable.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)
+CFLAGS_REMOVE_prom_init.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)
+CFLAGS_REMOVE_btext.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)
+CFLAGS_REMOVE_prom.o = -mno-sched-epilog $(CC_FLAGS_FTRACE) $(DISABLE_LATENT_ENTROPY_PLUGIN)
 # do not trace tracer code
 CFLAGS_REMOVE_ftrace.o = -mno-sched-epilog $(CC_FLAGS_FTRACE)
 # timers used by tracing
diff --git a/include/linux/random.h b/include/linux/random.h
index e47e533..752e7df 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -18,6 +18,16 @@  struct random_ready_callback {
 };
 
 extern void add_device_randomness(const void *, unsigned int);
+
+#if defined(CONFIG_GCC_PLUGIN_LATENT_ENTROPY) && !defined(__CHECKER__)
+static inline void add_latent_entropy(void)
+{
+	add_device_randomness((const void *)&latent_entropy, sizeof(latent_entropy));
+}
+#else
+static inline void add_latent_entropy(void) {}
+#endif
+
 extern void add_input_randomness(unsigned int type, unsigned int code,
 				 unsigned int value);
 extern void add_interrupt_randomness(int irq, int irq_flags);
diff --git a/init/main.c b/init/main.c
index 4c17fda..07e4174 100644
--- a/init/main.c
+++ b/init/main.c
@@ -781,6 +781,7 @@  int __init_or_module do_one_initcall(initcall_t fn)
 	}
 	WARN(msgbuf[0], "initcall %pF returned with %s\n", fn, msgbuf);
 
+	add_latent_entropy();
 	return ret;
 }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index cdf520f..d07d5a6 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1766,6 +1766,7 @@  long _do_fork(unsigned long clone_flags,
 
 	p = copy_process(clone_flags, stack_start, stack_size,
 			 child_tidptr, NULL, trace, tls, NUMA_NO_NODE);
+	add_latent_entropy();
 	/*
 	 * Do this prior waking up the new thread - the thread pointer
 	 * might get invalid after that point, if the thread exits quickly.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f8f3bfc..d10324e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1234,6 +1234,11 @@  static void __free_pages_ok(struct page *page, unsigned int order)
 	local_irq_restore(flags);
 }
 
+#ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
+volatile u64 latent_entropy;
+EXPORT_SYMBOL(latent_entropy);
+#endif
+
 static void __init __free_pages_boot_core(struct page *page, unsigned int order)
 {
 	unsigned int nr_pages = 1 << order;
diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index da7f86c..cd7902c 100644
--- a/scripts/Makefile.gcc-plugins
+++ b/scripts/Makefile.gcc-plugins
@@ -6,6 +6,12 @@  ifdef CONFIG_GCC_PLUGINS
 
   gcc-plugin-$(CONFIG_GCC_PLUGIN_CYC_COMPLEXITY)	+= cyc_complexity_plugin.so
 
+  gcc-plugin-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY)	+= latent_entropy_plugin.so
+  gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_LATENT_ENTROPY)	+= -DLATENT_ENTROPY_PLUGIN
+  ifdef CONFIG_PAX_LATENT_ENTROPY
+    DISABLE_LATENT_ENTROPY_PLUGIN			+= -fplugin-arg-latent_entropy_plugin-disable
+  endif
+
   ifdef CONFIG_GCC_PLUGIN_SANCOV
     ifeq ($(CFLAGS_KCOV),)
       # It is needed because of the gcc-plugin.sh and gcc version checks.
@@ -21,7 +27,7 @@  ifdef CONFIG_GCC_PLUGINS
 
   GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
 
-  export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN SANCOV_PLUGIN
+  export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN
 
   ifeq ($(PLUGINCC),)
     ifneq ($(GCC_PLUGINS_CFLAGS),)
diff --git a/scripts/gcc-plugins/Makefile b/scripts/gcc-plugins/Makefile
index 88c8ec4..a3f8ca4a 100644
--- a/scripts/gcc-plugins/Makefile
+++ b/scripts/gcc-plugins/Makefile
@@ -23,5 +23,6 @@  always := $($(HOSTLIBS)-y)
 
 cyc_complexity_plugin-objs := cyc_complexity_plugin.o
 sancov_plugin-objs := sancov_plugin.o
+latent_entropy_plugin-objs := latent_entropy_plugin.o
 
