drivers/misc/lkdtm_usercopy.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * This is for all the tests related to copy_to_user() and copy_from_user()
   4  * hardening.
   5  */
   6 #include "lkdtm.h"
   7 #include <linux/slab.h>
   8 #include <linux/vmalloc.h>
   9 #include <linux/sched/task_stack.h>
  10 #include <linux/mman.h>
  11 #include <linux/uaccess.h>
  12 #include <asm/cacheflush.h>
  13
  14 /*
  15  * Many of the tests here end up using const sizes, but those would
  16  * normally be ignored by hardened usercopy, so force the compiler
  17  * into choosing the non-const path to make sure we trigger the
  18  * hardened usercopy checks by added "unconst" to all the const copies,
  19  * and making sure "cache_size" isn't optimized into a const.
  20  */
  21 static volatile size_t unconst = 0;
  22 static volatile size_t cache_size = 1024;
  23 static struct kmem_cache *bad_cache;
  24
  25 static const unsigned char test_text[] = "This is a test.\n";
  26
  27 /*
  28  * Instead of adding -Wno-return-local-addr, just pass the stack address
  29  * through a function to obfuscate it from the compiler.
  30  */
  31 static noinline unsigned char *trick_compiler(unsigned char *stack)
  32 {
  33         return stack + 0;
  34 }
  35
  36 static noinline unsigned char *do_usercopy_stack_callee(int value)
  37 {
  38         unsigned char buf[32];
  39         int i;
  40
  41         /* Exercise stack to avoid everything living in registers. */
  42         for (i = 0; i < sizeof(buf); i++) {
  43                 buf[i] = value & 0xff;
  44         }
  45
  46         return trick_compiler(buf);
  47 }
  48
  49 static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
  50 {
  51         unsigned long user_addr;
  52         unsigned char good_stack[32];
  53         unsigned char *bad_stack;
  54         int i;
  55
  56         /* Exercise stack to avoid everything living in registers. */
  57         for (i = 0; i < sizeof(good_stack); i++)
  58                 good_stack[i] = test_text[i % sizeof(test_text)];
  59
  60         /* This is a pointer to outside our current stack frame. */
  61         if (bad_frame) {
  62                 bad_stack = do_usercopy_stack_callee((uintptr_t)&bad_stack);
  63         } else {
  64                 /* Put start address just inside stack. */
  65                 bad_stack = task_stack_page(current) + THREAD_SIZE;
  66                 bad_stack -= sizeof(unsigned long);
  67         }
  68
  69         user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
  70                             PROT_READ | PROT_WRITE | PROT_EXEC,
  71                             MAP_ANONYMOUS | MAP_PRIVATE, 0);
  72         if (user_addr >= TASK_SIZE) {
  73                 pr_warn("Failed to allocate user memory\n");
  74                 return;
  75         }
  76
  77         if (to_user) {
  78                 pr_info("attempting good copy_to_user of local stack\n");
  79                 if (copy_to_user((void __user *)user_addr, good_stack,
  80                                  unconst + sizeof(good_stack))) {
  81                         pr_warn("copy_to_user failed unexpectedly?!\n");
  82                         goto free_user;
  83                 }
  84
  85                 pr_info("attempting bad copy_to_user of distant stack\n");
  86                 if (copy_to_user((void __user *)user_addr, bad_stack,
  87                                  unconst + sizeof(good_stack))) {
  88                         pr_warn("copy_to_user failed, but lacked Oops\n");
  89                         goto free_user;
  90                 }
  91         } else {
  92                 /*
  93                  * There isn't a safe way to not be protected by usercopy
  94                  * if we're going to write to another thread's stack.
