kernel/cfi.c

   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
 285         if (is_kernel_text(ptr))
 286                 return __cfi_check;
 287
 288         /*
 289          * Indirect call checks can happen when RCU is not watching. Both
 290          * the shadow and __module_address use RCU, so we need to wake it
 291          * up if necessary.
 292          */
 293         RCU_NONIDLE({
 294                 if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
 295                         fn = find_shadow_check_fn(ptr);
 296
 297                 if (!fn)
 298                         fn = find_module_check_fn(ptr);
 299         });
 300
 301         return fn;
 302 }
 303
 304 void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
 305 {
 306         cfi_check_fn fn = find_check_fn((unsigned long)ptr);
 307
 308         if (likely(fn))
 309                 fn(id, ptr, diag);
 310         else /* Don't allow unchecked modules */
 311                 handle_cfi_failure(ptr);
 312 }
 313 EXPORT_SYMBOL(__cfi_slowpath_diag);
 314
 315 #else /* !CONFIG_MODULES */
 316
 317 void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
 318 {
 319         handle_cfi_failure(ptr); /* No modules */
 320 }
 321 EXPORT_SYMBOL(__cfi_slowpath_diag);
 322
 323 #endif /* CONFIG_MODULES */
 324
 325 void cfi_failure_handler(void *data, void *ptr, void *vtable)
 326 {
 327         handle_cfi_failure(ptr);
 328 }
 329 EXPORT_SYMBOL(cfi_failure_handler);