 clean-files += *.so
diff --git a/scripts/gcc-plugins/latent_entropy_plugin.c b/scripts/gcc-plugins/latent_entropy_plugin.c
new file mode 100644
index 0000000..8a70e38
--- /dev/null
+++ b/scripts/gcc-plugins/latent_entropy_plugin.c
@@ -0,0 +1,597 @@ 
+/*
+ * Copyright 2012-2016 by the PaX Team <pageexec@freemail.hu>
+ * Copyright 2016 by Emese Revfy <re.emese@gmail.com>
+ * 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
+ *
+ * This gcc plugin helps generate a little bit of entropy from program state,
+ * used throughout the uptime of the kernel. Here is an instrumentation example:
+ *
+ * before:
+ * void __latent_entropy test(int argc, char *argv[])
+ * {
+ *	printf("%u %s\n", argc, *argv);
+ * }
+ *
+ * after:
+ * void __latent_entropy test(int argc, char *argv[])
+ * {
+ *	// latent_entropy_execute() 1.
+ *	unsigned long local_entropy;
+ *	// init_local_entropy() 1.
+ *	void *local_entropy_frame_addr;
+ *	// init_local_entropy() 3.
+ *	unsigned long temp_latent_entropy;
+ *
+ *	// init_local_entropy() 2.
+ *	local_entropy_frame_addr = __builtin_frame_address(0);
+ *	local_entropy = (unsigned long) local_entropy_frame_addr;
+ *
+ *	// init_local_entropy() 4.
+ *	temp_latent_entropy = latent_entropy;
+ *	// init_local_entropy() 5.
+ *	local_entropy ^= temp_latent_entropy;
+ *
+ *	// latent_entropy_execute() 3.
+ *	local_entropy += 4623067384293424948;
+ *
+ *	printf("%u %s\n", argc, *argv);
+ *
+ *	// latent_entropy_execute() 4.
+ *	temp_latent_entropy = rol(temp_latent_entropy, local_entropy);
+ *	latent_entropy = temp_latent_entropy;
+ * }
+ *
+ * It would look like this in C:
+ *
+ * unsigned long local_entropy = latent_entropy;
+ * local_entropy ^= 1234567890;
+ * local_entropy ^= (unsigned long)__builtin_frame_address(0);
+ * local_entropy += 9876543210;
+ * latent_entropy = rol(local_entropy, 1234509876);
+ *
+ * TODO:
+ * - add ipa pass to identify not explicitly marked candidate functions
+ * - mix in more program state (function arguments/return values,
+ *   loop variables, etc)
+ * - more instrumentation control via attribute parameters
+ *
+ * BUGS:
+ * - none known
+ *
+ * Options:
+ * -fplugin-arg-latent_entropy_plugin-disable
+ *
+ * Attribute: __attribute__((latent_entropy))
+ *  The latent_entropy gcc attribute can be only on functions and variables.
+ *  If it is on a function then the plugin will instrument it. If the attribute
+ *  is on a variable then the plugin will initialize it with a random value.
+ *  The variable must be an integer, an integer array type or a structure
+ *  with integer fields.
+ */
+
+#include "gcc-common.h"
+
+int plugin_is_GPL_compatible;
+
+static GTY(()) tree latent_entropy_decl;
+
+static struct plugin_info latent_entropy_plugin_info = {
+	.version	= "201606141920",
+	.help		= "disable\tturn off latent entropy instrumentation\n",
+};
+
+static unsigned HOST_WIDE_INT seed;
+/*
+ * get_random_seed() (this is a GCC function) generates the seed.
+ * This is a simple random generator without any cryptographic security because
+ * the entropy doesn't come from here.
+ */
+static unsigned HOST_WIDE_INT get_random_const(void)
+{
+	unsigned int i;
+	unsigned HOST_WIDE_INT ret = 0;
+
+	for (i = 0; i < 8 * sizeof ret; i++) {
+		ret = (ret << 1) | (seed & 1);
+		seed >>= 1;
+		if (ret & 1)
+			seed ^= 0xD800000000000000ULL;
+	}
+
+	return ret;
+}
+
+static tree tree_get_random_const(tree type)
+{
+	unsigned long long mask;
+
+	mask = 1ULL << (TREE_INT_CST_LOW(TYPE_SIZE(type)) - 1);