  95                  */
  96                 if (!bad_frame)
  97                         goto free_user;
  98
  99                 pr_info("attempting good copy_from_user of local stack\n");
 100                 if (copy_from_user(good_stack, (void __user *)user_addr,
 101                                    unconst + sizeof(good_stack))) {
 102                         pr_warn("copy_from_user failed unexpectedly?!\n");
 103                         goto free_user;
 104                 }
 105
 106                 pr_info("attempting bad copy_from_user of distant stack\n");
 107                 if (copy_from_user(bad_stack, (void __user *)user_addr,
 108                                    unconst + sizeof(good_stack))) {
 109                         pr_warn("copy_from_user failed, but lacked Oops\n");
 110                         goto free_user;
 111                 }
 112         }
 113
 114 free_user:
 115         vm_munmap(user_addr, PAGE_SIZE);
 116 }
 117
 118 static void do_usercopy_heap_size(bool to_user)
 119 {
 120         unsigned long user_addr;
 121         unsigned char *one, *two;
 122         size_t size = unconst + 1024;
 123
 124         one = kmalloc(size, GFP_KERNEL);
 125         two = kmalloc(size, GFP_KERNEL);
 126         if (!one || !two) {
 127                 pr_warn("Failed to allocate kernel memory\n");
 128                 goto free_kernel;
 129         }
 130
 131         user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
 132                             PROT_READ | PROT_WRITE | PROT_EXEC,
 133                             MAP_ANONYMOUS | MAP_PRIVATE, 0);
 134         if (user_addr >= TASK_SIZE) {
 135                 pr_warn("Failed to allocate user memory\n");
 136                 goto free_kernel;
 137         }
 138
 139         memset(one, 'A', size);
 140         memset(two, 'B', size);
 141
 142         if (to_user) {
 143                 pr_info("attempting good copy_to_user of correct size\n");
 144                 if (copy_to_user((void __user *)user_addr, one, size)) {
 145                         pr_warn("copy_to_user failed unexpectedly?!\n");
 146                         goto free_user;
 147                 }
 148
 149                 pr_info("attempting bad copy_to_user of too large size\n");
 150                 if (copy_to_user((void __user *)user_addr, one, 2 * size)) {
 151                         pr_warn("copy_to_user failed, but lacked Oops\n");
 152                         goto free_user;
 153                 }
 154         } else {
 155                 pr_info("attempting good copy_from_user of correct size\n");
 156                 if (copy_from_user(one, (void __user *)user_addr, size)) {
 157                         pr_warn("copy_from_user failed unexpectedly?!\n");
 158                         goto free_user;
 159                 }
 160
 161                 pr_info("attempting bad copy_from_user of too large size\n");
 162                 if (copy_from_user(one, (void __user *)user_addr, 2 * size)) {
 163                         pr_warn("copy_from_user failed, but lacked Oops\n");
 164                         goto free_user;
 165                 }
 166         }
 167
 168 free_user:
 169         vm_munmap(user_addr, PAGE_SIZE);
 170 free_kernel:
 171         kfree(one);
 172         kfree(two);
 173 }
 174
 175 static void do_usercopy_heap_flag(bool to_user)
 176 {
 177         unsigned long user_addr;
 178         unsigned char *good_buf = NULL;
 179         unsigned char *bad_buf = NULL;
 180
 181         /* Make sure cache was prepared. */
 182         if (!bad_cache) {
 183                 pr_warn("Failed to allocate kernel cache\n");
 184                 return;
 185         }
 186
 187         /*
 188          * Allocate one buffer from each cache (kmalloc will have the
 189          * SLAB_USERCOPY flag already, but "bad_cache" won't).
