aboutsummaryrefslogtreecommitdiff
path: root/kernel/cfi.c
blob: 9594cfd1cf2cf7ea7be1a44ad048d3b25b624acc (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
// SPDX-License-Identifier: GPL-2.0
/*
 * Clang Control Flow Integrity (CFI) error and slowpath handling.
 *
 * Copyright (C) 2021 Google LLC
 */

#include <linux/hardirq.h>
#include <linux/kallsyms.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/printk.h>
#include <linux/ratelimit.h>
#include <linux/rcupdate.h>
#include <linux/vmalloc.h>
#include <asm/cacheflush.h>
#include <asm/set_memory.h>

/* Compiler-defined handler names */
#ifdef CONFIG_CFI_PERMISSIVE
#define cfi_failure_handler	__ubsan_handle_cfi_check_fail
#else
#define cfi_failure_handler	__ubsan_handle_cfi_check_fail_abort
#endif

static inline void handle_cfi_failure(void *ptr)
{
	if (IS_ENABLED(CONFIG_CFI_PERMISSIVE))
		WARN_RATELIMIT(1, "CFI failure (target: %pS):\n", ptr);
	else
		panic("CFI failure (target: %pS)\n", ptr);
}

#ifdef CONFIG_MODULES
#ifdef CONFIG_CFI_CLANG_SHADOW
/*
 * Index type. A 16-bit index can address at most (2^16)-2 pages (taking
 * into account SHADOW_INVALID), i.e. ~256M with 4k pages.
 */
typedef u16 shadow_t;
#define SHADOW_INVALID		((shadow_t)~0UL)

struct cfi_shadow {
	/* Page index for the beginning of the shadow */
	unsigned long base;
	/* An array of __cfi_check locations (as indices to the shadow) */
	shadow_t shadow[1];
} __packed;

/*
 * The shadow covers ~128M from the beginning of the module region. If
 * the region is larger, we fall back to __module_address for the rest.
 */
#define __SHADOW_RANGE		(_UL(SZ_128M) >> PAGE_SHIFT)

/* The in-memory size of struct cfi_shadow, always at least one page */
#define __SHADOW_PAGES		((__SHADOW_RANGE * sizeof(shadow_t)) >> PAGE_SHIFT)
#define SHADOW_PAGES		max(1UL, __SHADOW_PAGES)
#define SHADOW_SIZE		(SHADOW_PAGES << PAGE_SHIFT)

/* The actual size of the shadow array, minus metadata */
#define SHADOW_ARR_SIZE		(SHADOW_SIZE - offsetof(struct cfi_shadow, shadow))
#define SHADOW_ARR_SLOTS	(SHADOW_ARR_SIZE / sizeof(shadow_t))

static DEFINE_MUTEX(shadow_update_lock);
static struct cfi_shadow __rcu *cfi_shadow __read_mostly;

/* Returns the index in the shadow for the given address */
static inline int ptr_to_shadow(const struct cfi_shadow *s, unsigned long ptr)
{
	unsigned long index;
	unsigned long page = ptr >> PAGE_SHIFT;

	if (unlikely(page < s->base))
		return -1; /* Outside of module area */

	index = page - s->base;

	if (index >= SHADOW_ARR_SLOTS)
		return -1; /* Cannot be addressed with shadow */

	return (int)index;
}

/* Returns the page address for an index in the shadow */
static inline unsigned long shadow_to_ptr(const struct cfi_shadow *s,
	int index)
{
	if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
		return 0;

	return (s->base + index) << PAGE_SHIFT;
}

/* Returns the __cfi_check function address for the given shadow location */
static inline unsigned long shadow_to_check_fn(const struct cfi_shadow *s,
	int index)
{
	if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
		return 0;

	if (unlikely(s->shadow[index] == SHADOW_INVALID))
		return 0;

	/* __cfi_check is always page aligned */
	return (s->base + s->shadow[index]) << PAGE_SHIFT;
}

static void prepare_next_shadow(const struct cfi_shadow __rcu *prev,
		struct cfi_shadow *next)
{
	int i, index, check;

	/* Mark everything invalid */
	memset(next->shadow, 0xFF, SHADOW_ARR_SIZE);

	if (!prev)
		return; /* No previous shadow */

	/* If the base address didn't change, an update is not needed */
	if (prev->base == next->base) {
		memcpy(next->shadow, prev->shadow, SHADOW_ARR_SIZE);
		return;
	}

	/* Convert the previous shadow to the new address range */
	for (i = 0; i < SHADOW_ARR_SLOTS; ++i) {
		if (prev->shadow[i] == SHADOW_INVALID)
			continue;

		index = ptr_to_shadow(next, shadow_to_ptr(prev, i));
		if (index < 0)
			continue;

		check = ptr_to_shadow(next,
				shadow_to_check_fn(prev, prev->shadow[i]));
		if (check < 0)
			continue;

		next->shadow[index] = (shadow_t)check;
	}
}

static void add_module_to_shadow(struct cfi_shadow *s, struct module *mod,
			unsigned long min_addr, unsigned long max_addr)
{
	int check_index;
	unsigned long check = (unsigned long)mod->cfi_check;
	unsigned long ptr;

	if (unlikely(!PAGE_ALIGNED(check))) {
		pr_warn("cfi: not using shadow for module %s\n", mod->name);
		return;
	}

	check_index = ptr_to_shadow(s, check);
	if (check_index < 0)
		return; /* Module not addressable with shadow */