+	mask = 2 * (mask - 1) + 1;
+
+	if (TYPE_UNSIGNED(type))
+		return build_int_cstu(type, mask & get_random_const());
+	return build_int_cst(type, mask & get_random_const());
+}
+
+static tree handle_latent_entropy_attribute(tree *node, tree name, tree args __unused, int flags __unused, bool *no_add_attrs)
+{
+	tree type;
+#if BUILDING_GCC_VERSION <= 4007
+	VEC(constructor_elt, gc) *vals;
+#else
+	vec<constructor_elt, va_gc> *vals;
+#endif
+
+	switch (TREE_CODE(*node)) {
+	default:
+		*no_add_attrs = true;
+		error("%qE attribute only applies to functions and variables", name);
+		break;
+
+	case VAR_DECL:
+		if (DECL_INITIAL(*node)) {
+			*no_add_attrs = true;
+			error("variable %qD with %qE attribute must not be initialized", *node, name);
+			break;
+		}
+
+		if (!TREE_STATIC(*node)) {
+			*no_add_attrs = true;
+			error("variable %qD with %qE attribute must not be local", *node, name);
+			break;
+		}
+
+		type = TREE_TYPE(*node);
+		switch (TREE_CODE(type)) {
+		default:
+			*no_add_attrs = true;
+			error("variable %qD with %qE attribute must be an integer"
+				" or a fixed length integer array type"
+				" or a fixed sized structure with integer fields", *node, name);
+			break;
+
+		case RECORD_TYPE: {
+			tree field;
+			unsigned int nelt = 0;
+
+			for (field = TYPE_FIELDS(type); field; nelt++, field = TREE_CHAIN(field)) {
+				tree fieldtype;
+
+				fieldtype = TREE_TYPE(field);
+				if (TREE_CODE(fieldtype) == INTEGER_TYPE)
+					continue;
+
+				*no_add_attrs = true;
+				error("structure variable %qD with %qE attribute has"
+					" a non-integer field %qE", *node, name, field);
+				break;
+			}
+
+			if (field)
+				break;
+
+#if BUILDING_GCC_VERSION <= 4007
+			vals = VEC_alloc(constructor_elt, gc, nelt);
+#else
+			vec_alloc(vals, nelt);
+#endif
+
+			for (field = TYPE_FIELDS(type); field; field = TREE_CHAIN(field)) {
+				tree random_const;
+
+				random_const = tree_get_random_const(TREE_TYPE(field));
+				CONSTRUCTOR_APPEND_ELT(vals, field, random_const);
+			}
+
+			/* Initialize the fields with random constants */
+			DECL_INITIAL(*node) = build_constructor(type, vals);
+			break;
+		}
+
+		/* Initialize the variable with a random constant */
+		case INTEGER_TYPE:
+			DECL_INITIAL(*node) = tree_get_random_const(type);
+			break;
+
+		case ARRAY_TYPE: {
+			tree elt_type, array_size, elt_size;
+			unsigned int i, nelt;
+
+			elt_type = TREE_TYPE(type);
+			elt_size = TYPE_SIZE_UNIT(TREE_TYPE(type));
+			array_size = TYPE_SIZE_UNIT(type);
+
+			if (TREE_CODE(elt_type) != INTEGER_TYPE || !array_size || TREE_CODE(array_size) != INTEGER_CST) {
+				*no_add_attrs = true;
+				error("array variable %qD with %qE attribute must be"
+					" a fixed length integer array type", *node, name);
+				break;
+			}
+
+			nelt = TREE_INT_CST_LOW(array_size) / TREE_INT_CST_LOW(elt_size);
+#if BUILDING_GCC_VERSION <= 4007
+			vals = VEC_alloc(constructor_elt, gc, nelt);
+#else
+			vec_alloc(vals, nelt);
+#endif
+
+			for (i = 0; i < nelt; i++) {
+				tree random_const = tree_get_random_const(elt_type);
+
+				CONSTRUCTOR_APPEND_ELT(vals, size_int(i), random_const);
+			}
+
+			/* Initialize the elements of the array with random constants */
+			DECL_INITIAL(*node) = build_constructor(type, vals);
+			break;
+		}
+		}
+		break;
+
+	case FUNCTION_DECL:
+		break;
+	}
+
+	return NULL_TREE;
+}
+
+static struct attribute_spec latent_entropy_attr = {
+	.name				= "latent_entropy",
+	.min_length			= 0,
+	.max_length			= 0,
