xref: /openbmc/linux/kernel/cfi.c (revision e6e8c6c2) 
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Clang Control Flow Integrity (CFI) error and slowpath handling.
4  *
5  * Copyright (C) 2021 Google LLC
6  */
7 
8 #include <linux/hardirq.h>
9 #include <linux/kallsyms.h>
10 #include <linux/module.h>
11 #include <linux/mutex.h>
12 #include <linux/printk.h>
13 #include <linux/ratelimit.h>
14 #include <linux/rcupdate.h>
15 #include <linux/vmalloc.h>
16 #include <asm/cacheflush.h>
17 #include <asm/set_memory.h>
18 
19 /* Compiler-defined handler names */
20 #ifdef CONFIG_CFI_PERMISSIVE
21 #define cfi_failure_handler	__ubsan_handle_cfi_check_fail
22 #else
23 #define cfi_failure_handler	__ubsan_handle_cfi_check_fail_abort
24 #endif
25 
26 static inline void handle_cfi_failure(void *ptr)
27 {
28 	if (IS_ENABLED(CONFIG_CFI_PERMISSIVE))
29 		WARN_RATELIMIT(1, "CFI failure (target: %pS):\n", ptr);
30 	else
31 		panic("CFI failure (target: %pS)\n", ptr);
32 }
33 
34 #ifdef CONFIG_MODULES
35 #ifdef CONFIG_CFI_CLANG_SHADOW
36 /*
37  * Index type. A 16-bit index can address at most (2^16)-2 pages (taking
38  * into account SHADOW_INVALID), i.e. ~256M with 4k pages.
39  */
40 typedef u16 shadow_t;
41 #define SHADOW_INVALID		((shadow_t)~0UL)
42 
43 struct cfi_shadow {
44 	/* Page index for the beginning of the shadow */
45 	unsigned long base;
46 	/* An array of __cfi_check locations (as indices to the shadow) */
47 	shadow_t shadow[1];
48 } __packed;
49 
50 /*
51  * The shadow covers ~128M from the beginning of the module region. If
52  * the region is larger, we fall back to __module_address for the rest.
53  */
54 #define __SHADOW_RANGE		(_UL(SZ_128M) >> PAGE_SHIFT)
55 
56 /* The in-memory size of struct cfi_shadow, always at least one page */
57 #define __SHADOW_PAGES		((__SHADOW_RANGE * sizeof(shadow_t)) >> PAGE_SHIFT)
58 #define SHADOW_PAGES		max(1UL, __SHADOW_PAGES)
59 #define SHADOW_SIZE		(SHADOW_PAGES << PAGE_SHIFT)
60 
61 /* The actual size of the shadow array, minus metadata */
62 #define SHADOW_ARR_SIZE		(SHADOW_SIZE - offsetof(struct cfi_shadow, shadow))
63 #define SHADOW_ARR_SLOTS	(SHADOW_ARR_SIZE / sizeof(shadow_t))
64 
65 static DEFINE_MUTEX(shadow_update_lock);
66 static struct cfi_shadow __rcu *cfi_shadow __read_mostly;
67 
68 /* Returns the index in the shadow for the given address */
69 static inline int ptr_to_shadow(const struct cfi_shadow *s, unsigned long ptr)
70 {
71 	unsigned long index;
72 	unsigned long page = ptr >> PAGE_SHIFT;
73 
74 	if (unlikely(page < s->base))
75 		return -1; /* Outside of module area */
76 
77 	index = page - s->base;
78 
79 	if (index >= SHADOW_ARR_SLOTS)
80 		return -1; /* Cannot be addressed with shadow */
81 
82 	return (int)index;
83 }
84 
85 /* Returns the page address for an index in the shadow */
86 static inline unsigned long shadow_to_ptr(const struct cfi_shadow *s,
87 	int index)
88 {
89 	if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
90 		return 0;
91 
92 	return (s->base + index) << PAGE_SHIFT;
93 }
94 
95 /* Returns the __cfi_check function address for the given shadow location */
96 static inline unsigned long shadow_to_check_fn(const struct cfi_shadow *s,
97 	int index)
98 {
99 	if (unlikely(index < 0 || index >= SHADOW_ARR_SLOTS))
