1 /*
2  * Copyright 2013-2017 by PaX Team <pageexec@freemail.hu>
3  * Licensed under the GPL v2
4  *
5  * Note: the choice of the license means that the compilation process is
6  *       NOT 'eligible' as defined by gcc's library exception to the GPL v3,
7  *       but for the kernel it doesn't matter since it doesn't link against
8  *       any of the gcc libraries
9  *
10  * gcc plugin to forcibly initialize certain local variables that could
11  * otherwise leak kernel stack to userland if they aren't properly initialized
12  * by later code
13  *
14  * Homepage: https://pax.grsecurity.net/
15  *
16  * Options:
17  * -fplugin-arg-structleak_plugin-disable
18  * -fplugin-arg-structleak_plugin-verbose
19  * -fplugin-arg-structleak_plugin-byref
20  * -fplugin-arg-structleak_plugin-byref-all
21  *
22  * Usage:
23  * $ # for 4.5/4.6/C based 4.7
24  * $ 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
25  * $ # for C++ based 4.7/4.8+
26  * $ 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
27  * $ gcc -fplugin=./structleak_plugin.so test.c -O2
28  *
29  * TODO: eliminate redundant initializers
30  */
31 
32 #include "gcc-common.h"
33 
34 /* unused C type flag in all versions 4.5-6 */
35 #define TYPE_USERSPACE(TYPE) TYPE_LANG_FLAG_5(TYPE)
36 
37 __visible int plugin_is_GPL_compatible;
38 
39 static struct plugin_info structleak_plugin_info = {
40 	.version	= PLUGIN_VERSION,
41 	.help		= "disable\tdo not activate plugin\n"
42 			  "byref\tinit structs passed by reference\n"
43 			  "byref-all\tinit anything passed by reference\n"
44 			  "verbose\tprint all initialized variables\n",
45 };
46 
47 #define BYREF_STRUCT	1
48 #define BYREF_ALL	2
49 
50 static bool verbose;
51 static int byref;
52 
53 static tree handle_user_attribute(tree *node, tree name, tree args, int flags, bool *no_add_attrs)
54 {
55 	*no_add_attrs = true;
56 
57 	/* check for types? for now accept everything linux has to offer */
58 	if (TREE_CODE(*node) != FIELD_DECL)
59 		return NULL_TREE;
60 
61 	*no_add_attrs = false;
62 	return NULL_TREE;
63 }
64 
65 static struct attribute_spec user_attr = { };
66 
67 static void register_attributes(void *event_data, void *data)
68 {
69 	user_attr.name			= "user";
70 	user_attr.handler		= handle_user_attribute;
71 	user_attr.affects_type_identity	= true;
72 
73 	register_attribute(&user_attr);
74 }
75 
76 static tree get_field_type(tree field)
77 {
78 	return strip_array_types(TREE_TYPE(field));
79 }
80 
81 static bool is_userspace_type(tree type)
82 {
83 	tree field;
84 
85 	for (field = TYPE_FIELDS(type); field; field = TREE_CHAIN(field)) {
86 		tree fieldtype = get_field_type(field);
87 		enum tree_code code = TREE_CODE(fieldtype);
88 
89 		if (code == RECORD_TYPE || code == UNION_TYPE)
90 			if (is_userspace_type(fieldtype))
91 				return true;
92 
93 		if (lookup_attribute("user", DECL_ATTRIBUTES(field)))
94 			return true;
95 	}
96 	return false;
97 }
98 
99 static void finish_type(void *event_data, void *data)
100 {
101 	tree type = (tree)event_data;
102 
103 	if (type == NULL_TREE || type == error_mark_node)
104 		return;
105 
106 	if (TREE_CODE(type) == ENUMERAL_TYPE)
107 		return;
108 
109 	if (TYPE_USERSPACE(type))
110 		return;
111 
112 	if (is_userspace_type(type))
113 		TYPE_USERSPACE(type) = 1;
114 }
115 
116 static void initialize(tree var)
117 {
118 	basic_block bb;
119 	gimple_stmt_iterator gsi;
120 	tree initializer;
121 	gimple init_stmt;
122 	tree type;
123 
124 	/* this is the original entry bb before the forced split */
125 	bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
126 
127 	/* first check if variable is already initialized, warn otherwise */
128 	for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi)) {
129 		gimple stmt = gsi_stmt(gsi);
130 		tree rhs1;
131 
132 		/* we're looking for an assignment of a single rhs... */
133 		if (!gimple_assign_single_p(stmt))
134 			continue;
135 		rhs1 = gimple_assign_rhs1(stmt);
136 		/* ... of a non-clobbering expression... */