+	.decl_required			= true,
+	.type_required			= false,
+	.function_type_required		= false,
+	.handler			= handle_latent_entropy_attribute,
+#if BUILDING_GCC_VERSION >= 4007
+	.affects_type_identity		= false
+#endif
+};
+
+static void register_attributes(void *event_data __unused, void *data __unused)
+{
+	register_attribute(&latent_entropy_attr);
+}
+
+static bool latent_entropy_gate(void)
+{
+	/* don't bother with noreturn functions for now */
+	if (TREE_THIS_VOLATILE(current_function_decl))
+		return false;
+
+	/* gcc-4.5 doesn't discover some trivial noreturn functions */
+	if (EDGE_COUNT(EXIT_BLOCK_PTR_FOR_FN(cfun)->preds) == 0)
+		return false;
+
+	return lookup_attribute("latent_entropy", DECL_ATTRIBUTES(current_function_decl)) != NULL_TREE;
+}
+
+static tree create_a_tmp_var(tree type, const char *name)
+{
+	tree var;
+
+	var = create_tmp_var(type, name);
+	add_referenced_var(var);
+	mark_sym_for_renaming(var);
+	return var;
+}
+
+/*
+ * Set up the next operation and its constant operand to use in the latent
+ * entropy PRNG. When RHS is specified, the request is for perturbing the
+ * local latent entropy variable, otherwise it is for perturbing the global
+ * latent entropy variable where the two operands are already given by the
+ * local and global latent entropy variables themselves.
+ *
+ * The operation is one of add/xor/rol when instrumenting the local entropy
+ * variable and one of add/xor when perturbing the global entropy variable.
+ * Rotation is not used for the latter case because it would transmit less
+ * entropy to the global variable than the other two operations.
+ */
+static enum tree_code get_op(tree *rhs)
+{
+	static enum tree_code op;
+	unsigned HOST_WIDE_INT random_const;
+
+	random_const = get_random_const();
+
+	switch (op) {
+	case BIT_XOR_EXPR:
+		op = PLUS_EXPR;
+		break;
+
+	case PLUS_EXPR:
+		if (rhs) {
+			op = LROTATE_EXPR;
+			/*
+			 * This code limits the value of random_const to
+			 * the size of a wide int for the rotation
+			 */
+			random_const &= HOST_BITS_PER_WIDE_INT - 1;
+			break;
+		}
+
+	case LROTATE_EXPR:
+	default:
+		op = BIT_XOR_EXPR;
+		break;
+	}
+	if (rhs)
+		*rhs = build_int_cstu(unsigned_intDI_type_node, random_const);
+	return op;
+}
+
+static void perturb_local_entropy(basic_block bb, tree local_entropy)
+{
+	gimple_stmt_iterator gsi;
+	gimple assign;
+	tree rhs;
+	enum tree_code op;
+
+	op = get_op(&rhs);
+	assign = gimple_build_assign_with_ops(op, local_entropy, local_entropy, rhs);
+	gsi = gsi_after_labels(bb);
+	gsi_insert_before(&gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+}
+
+static void __perturb_latent_entropy(gimple_stmt_iterator *gsi, tree local_entropy)
+{
+	gimple assign;
+	tree temp;
+	enum tree_code op;
+
+	/* 1. create temporary copy of latent_entropy */
+	temp = create_a_tmp_var(unsigned_intDI_type_node, "temp_latent_entropy");
+
+	/* 2. read... */
+	add_referenced_var(latent_entropy_decl);
+	mark_sym_for_renaming(latent_entropy_decl);
+	assign = gimple_build_assign(temp, latent_entropy_decl);
+	gsi_insert_before(gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+
+	/* 3. ...modify... */
+	op = get_op(NULL);
+	assign = gimple_build_assign_with_ops(op, temp, temp, local_entropy);
+	gsi_insert_after(gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+
+	/* 4. ...write latent_entropy */
+	assign = gimple_build_assign(latent_entropy_decl, temp);
+	gsi_insert_after(gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+}
