security/loadpin/loadpin.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Module and Firmware Pinning Security Module
   4  *
   5  * Copyright 2011-2016 Google Inc.
   6  *
   7  * Author: Kees Cook <keescook@chromium.org>
   8  */
   9
  10 #define pr_fmt(fmt) "LoadPin: " fmt
  11
  12 #include <linux/module.h>
  13 #include <linux/fs.h>
  14 #include <linux/kernel_read_file.h>
  15 #include <linux/lsm_hooks.h>
  16 #include <linux/mount.h>
  17 #include <linux/blkdev.h>
  18 #include <linux/path.h>
  19 #include <linux/sched.h>        /* current */
  20 #include <linux/string_helpers.h>
  21 #include <linux/dm-verity-loadpin.h>
  22 #include <uapi/linux/loadpin.h>
  23
  24 #define VERITY_DIGEST_FILE_HEADER "# LOADPIN_TRUSTED_VERITY_ROOT_DIGESTS"
  25
  26 static void report_load(const char *origin, struct file *file, char *operation)
  27 {
  28         char *cmdline, *pathname;
  29
  30         pathname = kstrdup_quotable_file(file, GFP_KERNEL);
  31         cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
  32
  33         pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
  34                   origin, operation,
  35                   (pathname && pathname[0] != '<') ? "\"" : "",
  36                   pathname,
  37                   (pathname && pathname[0] != '<') ? "\"" : "",
  38                   task_pid_nr(current),
  39                   cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
  40
  41         kfree(cmdline);
  42         kfree(pathname);
  43 }
  44
  45 static int enforce = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENFORCE);
  46 static char *exclude_read_files[READING_MAX_ID];
  47 static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
  48 static struct super_block *pinned_root;
  49 static DEFINE_SPINLOCK(pinned_root_spinlock);
  50 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
  51 static bool deny_reading_verity_digests;
  52 #endif
  53
  54 #ifdef CONFIG_SYSCTL
  55 static struct ctl_table loadpin_sysctl_table[] = {
  56         {
  57                 .procname       = "enforce",
  58                 .data           = &enforce,
  59                 .maxlen         = sizeof(int),
  60                 .mode           = 0644,
  61                 .proc_handler   = proc_dointvec_minmax,
  62                 .extra1         = SYSCTL_ONE,
  63                 .extra2         = SYSCTL_ONE,
  64         },
  65         { }
  66 };
  67
  68 static void set_sysctl(bool is_writable)
  69 {
  70         /*
  71          * If load pinning is not enforced via a read-only block
  72          * device, allow sysctl to change modes for testing.
  73          */
  74         if (is_writable)
  75                 loadpin_sysctl_table[0].extra1 = SYSCTL_ZERO;
  76         else
  77                 loadpin_sysctl_table[0].extra1 = SYSCTL_ONE;
  78 }
  79 #else
  80 static inline void set_sysctl(bool is_writable) { }
  81 #endif
  82
  83 static void report_writable(struct super_block *mnt_sb, bool writable)
  84 {
  85         if (mnt_sb->s_bdev) {
  86                 pr_info("%pg (%u:%u): %s\n", mnt_sb->s_bdev,
  87                         MAJOR(mnt_sb->s_bdev->bd_dev),
  88                         MINOR(mnt_sb->s_bdev->bd_dev),
  89                         writable ? "writable" : "read-only");
  90         } else
  91                 pr_info("mnt_sb lacks block device, treating as: writable\n");
  92
  93         if (!writable)
  94                 pr_info("load pinning engaged.\n");
  95 }
  96
  97 /*
  98  * This must be called after early kernel init, since then the rootdev
  99  * is available.
 100  */
 101 static bool sb_is_writable(struct super_block *mnt_sb)
 102 {
 103         bool writable = true;
 104
 105         if (mnt_sb->s_bdev)
 106                 writable = !bdev_read_only(mnt_sb->s_bdev);
 107
 108         return writable;
 109 }
 110
 111 static void loadpin_sb_free_security(struct super_block *mnt_sb)
 112 {
 113         /*
 114          * When unmounting the filesystem we were using for load
 115          * pinning, we acknowledge the superblock release, but make sure
 116          * no other modules or firmware can be loaded when we are in
 117          * enforcing mode. Otherwise, allow the root to be reestablished.