	/* For each page, store the check function index in the shadow */
	for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
		int index = ptr_to_shadow(s, ptr);

		if (index >= 0) {
			/* Each page must only contain one module */
			WARN_ON_ONCE(s->shadow[index] != SHADOW_INVALID);
			s->shadow[index] = (shadow_t)check_index;
		}
	}
}

static void remove_module_from_shadow(struct cfi_shadow *s, struct module *mod,
		unsigned long min_addr, unsigned long max_addr)
{
	unsigned long ptr;

	for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
		int index = ptr_to_shadow(s, ptr);

		if (index >= 0)
			s->shadow[index] = SHADOW_INVALID;
	}
}

typedef void (*update_shadow_fn)(struct cfi_shadow *, struct module *,
			unsigned long min_addr, unsigned long max_addr);

static void update_shadow(struct module *mod, unsigned long base_addr,
		update_shadow_fn fn)
{
	struct cfi_shadow *prev;
	struct cfi_shadow *next;
	unsigned long min_addr, max_addr;

	next = vmalloc(SHADOW_SIZE);

	mutex_lock(&shadow_update_lock);
	prev = rcu_dereference_protected(cfi_shadow,
					 mutex_is_locked(&shadow_update_lock));

	if (next) {
		next->base = base_addr >> PAGE_SHIFT;
		prepare_next_shadow(prev, next);

		min_addr = (unsigned long)mod->core_layout.base;
		max_addr = min_addr + mod->core_layout.text_size;
		fn(next, mod, min_addr & PAGE_MASK, max_addr & PAGE_MASK);

		set_memory_ro((unsigned long)next, SHADOW_PAGES);
	}

	rcu_assign_pointer(cfi_shadow, next);
	mutex_unlock(&shadow_update_lock);
	synchronize_rcu();

	if (prev) {
		set_memory_rw((unsigned long)prev, SHADOW_PAGES);
		vfree(prev);
	}
}

void cfi_module_add(struct module *mod, unsigned long base_addr)
{
	update_shadow(mod, base_addr, add_module_to_shadow);
}

void cfi_module_remove(struct module *mod, unsigned long base_addr)
{
	update_shadow(mod, base_addr, remove_module_from_shadow);
}

static inline cfi_check_fn ptr_to_check_fn(const struct cfi_shadow __rcu *s,
	unsigned long ptr)
{
	int index;

	if (unlikely(!s))
		return NULL; /* No shadow available */

	index = ptr_to_shadow(s, ptr);
	if (index < 0)
		return NULL; /* Cannot be addressed with shadow */

	return (cfi_check_fn)shadow_to_check_fn(s, index);
}

static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
{
	cfi_check_fn fn;

	rcu_read_lock_sched_notrace();
	fn = ptr_to_check_fn(rcu_dereference_sched(cfi_shadow), ptr);
	rcu_read_unlock_sched_notrace();

	return fn;
}

#else /* !CONFIG_CFI_CLANG_SHADOW */

static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
{
	return NULL;
}

#endif /* CONFIG_CFI_CLANG_SHADOW */

static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
{
	cfi_check_fn fn = NULL;
	struct module *mod;

	rcu_read_lock_sched_notrace();
	mod = __module_address(ptr);
	if (mod)
		fn = mod->cfi_check;
	rcu_read_unlock_sched_notrace();

	return fn;
}

static inline cfi_check_fn find_check_fn(unsigned long ptr)
{
	cfi_check_fn fn = NULL;

	if (is_kernel_text(ptr))
		return __cfi_check;

	/*
	 * Indirect call checks can happen when RCU is not watching. Both
	 * the shadow and __module_address use RCU, so we need to wake it
	 * up if necessary.
	 */
	RCU_NONIDLE({
		if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
			fn = find_shadow_check_fn(ptr);

		if (!fn)
			fn = find_module_check_fn(ptr);
	});

	return fn;
}

void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
{
	cfi_check_fn fn = find_check_fn((unsigned long)ptr);

	if (likely(fn))
		fn(id, ptr, diag);
	else /* Don't allow unchecked modules */
		handle_cfi_failure(ptr);
}
EXPORT_SYMBOL(__cfi_slowpath_diag);

#else /* !CONFIG_MODULES */

void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
{
	handle_cfi_failure(ptr); /* No modules */
}
EXPORT_SYMBOL(__cfi_slowpath_diag);

#endif /* CONFIG_MODULES */

void cfi_failure_handler(void *data, void *ptr, void *vtable)
{
	handle_cfi_failure(ptr);
}
EXPORT_SYMBOL(cfi_failure_handler);