+
+static bool handle_tail_calls(basic_block bb, tree local_entropy)
+{
+	gimple_stmt_iterator gsi;
+
+	for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi)) {
+		gcall *call;
+		gimple stmt = gsi_stmt(gsi);
+
+		if (!is_gimple_call(stmt))
+			continue;
+
+		call = as_a_gcall(stmt);
+		if (!gimple_call_tail_p(call))
+			continue;
+
+		__perturb_latent_entropy(&gsi, local_entropy);
+		return true;
+	}
+
+	return false;
+}
+
+static void perturb_latent_entropy(tree local_entropy)
+{
+	edge_iterator ei;
+	edge e, last_bb_e;
+	basic_block last_bb;
+
+	gcc_assert(single_pred_p(EXIT_BLOCK_PTR_FOR_FN(cfun)));
+	last_bb_e = single_pred_edge(EXIT_BLOCK_PTR_FOR_FN(cfun));
+
+	FOR_EACH_EDGE(e, ei, last_bb_e->src->preds) {
+		if (ENTRY_BLOCK_PTR_FOR_FN(cfun) == e->src)
+			continue;
+		if (EXIT_BLOCK_PTR_FOR_FN(cfun) == e->src)
+			continue;
+
+		handle_tail_calls(e->src, local_entropy);
+	}
+
+	last_bb = single_pred(EXIT_BLOCK_PTR_FOR_FN(cfun));
+	if (!handle_tail_calls(last_bb, local_entropy)) {
+		gimple_stmt_iterator gsi = gsi_last_bb(last_bb);
+
+		__perturb_latent_entropy(&gsi, local_entropy);
+	}
+}
+
+static void init_local_entropy(basic_block bb, tree local_entropy)
+{
+	gimple assign, call;
+	tree frame_addr, rand_const, temp, fndecl, udi_frame_addr;
+	enum tree_code op;
+	gimple_stmt_iterator gsi = gsi_after_labels(bb);
+
+	/* 1. create local_entropy_frame_addr */
+	frame_addr = create_a_tmp_var(ptr_type_node, "local_entropy_frame_addr");
+
+	/* 2. local_entropy_frame_addr = __builtin_frame_address() */
+	fndecl = builtin_decl_implicit(BUILT_IN_FRAME_ADDRESS);
+	call = gimple_build_call(fndecl, 1, integer_zero_node);
+	gimple_call_set_lhs(call, frame_addr);
+	gsi_insert_before(&gsi, call, GSI_NEW_STMT);
+	update_stmt(call);
+
+	udi_frame_addr = fold_convert(unsigned_intDI_type_node, frame_addr);
+	assign = gimple_build_assign(local_entropy, udi_frame_addr);
+	gsi_insert_after(&gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+
+	/* 3. create temporary copy of latent_entropy */
+	temp = create_a_tmp_var(unsigned_intDI_type_node, "temp_latent_entropy");
+
+	/* 4. read the global entropy variable into local entropy */
+	add_referenced_var(latent_entropy_decl);
+	mark_sym_for_renaming(latent_entropy_decl);
+	assign = gimple_build_assign(temp, latent_entropy_decl);
+	gsi_insert_after(&gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+
+	/* 5. mix local_entropy_frame_addr into local entropy */
+	assign = gimple_build_assign_with_ops(BIT_XOR_EXPR, local_entropy, local_entropy, temp);
+	gsi_insert_after(&gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+
+	rand_const = build_int_cstu(unsigned_intDI_type_node, get_random_const());
+	op = get_op(NULL);
+	assign = gimple_build_assign_with_ops(op, local_entropy, local_entropy, rand_const);
+	gsi_insert_after(&gsi, assign, GSI_NEW_STMT);
+	update_stmt(assign);
+}
+
+static bool create_latent_entropy_decl(void)
+{
+	varpool_node_ptr node;
+
+	if (latent_entropy_decl != NULL_TREE)
+		return true;
+
+	FOR_EACH_VARIABLE(node) {
+		tree var = NODE_DECL(node);
+
+		if (DECL_NAME_LENGTH(var) < sizeof("latent_entropy") - 1)
+			continue;
+		if (strcmp(IDENTIFIER_POINTER(DECL_NAME(var)), "latent_entropy"))
+			continue;
+
+		latent_entropy_decl = var;
+		break;
+	}
+
+	return latent_entropy_decl != NULL_TREE;
+}
+
+static unsigned int latent_entropy_execute(void)
+{
+	basic_block bb;
+	tree local_entropy;
+
+	if (!create_latent_entropy_decl())
+		return 0;
+
