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 #include <uapi/linux/lsm.h>
  24
  25 #define VERITY_DIGEST_FILE_HEADER "# LOADPIN_TRUSTED_VERITY_ROOT_DIGESTS"
  26
  27 static void report_load(const char *origin, struct file *file, char *operation)
  28 {
  29         char *cmdline, *pathname;
  30
  31         pathname = kstrdup_quotable_file(file, GFP_KERNEL);
  32         cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
  33
  34         pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
  35                   origin, operation,
  36                   (pathname && pathname[0] != '<') ? "\"" : "",
  37                   pathname,
  38                   (pathname && pathname[0] != '<') ? "\"" : "",
  39                   task_pid_nr(current),
  40                   cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
  41
  42         kfree(cmdline);
  43         kfree(pathname);
  44 }
  45
  46 static int enforce = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENFORCE);
  47 static char *exclude_read_files[READING_MAX_ID];
  48 static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
  49 static struct super_block *pinned_root;
  50 static DEFINE_SPINLOCK(pinned_root_spinlock);
  51 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
  52 static bool deny_reading_verity_digests;
  53 #endif
  54
  55 #ifdef CONFIG_SYSCTL
  56 static struct ctl_table loadpin_sysctl_table[] = {
  57         {
  58                 .procname       = "enforce",
  59                 .data           = &enforce,
  60                 .maxlen         = sizeof(int),
  61                 .mode           = 0644,
  62                 .proc_handler   = proc_dointvec_minmax,
  63                 .extra1         = SYSCTL_ONE,
  64                 .extra2         = SYSCTL_ONE,
  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 const struct lsm_id loadpin_lsmid = {
 212         .name = "loadpin",
 213         .id = LSM_ID_LOADPIN,
 214 };
 215
 216 static struct security_hook_list loadpin_hooks[] __ro_after_init = {
 217         LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
 218         LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
 219         LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
 220 };
 221
 222 static void __init parse_exclude(void)
 223 {
 224         int i, j;
 225         char *cur;
 226
 227         /*
 228          * Make sure all the arrays stay within expected sizes. This
 229          * is slightly weird because kernel_read_file_str[] includes
 230          * READING_MAX_ID, which isn't actually meaningful here.
 231          */
 232         BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
 233                      ARRAY_SIZE(ignore_read_file_id));
 234         BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
 235                      ARRAY_SIZE(ignore_read_file_id));
 236
 237         for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
 238                 cur = exclude_read_files[i];
 239                 if (!cur)
 240                         break;
 241                 if (*cur == '\0')
 242                         continue;
 243
 244                 for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
 245                         if (strcmp(cur, kernel_read_file_str[j]) == 0) {
 246                                 pr_info("excluding: %s\n",
 247                                         kernel_read_file_str[j]);
 248                                 ignore_read_file_id[j] = 1;
 249                                 /*
 250                                  * Can not break, because one read_file_str
 251                                  * may map to more than on read_file_id.
 252                                  */
 253                         }
 254                 }
 255         }
 256 }
 257
 258 static int __init loadpin_init(void)
 259 {
 260         pr_info("ready to pin (currently %senforcing)\n",
 261                 enforce ? "" : "not ");
 262         parse_exclude();
 263 #ifdef CONFIG_SYSCTL
 264         if (!register_sysctl("kernel/loadpin", loadpin_sysctl_table))
 265                 pr_notice("sysctl registration failed!\n");
 266 #endif
 267         security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks),
 268                            &loadpin_lsmid);
 269
 270         return 0;
 271 }
 272
 273 DEFINE_LSM(loadpin) = {
 274         .name = "loadpin",
 275         .init = loadpin_init,
 276 };
 277
 278 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
 279
 280 enum loadpin_securityfs_interface_index {
 281         LOADPIN_DM_VERITY,
 282 };
 283
 284 static int read_trusted_verity_root_digests(unsigned int fd)
 285 {
 286         void *data;
 287         int rc;
 288         char *p, *d;
 289
 290         if (deny_reading_verity_digests)
 291                 return -EPERM;
 292
 293         /* The list of trusted root digests can only be set up once */
 294         if (!list_empty(&dm_verity_loadpin_trusted_root_digests))
 295                 return -EPERM;
 296
 297         CLASS(fd, f)(fd);