100 		return 0;
101 
102 	if (unlikely(s->shadow[index] == SHADOW_INVALID))
103 		return 0;
104 
105 	/* __cfi_check is always page aligned */
106 	return (s->base + s->shadow[index]) << PAGE_SHIFT;
107 }
108 
109 static void prepare_next_shadow(const struct cfi_shadow __rcu *prev,
110 		struct cfi_shadow *next)
111 {
112 	int i, index, check;
113 
114 	/* Mark everything invalid */
115 	memset(next->shadow, 0xFF, SHADOW_ARR_SIZE);
116 
117 	if (!prev)
118 		return; /* No previous shadow */
119 
120 	/* If the base address didn't change, an update is not needed */
121 	if (prev->base == next->base) {
122 		memcpy(next->shadow, prev->shadow, SHADOW_ARR_SIZE);
123 		return;
124 	}
125 
126 	/* Convert the previous shadow to the new address range */
127 	for (i = 0; i < SHADOW_ARR_SLOTS; ++i) {
128 		if (prev->shadow[i] == SHADOW_INVALID)
129 			continue;
130 
131 		index = ptr_to_shadow(next, shadow_to_ptr(prev, i));
132 		if (index < 0)
133 			continue;
134 
135 		check = ptr_to_shadow(next,
136 				shadow_to_check_fn(prev, prev->shadow[i]));
137 		if (check < 0)
138 			continue;
139 
140 		next->shadow[index] = (shadow_t)check;
141 	}
142 }
143 
144 static void add_module_to_shadow(struct cfi_shadow *s, struct module *mod,
145 			unsigned long min_addr, unsigned long max_addr)
146 {
147 	int check_index;
148 	unsigned long check = (unsigned long)mod->cfi_check;
149 	unsigned long ptr;
150 
151 	if (unlikely(!PAGE_ALIGNED(check))) {
152 		pr_warn("cfi: not using shadow for module %s\n", mod->name);
153 		return;
154 	}
155 
156 	check_index = ptr_to_shadow(s, check);
157 	if (check_index < 0)
158 		return; /* Module not addressable with shadow */
159 
160 	/* For each page, store the check function index in the shadow */
161 	for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
162 		int index = ptr_to_shadow(s, ptr);
163 
164 		if (index >= 0) {
165 			/* Each page must only contain one module */
166 			WARN_ON_ONCE(s->shadow[index] != SHADOW_INVALID);
167 			s->shadow[index] = (shadow_t)check_index;
168 		}
169 	}
170 }
171 
172 static void remove_module_from_shadow(struct cfi_shadow *s, struct module *mod,
173 		unsigned long min_addr, unsigned long max_addr)
174 {
175 	unsigned long ptr;
176 
177 	for (ptr = min_addr; ptr <= max_addr; ptr += PAGE_SIZE) {
178 		int index = ptr_to_shadow(s, ptr);
179 
180 		if (index >= 0)
181 			s->shadow[index] = SHADOW_INVALID;
182 	}
183 }
184 
185 typedef void (*update_shadow_fn)(struct cfi_shadow *, struct module *,
186 			unsigned long min_addr, unsigned long max_addr);
187 
188 static void update_shadow(struct module *mod, unsigned long base_addr,
189 		update_shadow_fn fn)
190 {
191 	struct cfi_shadow *prev;
192 	struct cfi_shadow *next;
193 	unsigned long min_addr, max_addr;
194 
195 	next = vmalloc(SHADOW_SIZE);
196 
197 	mutex_lock(&shadow_update_lock);
198 	prev = rcu_dereference_protected(cfi_shadow,
199 					 mutex_is_locked(&shadow_update_lock));
200 
201 	if (next) {
202 		next->base = base_addr >> PAGE_SHIFT;
203 		prepare_next_shadow(prev, next);
204 
205 		min_addr = (unsigned long)mod->core_layout.base;
206 		max_addr = min_addr + mod->core_layout.text_size;
207 		fn(next, mod, min_addr & PAGE_MASK, max_addr & PAGE_MASK);