+	/* 2. initialize local entropy variable */
+	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)));
+		bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
+	}
+
+	/* 1. create the local entropy variable */
+	local_entropy = create_a_tmp_var(unsigned_intDI_type_node, "local_entropy");
+
+	/* 2. initialize the local entropy variable */
+	init_local_entropy(bb, local_entropy);
+
+	bb = bb->next_bb;
+
+	/* 3. instrument each BB with an operation on the local entropy variable */
+	while (bb != EXIT_BLOCK_PTR_FOR_FN(cfun)) {
+		perturb_local_entropy(bb, local_entropy);
+		bb = bb->next_bb;
+	};
+
+	/* 4. mix local entropy into the global entropy variable */
+	perturb_latent_entropy(local_entropy);
+	return 0;
+}
+
+static void latent_entropy_start_unit(void *gcc_data __unused, void *user_data __unused)
+{
+	tree latent_entropy_type;
+
+	seed = get_random_seed(false);
+
+	if (in_lto_p)
+		return;
+
+	/* extern volatile u64 latent_entropy */
+	gcc_assert(TYPE_PRECISION(long_long_unsigned_type_node) == 64);
+	latent_entropy_type = build_qualified_type(long_long_unsigned_type_node, TYPE_QUALS(long_long_unsigned_type_node) | TYPE_QUAL_VOLATILE);
+	latent_entropy_decl = build_decl(UNKNOWN_LOCATION, VAR_DECL, get_identifier("latent_entropy"), latent_entropy_type);
+
+	TREE_STATIC(latent_entropy_decl) = 1;
+	TREE_PUBLIC(latent_entropy_decl) = 1;
+	TREE_USED(latent_entropy_decl) = 1;
+	DECL_PRESERVE_P(latent_entropy_decl) = 1;
+	TREE_THIS_VOLATILE(latent_entropy_decl) = 1;
+	DECL_EXTERNAL(latent_entropy_decl) = 1;
+	DECL_ARTIFICIAL(latent_entropy_decl) = 1;
+	lang_hooks.decls.pushdecl(latent_entropy_decl);
+}
+
+#define PASS_NAME latent_entropy
+#define PROPERTIES_REQUIRED PROP_gimple_leh | PROP_cfg
+#define TODO_FLAGS_FINISH TODO_verify_ssa | TODO_verify_stmts | TODO_dump_func | TODO_update_ssa
+#include "gcc-generate-gimple-pass.h"
+
+int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
+{
+	bool enabled = true;
+	const char * const plugin_name = plugin_info->base_name;
+	const int argc = plugin_info->argc;
+	const struct plugin_argument * const argv = plugin_info->argv;
+	int i;
+
+	struct register_pass_info latent_entropy_pass_info;
+
+	latent_entropy_pass_info.pass				= make_latent_entropy_pass();
+	latent_entropy_pass_info.reference_pass_name		= "optimized";
+	latent_entropy_pass_info.ref_pass_instance_number	= 1;
+	latent_entropy_pass_info.pos_op				= PASS_POS_INSERT_BEFORE;
+	static const struct ggc_root_tab gt_ggc_r_gt_latent_entropy[] = {
+		{
+			.base = &latent_entropy_decl,
+			.nelt = 1,
+			.stride = sizeof(latent_entropy_decl),
+			.cb = &gt_ggc_mx_tree_node,
+			.pchw = &gt_pch_nx_tree_node
+		},
+		LAST_GGC_ROOT_TAB
+	};
+
+	if (!plugin_default_version_check(version, &gcc_version)) {
+		error(G_("incompatible gcc/plugin versions"));
+		return 1;
+	}
+
+	for (i = 0; i < argc; ++i) {
+		if (!(strcmp(argv[i].key, "disable"))) {
+			enabled = false;
+			continue;
+		}
+		error(G_("unkown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
+	}
+
+	register_callback(plugin_name, PLUGIN_INFO, NULL, &latent_entropy_plugin_info);
+	if (enabled) {
+		register_callback(plugin_name, PLUGIN_START_UNIT, &latent_entropy_start_unit, NULL);
+		register_callback(plugin_name, PLUGIN_REGISTER_GGC_ROOTS, NULL, (void *)&gt_ggc_r_gt_latent_entropy);
+		register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &latent_entropy_pass_info);
+	}
+	register_callback(plugin_name, PLUGIN_ATTRIBUTES, register_attributes, NULL);
+
+	return 0;
+}