 118          */
 119         if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
 120                 if (enforce) {
 121                         pinned_root = ERR_PTR(-EIO);
 122                         pr_info("umount pinned fs: refusing further loads\n");
 123                 } else {
 124                         pinned_root = NULL;
 125                 }
 126         }
 127 }
 128
 129 static int loadpin_check(struct file *file, enum kernel_read_file_id id)
 130 {
 131         struct super_block *load_root;
 132         const char *origin = kernel_read_file_id_str(id);
 133         bool first_root_pin = false;
 134         bool load_root_writable;
 135
 136         /* If the file id is excluded, ignore the pinning. */
 137         if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
 138             ignore_read_file_id[id]) {
 139                 report_load(origin, file, "pinning-excluded");
 140                 return 0;
 141         }
 142
 143         /* This handles the older init_module API that has a NULL file. */
 144         if (!file) {
 145                 if (!enforce) {
 146                         report_load(origin, NULL, "old-api-pinning-ignored");
 147                         return 0;
 148                 }
 149
 150                 report_load(origin, NULL, "old-api-denied");
 151                 return -EPERM;
 152         }
 153
 154         load_root = file->f_path.mnt->mnt_sb;
 155         load_root_writable = sb_is_writable(load_root);
 156
 157         /* First loaded module/firmware defines the root for all others. */
 158         spin_lock(&pinned_root_spinlock);
 159         /*
 160          * pinned_root is only NULL at startup or when the pinned root has
 161          * been unmounted while we are not in enforcing mode. Otherwise, it
 162          * is either a valid reference, or an ERR_PTR.
 163          */
 164         if (!pinned_root) {
 165                 pinned_root = load_root;
 166                 first_root_pin = true;
 167         }
 168         spin_unlock(&pinned_root_spinlock);
 169
 170         if (first_root_pin) {
 171                 report_writable(pinned_root, load_root_writable);
 172                 set_sysctl(load_root_writable);
 173                 report_load(origin, file, "pinned");
 174         }
 175
 176         if (IS_ERR_OR_NULL(pinned_root) ||
 177             ((load_root != pinned_root) && !dm_verity_loadpin_is_bdev_trusted(load_root->s_bdev))) {
 178                 if (unlikely(!enforce)) {
 179                         report_load(origin, file, "pinning-ignored");
 180                         return 0;
 181                 }
 182
 183                 report_load(origin, file, "denied");
 184                 return -EPERM;
 185         }
 186
 187         return 0;
 188 }
 189
 190 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
 191                              bool contents)
 192 {
 193         /*
 194          * LoadPin only cares about the _origin_ of a file, not its
 195          * contents, so we can ignore the "are full contents available"
 196          * argument here.
 197          */
 198         return loadpin_check(file, id);
 199 }
 200
 201 static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
 202 {
 203         /*
 204          * LoadPin only cares about the _origin_ of a file, not its
 205          * contents, so a NULL file is passed, and we can ignore the
 206          * state of "contents".
 207          */
 208         return loadpin_check(NULL, (enum kernel_read_file_id) id);
 209 }
 210
 211 static struct security_hook_list loadpin_hooks[] __ro_after_init = {
 212         LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
 213         LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
 214         LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
 215 };
 216
 217 static void __init parse_exclude(void)
 218 {
 219         int i, j;
 220         char *cur;
 221
 222         /*
 223          * Make sure all the arrays stay within expected sizes. This
 224          * is slightly weird because kernel_read_file_str[] includes
 225          * READING_MAX_ID, which isn't actually meaningful here.
 226          */
 227         BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
 228                      ARRAY_SIZE(ignore_read_file_id));
 229         BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
 230                      ARRAY_SIZE(ignore_read_file_id));
 231
 232         for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
 233                 cur = exclude_read_files[i];
 234                 if (!cur)
 235                         break;
 236                 if (*cur == '\0')
 237                         continue;
 238
 239                 for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
 240                         if (strcmp(cur, kernel_read_file_str[j]) == 0) {
 241                                 pr_info("excluding: %s\n",
 242                                         kernel_read_file_str[j]);
 243                                 ignore_read_file_id[j] = 1;
 244                                 /*
 245                                  * Can not break, because one read_file_str
 246                                  * may map to more than on read_file_id.