 190          */
 191         good_buf = kmalloc(cache_size, GFP_KERNEL);
 192         bad_buf = kmem_cache_alloc(bad_cache, GFP_KERNEL);
 193         if (!good_buf || !bad_buf) {
 194                 pr_warn("Failed to allocate buffers from caches\n");
 195                 goto free_alloc;
 196         }
 197
 198         /* Allocate user memory we'll poke at. */
 199         user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
 200                             PROT_READ | PROT_WRITE | PROT_EXEC,
 201                             MAP_ANONYMOUS | MAP_PRIVATE, 0);
 202         if (user_addr >= TASK_SIZE) {
 203                 pr_warn("Failed to allocate user memory\n");
 204                 goto free_alloc;
 205         }
 206
 207         memset(good_buf, 'A', cache_size);
 208         memset(bad_buf, 'B', cache_size);
 209
 210         if (to_user) {
 211                 pr_info("attempting good copy_to_user with SLAB_USERCOPY\n");
 212                 if (copy_to_user((void __user *)user_addr, good_buf,
 213                                  cache_size)) {
 214                         pr_warn("copy_to_user failed unexpectedly?!\n");
 215                         goto free_user;
 216                 }
 217
 218                 pr_info("attempting bad copy_to_user w/o SLAB_USERCOPY\n");
 219                 if (copy_to_user((void __user *)user_addr, bad_buf,
 220                                  cache_size)) {
 221                         pr_warn("copy_to_user failed, but lacked Oops\n");
 222                         goto free_user;
 223                 }
 224         } else {
 225                 pr_info("attempting good copy_from_user with SLAB_USERCOPY\n");
 226                 if (copy_from_user(good_buf, (void __user *)user_addr,
 227                                    cache_size)) {
 228                         pr_warn("copy_from_user failed unexpectedly?!\n");
 229                         goto free_user;
 230                 }
 231
 232                 pr_info("attempting bad copy_from_user w/o SLAB_USERCOPY\n");
 233                 if (copy_from_user(bad_buf, (void __user *)user_addr,
 234                                    cache_size)) {
 235                         pr_warn("copy_from_user failed, but lacked Oops\n");
 236                         goto free_user;
 237                 }
 238         }
 239
 240 free_user:
 241         vm_munmap(user_addr, PAGE_SIZE);
 242 free_alloc:
 243         if (bad_buf)
 244                 kmem_cache_free(bad_cache, bad_buf);
 245         kfree(good_buf);
 246 }
 247
 248 /* Callable tests. */
 249 void lkdtm_USERCOPY_HEAP_SIZE_TO(void)
 250 {
 251         do_usercopy_heap_size(true);
 252 }
 253
 254 void lkdtm_USERCOPY_HEAP_SIZE_FROM(void)
 255 {
 256         do_usercopy_heap_size(false);
 257 }
 258
 259 void lkdtm_USERCOPY_HEAP_FLAG_TO(void)
 260 {
 261         do_usercopy_heap_flag(true);
 262 }
 263
 264 void lkdtm_USERCOPY_HEAP_FLAG_FROM(void)
 265 {
 266         do_usercopy_heap_flag(false);
 267 }
 268
 269 void lkdtm_USERCOPY_STACK_FRAME_TO(void)
 270 {
 271         do_usercopy_stack(true, true);
 272 }
 273
 274 void lkdtm_USERCOPY_STACK_FRAME_FROM(void)
 275 {
 276         do_usercopy_stack(false, true);
 277 }
 278
 279 void lkdtm_USERCOPY_STACK_BEYOND(void)
 280 {
 281         do_usercopy_stack(true, false);
 282 }
 283
 284 void lkdtm_USERCOPY_KERNEL(void)
 285 {
 286         unsigned long user_addr;
 287
 288         user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
 289                             PROT_READ | PROT_WRITE | PROT_EXEC,
 290                             MAP_ANONYMOUS | MAP_PRIVATE, 0);
 291         if (user_addr >= TASK_SIZE) {
 292                 pr_warn("Failed to allocate user memory\n");
 293                 return;
 294         }
 295
 296         pr_info("attempting good copy_to_user from kernel rodata\n");
 297         if (copy_to_user((void __user *)user_addr, test_text,
 298                          unconst + sizeof(test_text))) {
 299                 pr_warn("copy_to_user failed unexpectedly?!\n");
 300                 goto free_user;
 301         }
 302
 303         pr_info("attempting bad copy_to_user from kernel text\n");
 304         if (copy_to_user((void __user *)user_addr, vm_mmap,
 305                          unconst + PAGE_SIZE)) {
 306                 pr_warn("copy_to_user failed, but lacked Oops\n");
 307                 goto free_user;
 308         }
 309
 310 free_user:
 311         vm_munmap(user_addr, PAGE_SIZE);
 312 }
 313
 314 void __init lkdtm_usercopy_init(void)
 315 {
 316         /* Prepare cache that lacks SLAB_USERCOPY flag. */
 317         bad_cache = kmem_cache_create("lkdtm-no-usercopy", cache_size, 0,
 318                                       0, NULL);
 319 }
 320
 321 void __exit lkdtm_usercopy_exit(void)
 322 {
 323         kmem_cache_destroy(bad_cache);
 324 }