137 		if (TREE_CLOBBER_P(rhs1))
138 			continue;
139 		/* ... to our variable... */
140 		if (gimple_get_lhs(stmt) != var)
141 			continue;
142 		/* if it's an initializer then we're good */
143 		if (TREE_CODE(rhs1) == CONSTRUCTOR)
144 			return;
145 	}
146 
147 	/* these aren't the 0days you're looking for */
148 	if (verbose)
149 		inform(DECL_SOURCE_LOCATION(var),
150 			"%s variable will be forcibly initialized",
151 			(byref && TREE_ADDRESSABLE(var)) ? "byref"
152 							 : "userspace");
153 
154 	/* build the initializer expression */
155 	type = TREE_TYPE(var);
156 	if (AGGREGATE_TYPE_P(type))
157 		initializer = build_constructor(type, NULL);
158 	else
159 		initializer = fold_convert(type, integer_zero_node);
160 
161 	/* build the initializer stmt */
162 	init_stmt = gimple_build_assign(var, initializer);
163 	gsi = gsi_after_labels(single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
164 	gsi_insert_before(&gsi, init_stmt, GSI_NEW_STMT);
165 	update_stmt(init_stmt);
166 }
167 
168 static unsigned int structleak_execute(void)
169 {
170 	basic_block bb;
171 	tree var;
172 	unsigned int i;
173 
174 	/* split the first bb where we can put the forced initializers */
175 	gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
176 	bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
177 	if (!single_pred_p(bb)) {
178 		split_edge(single_succ_edge(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
179 		gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
180 	}
181 
182 	/* enumerate all local variables and forcibly initialize our targets */
183 	FOR_EACH_LOCAL_DECL(cfun, i, var) {
184 		tree type = TREE_TYPE(var);
185 
186 		gcc_assert(DECL_P(var));
187 		if (!auto_var_in_fn_p(var, current_function_decl))
188 			continue;
189 
190 		/* only care about structure types unless byref-all */
191 		if (byref != BYREF_ALL && TREE_CODE(type) != RECORD_TYPE && TREE_CODE(type) != UNION_TYPE)
192 			continue;
193 
194 		/* if the type is of interest, examine the variable */
195 		if (TYPE_USERSPACE(type) ||
196 		    (byref && TREE_ADDRESSABLE(var)))
197 			initialize(var);
198 	}
199 
200 	return 0;
201 }
202 
203 #define PASS_NAME structleak
204 #define NO_GATE
205 #define PROPERTIES_REQUIRED PROP_cfg
206 #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
207 #include "gcc-generate-gimple-pass.h"
208 
209 __visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
210 {
211 	int i;
212 	const char * const plugin_name = plugin_info->base_name;
213 	const int argc = plugin_info->argc;
214 	const struct plugin_argument * const argv = plugin_info->argv;
215 	bool enable = true;
216 
217 	PASS_INFO(structleak, "early_optimizations", 1, PASS_POS_INSERT_BEFORE);
218 
219 	if (!plugin_default_version_check(version, &gcc_version)) {
220 		error(G_("incompatible gcc/plugin versions"));
221 		return 1;
222 	}
223 
224 	if (strncmp(lang_hooks.name, "GNU C", 5) && !strncmp(lang_hooks.name, "GNU C+", 6)) {
225 		inform(UNKNOWN_LOCATION, G_("%s supports C only, not %s"), plugin_name, lang_hooks.name);
226 		enable = false;
227 	}
228 
229 	for (i = 0; i < argc; ++i) {
230 		if (!strcmp(argv[i].key, "disable")) {
231 			enable = false;
232 			continue;
233 		}
234 		if (!strcmp(argv[i].key, "verbose")) {
235 			verbose = true;
236 			continue;
237 		}
238 		if (!strcmp(argv[i].key, "byref")) {
239 			byref = BYREF_STRUCT;
240 			continue;
241 		}
242 		if (!strcmp(argv[i].key, "byref-all")) {
243 			byref = BYREF_ALL;
244 			continue;
245 		}
246 		error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
247 	}
248 
249 	register_callback(plugin_name, PLUGIN_INFO, NULL, &structleak_plugin_info);
250 	if (enable) {
251 		register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &structleak_pass_info);
252 		register_callback(plugin_name, PLUGIN_FINISH_TYPE, finish_type, NULL);
253 	}
254 	register_callback(plugin_name, PLUGIN_ATTRIBUTES, register_attributes, NULL);
255 
256 	return 0;
257 }
258