208 
209 		set_memory_ro((unsigned long)next, SHADOW_PAGES);
210 	}
211 
212 	rcu_assign_pointer(cfi_shadow, next);
213 	mutex_unlock(&shadow_update_lock);
214 	synchronize_rcu();
215 
216 	if (prev) {
217 		set_memory_rw((unsigned long)prev, SHADOW_PAGES);
218 		vfree(prev);
219 	}
220 }
221 
222 void cfi_module_add(struct module *mod, unsigned long base_addr)
223 {
224 	update_shadow(mod, base_addr, add_module_to_shadow);
225 }
226 
227 void cfi_module_remove(struct module *mod, unsigned long base_addr)
228 {
229 	update_shadow(mod, base_addr, remove_module_from_shadow);
230 }
231 
232 static inline cfi_check_fn ptr_to_check_fn(const struct cfi_shadow __rcu *s,
233 	unsigned long ptr)
234 {
235 	int index;
236 
237 	if (unlikely(!s))
238 		return NULL; /* No shadow available */
239 
240 	index = ptr_to_shadow(s, ptr);
241 	if (index < 0)
242 		return NULL; /* Cannot be addressed with shadow */
243 
244 	return (cfi_check_fn)shadow_to_check_fn(s, index);
245 }
246 
247 static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
248 {
249 	cfi_check_fn fn;
250 
251 	rcu_read_lock_sched_notrace();
252 	fn = ptr_to_check_fn(rcu_dereference_sched(cfi_shadow), ptr);
253 	rcu_read_unlock_sched_notrace();
254 
255 	return fn;
256 }
257 
258 #else /* !CONFIG_CFI_CLANG_SHADOW */
259 
260 static inline cfi_check_fn find_shadow_check_fn(unsigned long ptr)
261 {
262 	return NULL;
263 }
264 
265 #endif /* CONFIG_CFI_CLANG_SHADOW */
266 
267 static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
268 {
269 	cfi_check_fn fn = NULL;
270 	struct module *mod;
271 
272 	rcu_read_lock_sched_notrace();
273 	mod = __module_address(ptr);
274 	if (mod)
275 		fn = mod->cfi_check;
276 	rcu_read_unlock_sched_notrace();
277 
278 	return fn;
279 }
280 
281 static inline cfi_check_fn find_check_fn(unsigned long ptr)
282 {
283 	cfi_check_fn fn = NULL;
284 	unsigned long flags;
285 	bool rcu_idle;
286 
287 	if (is_kernel_text(ptr))
288 		return __cfi_check;
289 
290 	/*
291 	 * Indirect call checks can happen when RCU is not watching. Both
292 	 * the shadow and __module_address use RCU, so we need to wake it
293 	 * up if necessary.
294 	 */
295 	rcu_idle = !rcu_is_watching();
296 	if (rcu_idle) {
297 		local_irq_save(flags);
298 		ct_irq_enter();
299 	}
300 
301 	if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
302 		fn = find_shadow_check_fn(ptr);
303 	if (!fn)
304 		fn = find_module_check_fn(ptr);
305 
306 	if (rcu_idle) {
307 		ct_irq_exit();
308 		local_irq_restore(flags);
309 	}
310 
311 	return fn;
312 }
313 
314 void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
315 {
316 	cfi_check_fn fn = find_check_fn((unsigned long)ptr);
317 
318 	if (likely(fn))
319 		fn(id, ptr, diag);
320 	else /* Don't allow unchecked modules */
321 		handle_cfi_failure(ptr);
322 }
323 EXPORT_SYMBOL(__cfi_slowpath_diag);
324 
325 #else /* !CONFIG_MODULES */
326 
327 void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
328 {
329 	handle_cfi_failure(ptr); /* No modules */
330 }
331 EXPORT_SYMBOL(__cfi_slowpath_diag);
332 
333 #endif /* CONFIG_MODULES */
334 
335 void cfi_failure_handler(void *data, void *ptr, void *vtable)
336 {
337 	handle_cfi_failure(ptr);
338 }
339 EXPORT_SYMBOL(cfi_failure_handler);
340