 298         if (fd_empty(f))
 299                 return -EINVAL;
 300
 301         data = kzalloc(SZ_4K, GFP_KERNEL);
 302         if (!data) {
 303                 rc = -ENOMEM;
 304                 goto err;
 305         }
 306
 307         rc = kernel_read_file(fd_file(f), 0, (void **)&data, SZ_4K - 1, NULL, READING_POLICY);
 308         if (rc < 0)
 309                 goto err;
 310
 311         p = data;
 312         p[rc] = '\0';
 313         p = strim(p);
 314
 315         p = strim(data);
 316         while ((d = strsep(&p, "\n")) != NULL) {
 317                 int len;
 318                 struct dm_verity_loadpin_trusted_root_digest *trd;
 319
 320                 if (d == data) {
 321                         /* first line, validate header */
 322                         if (strcmp(d, VERITY_DIGEST_FILE_HEADER)) {
 323                                 rc = -EPROTO;
 324                                 goto err;
 325                         }
 326
 327                         continue;
 328                 }
 329
 330                 len = strlen(d);
 331
 332                 if (len % 2) {
 333                         rc = -EPROTO;
 334                         goto err;
 335                 }
 336
 337                 len /= 2;
 338
 339                 trd = kzalloc(struct_size(trd, data, len), GFP_KERNEL);
 340                 if (!trd) {
 341                         rc = -ENOMEM;
 342                         goto err;
 343                 }
 344                 trd->len = len;
 345
 346                 if (hex2bin(trd->data, d, len)) {
 347                         kfree(trd);
 348                         rc = -EPROTO;
 349                         goto err;
 350                 }
 351
 352                 list_add_tail(&trd->node, &dm_verity_loadpin_trusted_root_digests);
 353         }
 354
 355         if (list_empty(&dm_verity_loadpin_trusted_root_digests)) {
 356                 rc = -EPROTO;
 357                 goto err;
 358         }
 359
 360         kfree(data);
 361
 362         return 0;
 363
 364 err:
 365         kfree(data);
 366
 367         /* any failure in loading/parsing invalidates the entire list */
 368         {
 369                 struct dm_verity_loadpin_trusted_root_digest *trd, *tmp;
 370
 371                 list_for_each_entry_safe(trd, tmp, &dm_verity_loadpin_trusted_root_digests, node) {
 372                         list_del(&trd->node);
 373                         kfree(trd);
 374                 }
 375         }
 376
 377         /* disallow further attempts after reading a corrupt/invalid file */
 378         deny_reading_verity_digests = true;
 379
 380         return rc;
 381 }
 382
 383 /******************************** securityfs ********************************/
 384
 385 static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 386 {
 387         void __user *uarg = (void __user *)arg;
 388         unsigned int fd;
 389
 390         switch (cmd) {
 391         case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
 392                 if (copy_from_user(&fd, uarg, sizeof(fd)))
 393                         return -EFAULT;
 394
 395                 return read_trusted_verity_root_digests(fd);
 396
 397         default:
 398                 return -EINVAL;
 399         }
 400 }
 401
 402 static const struct file_operations loadpin_dm_verity_ops = {
 403         .unlocked_ioctl = dm_verity_ioctl,
 404         .compat_ioctl = compat_ptr_ioctl,
 405 };
 406
 407 /**
 408  * init_loadpin_securityfs - create the securityfs directory for LoadPin
 409  *
 410  * We can not put this method normally under the loadpin_init() code path since
 411  * the security subsystem gets initialized before the vfs caches.
 412  *
 413  * Returns 0 if the securityfs directory creation was successful.
 414  */
 415 static int __init init_loadpin_securityfs(void)
 416 {
 417         struct dentry *loadpin_dir, *dentry;
 418
 419         loadpin_dir = securityfs_create_dir("loadpin", NULL);
 420         if (IS_ERR(loadpin_dir)) {
 421                 pr_err("LoadPin: could not create securityfs dir: %ld\n",
 422                        PTR_ERR(loadpin_dir));
 423                 return PTR_ERR(loadpin_dir);
 424         }
 425
 426         dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
 427                                         (void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
 428         if (IS_ERR(dentry)) {
 429                 pr_err("LoadPin: could not create securityfs entry 'dm-verity': %ld\n",
 430                        PTR_ERR(dentry));
 431                 return PTR_ERR(dentry);
 432         }
 433
 434         return 0;
 435 }
 436
 437 fs_initcall(init_loadpin_securityfs);
 438
 439 #endif /* CONFIG_SECURITY_LOADPIN_VERITY */
 440
 441 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
 442 module_param(enforce, int, 0);
 443 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
 444 module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
 445 MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");