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	= "20190125vanilla",
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 BUILDING_GCC_VERSION >= 5000
107 	if (TREE_CODE(type) == ENUMERAL_TYPE)
108 		return;
109 #endif
110 
111 	if (TYPE_USERSPACE(type))
112 		return;
113 
114 	if (is_userspace_type(type))
115 		TYPE_USERSPACE(type) = 1;
116 }
117 
118 static void initialize(tree var)
119 {
120 	basic_block bb;
121 	gimple_stmt_iterator gsi;
122 	tree initializer;
123 	gimple init_stmt;
124 	tree type;
125 
126 	/* this is the original entry bb before the forced split */
127 	bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
128 
129 	/* first check if variable is already initialized, warn otherwise */
130 	for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi)) {
131 		gimple stmt = gsi_stmt(gsi);
132 		tree rhs1;
133 
134 		/* we're looking for an assignment of a single rhs... */
135 		if (!gimple_assign_single_p(stmt))
136 			continue;
137 		rhs1 = gimple_assign_rhs1(stmt);
138 		/* ... of a non-clobbering expression... */
139 		if (TREE_CLOBBER_P(rhs1))
140 			continue;
141 		/* ... to our variable... */
142 		if (gimple_get_lhs(stmt) != var)
143 			continue;
144 		/* if it's an initializer then we're good */
145 		if (TREE_CODE(rhs1) == CONSTRUCTOR)
146 			return;
147 	}
148 
149 	/* these aren't the 0days you're looking for */
150 	if (verbose)
151 		inform(DECL_SOURCE_LOCATION(var),
152 			"%s variable will be forcibly initialized",
153 			(byref && TREE_ADDRESSABLE(var)) ? "byref"
154 							 : "userspace");
155 
156 	/* build the initializer expression */
157 	type = TREE_TYPE(var);
158 	if (AGGREGATE_TYPE_P(type))
159 		initializer = build_constructor(type, NULL);
160 	else
161 		initializer = fold_convert(type, integer_zero_node);
162 
163 	/* build the initializer stmt */
164 	init_stmt = gimple_build_assign(var, initializer);
165 	gsi = gsi_after_labels(single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
166 	gsi_insert_before(&gsi, init_stmt, GSI_NEW_STMT);
167 	update_stmt(init_stmt);
168 }
169 
170 static unsigned int structleak_execute(void)
171 {
172 	basic_block bb;
173 	tree var;
174 	unsigned int i;
175 
176 	/* split the first bb where we can put the forced initializers */
177 	gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
178 	bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
179 	if (!single_pred_p(bb)) {
180 		split_edge(single_succ_edge(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
181 		gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
182 	}
183 
184 	/* enumerate all local variables and forcibly initialize our targets */
185 	FOR_EACH_LOCAL_DECL(cfun, i, var) {
186 		tree type = TREE_TYPE(var);
187 
188 		gcc_assert(DECL_P(var));
189 		if (!auto_var_in_fn_p(var, current_function_decl))
190 			continue;
191 
192 		/* only care about structure types unless byref-all */
193 		if (byref != BYREF_ALL && TREE_CODE(type) != RECORD_TYPE && TREE_CODE(type) != UNION_TYPE)
194 			continue;
195 
196 		/* if the type is of interest, examine the variable */
197 		if (TYPE_USERSPACE(type) ||
198 		    (byref && TREE_ADDRESSABLE(var)))
199 			initialize(var);
200 	}
201 
202 	return 0;
203 }
204 
205 #define PASS_NAME structleak
206 #define NO_GATE
207 #define PROPERTIES_REQUIRED PROP_cfg
208 #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
209 #include "gcc-generate-gimple-pass.h"
210 
211 __visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
212 {
213 	int i;
214 	const char * const plugin_name = plugin_info->base_name;
215 	const int argc = plugin_info->argc;
216 	const struct plugin_argument * const argv = plugin_info->argv;
217 	bool enable = true;
218 
219 	PASS_INFO(structleak, "early_optimizations", 1, PASS_POS_INSERT_BEFORE);
220 
221 	if (!plugin_default_version_check(version, &gcc_version)) {
222 		error(G_("incompatible gcc/plugin versions"));
223 		return 1;
224 	}
225 
226 	if (strncmp(lang_hooks.name, "GNU C", 5) && !strncmp(lang_hooks.name, "GNU C+", 6)) {
227 		inform(UNKNOWN_LOCATION, G_("%s supports C only, not %s"), plugin_name, lang_hooks.name);
228 		enable = false;
229 	}
230 
231 	for (i = 0; i < argc; ++i) {
232 		if (!strcmp(argv[i].key, "disable")) {
233 			enable = false;
234 			continue;
235 		}
236 		if (!strcmp(argv[i].key, "verbose")) {
237 			verbose = true;
238 			continue;
239 		}
240 		if (!strcmp(argv[i].key, "byref")) {
241 			byref = BYREF_STRUCT;
242 			continue;
243 		}
244 		if (!strcmp(argv[i].key, "byref-all")) {
245 			byref = BYREF_ALL;
246 			continue;
247 		}
248 		error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
249 	}
250 
251 	register_callback(plugin_name, PLUGIN_INFO, NULL, &structleak_plugin_info);
252 	if (enable) {
253 		register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &structleak_pass_info);
254 		register_callback(plugin_name, PLUGIN_FINISH_TYPE, finish_type, NULL);
255 	}
256 	register_callback(plugin_name, PLUGIN_ATTRIBUTES, register_attributes, NULL);
257 
258 	return 0;
259 }
260