 247                                  */
 248                         }
 249                 }
 250         }
 251 }
 252
 253 static int __init loadpin_init(void)
 254 {
 255         pr_info("ready to pin (currently %senforcing)\n",
 256                 enforce ? "" : "not ");
 257         parse_exclude();
 258 #ifdef CONFIG_SYSCTL
 259         if (!register_sysctl("kernel/loadpin", loadpin_sysctl_table))
 260                 pr_notice("sysctl registration failed!\n");
 261 #endif
 262         security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
 263
 264         return 0;
 265 }
 266
 267 DEFINE_LSM(loadpin) = {
 268         .name = "loadpin",
 269         .init = loadpin_init,
 270 };
 271
 272 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
 273
 274 enum loadpin_securityfs_interface_index {
 275         LOADPIN_DM_VERITY,
 276 };
 277
 278 static int read_trusted_verity_root_digests(unsigned int fd)
 279 {
 280         struct fd f;
 281         void *data;
 282         int rc;
 283         char *p, *d;
 284
 285         if (deny_reading_verity_digests)
 286                 return -EPERM;
 287
 288         /* The list of trusted root digests can only be set up once */
 289         if (!list_empty(&dm_verity_loadpin_trusted_root_digests))
 290                 return -EPERM;
 291
 292         f = fdget(fd);
 293         if (!f.file)
 294                 return -EINVAL;
 295
 296         data = kzalloc(SZ_4K, GFP_KERNEL);
 297         if (!data) {
 298                 rc = -ENOMEM;
 299                 goto err;
 300         }
 301
 302         rc = kernel_read_file(f.file, 0, (void **)&data, SZ_4K - 1, NULL, READING_POLICY);
 303         if (rc < 0)
 304                 goto err;
 305
 306         p = data;
 307         p[rc] = '\0';
 308         p = strim(p);
 309
 310         p = strim(data);
 311         while ((d = strsep(&p, "\n")) != NULL) {
 312                 int len;
 313                 struct dm_verity_loadpin_trusted_root_digest *trd;
 314
 315                 if (d == data) {
 316                         /* first line, validate header */
 317                         if (strcmp(d, VERITY_DIGEST_FILE_HEADER)) {
 318                                 rc = -EPROTO;
 319                                 goto err;
 320                         }
 321
 322                         continue;
 323                 }
 324
 325                 len = strlen(d);
 326
 327                 if (len % 2) {
 328                         rc = -EPROTO;
 329                         goto err;
 330                 }
 331
 332                 len /= 2;
 333
 334                 trd = kzalloc(struct_size(trd, data, len), GFP_KERNEL);
 335                 if (!trd) {
 336                         rc = -ENOMEM;
 337                         goto err;
 338                 }
 339                 trd->len = len;
 340
 341                 if (hex2bin(trd->data, d, len)) {
 342                         kfree(trd);
 343                         rc = -EPROTO;
 344                         goto err;
 345                 }
 346
 347                 list_add_tail(&trd->node, &dm_verity_loadpin_trusted_root_digests);
 348         }
 349
 350         if (list_empty(&dm_verity_loadpin_trusted_root_digests)) {
 351                 rc = -EPROTO;
 352                 goto err;
 353         }
 354
 355         kfree(data);
 356         fdput(f);
 357
 358         return 0;
 359
 360 err:
 361         kfree(data);
 362
 363         /* any failure in loading/parsing invalidates the entire list */
 364         {
 365                 struct dm_verity_loadpin_trusted_root_digest *trd, *tmp;
 366
 367                 list_for_each_entry_safe(trd, tmp, &dm_verity_loadpin_trusted_root_digests, node) {
 368                         list_del(&trd->node);
 369                         kfree(trd);
 370                 }
 371         }
 372
 373         /* disallow further attempts after reading a corrupt/invalid file */
 374         deny_reading_verity_digests = true;
 375
 376         fdput(f);
 377
 378         return rc;
 379 }
 380
 381 /******************************** securityfs ********************************/
 382
 383 static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 384 {
 385         void __user *uarg = (void __user *)arg;
 386         unsigned int fd;
 387
 388         switch (cmd) {
 389         case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
 390                 if (copy_from_user(&fd, uarg, sizeof(fd)))
 391                         return -EFAULT;
 392
 393                 return read_trusted_verity_root_digests(fd);
 394
 395         default:
 396                 return -EINVAL;
 397         }
 398 }
 399
 400 static const struct file_operations loadpin_dm_verity_ops = {
 401         .unlocked_ioctl = dm_verity_ioctl,
 402         .compat_ioctl = compat_ptr_ioctl,
 403 };
 404
 405 /**
 406  * init_loadpin_securityfs - create the securityfs directory for LoadPin
 407  *
 408  * We can not put this method normally under the loadpin_init() code path since
 409  * the security subsystem gets initialized before the vfs caches.
 410  *
 411  * Returns 0 if the securityfs directory creation was successful.
 412  */
 413 static int __init init_loadpin_securityfs(void)
 414 {
 415         struct dentry *loadpin_dir, *dentry;
 416
 417         loadpin_dir = securityfs_create_dir("loadpin", NULL);
 418         if (IS_ERR(loadpin_dir)) {
 419                 pr_err("LoadPin: could not create securityfs dir: %ld\n",
 420                        PTR_ERR(loadpin_dir));
 421                 return PTR_ERR(loadpin_dir);
 422         }
 423
 424         dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
 425                                         (void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
 426         if (IS_ERR(dentry)) {
 427                 pr_err("LoadPin: could not create securityfs entry 'dm-verity': %ld\n",
 428                        PTR_ERR(dentry));
 429                 return PTR_ERR(dentry);
 430         }
 431
 432         return 0;
 433 }
 434
 435 fs_initcall(init_loadpin_securityfs);
 436
 437 #endif /* CONFIG_SECURITY_LOADPIN_VERITY */
 438
 439 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
 440 module_param(enforce, int, 0);
 441 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
 442 module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
 443 MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");