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
  69 static void set_sysctl(bool is_writable)
  70 {
  71         /*
  72          * If load pinning is not enforced via a read-only block
  73          * device, allow sysctl to change modes for testing.
  74          */
  75         if (is_writable)
  76                 loadpin_sysctl_table[0].extra1 = SYSCTL_ZERO;
  77         else
  78                 loadpin_sysctl_table[0].extra1 = SYSCTL_ONE;
  79 }
  80 #else
  81 static inline void set_sysctl(bool is_writable) { }
  82 #endif
  83
  84 static void report_writable(struct super_block *mnt_sb, bool writable)
  85 {
  86         if (mnt_sb->s_bdev) {
  87                 pr_info("%pg (%u:%u): %s\n", mnt_sb->s_bdev,
  88                         MAJOR(mnt_sb->s_bdev->bd_dev),
  89                         MINOR(mnt_sb->s_bdev->bd_dev),
  90                         writable ? "writable" : "read-only");
  91         } else
  92                 pr_info("mnt_sb lacks block device, treating as: writable\n");
  93
  94         if (!writable)
  95                 pr_info("load pinning engaged.\n");
  96 }
  97
  98 /*
  99  * This must be called after early kernel init, since then the rootdev
 100  * is available.
 101  */
 102 static bool sb_is_writable(struct super_block *mnt_sb)
 103 {
 104         bool writable = true;
 105
 106         if (mnt_sb->s_bdev)
 107                 writable = !bdev_read_only(mnt_sb->s_bdev);
 108
 109         return writable;
 110 }
 111
 112 static void loadpin_sb_free_security(struct super_block *mnt_sb)
 113 {
 114         /*
 115          * When unmounting the filesystem we were using for load
 116          * pinning, we acknowledge the superblock release, but make sure
 117          * no other modules or firmware can be loaded when we are in
 118          * enforcing mode. Otherwise, allow the root to be reestablished.
 119          */
 120         if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
 121                 if (enforce) {
 122                         pinned_root = ERR_PTR(-EIO);
 123                         pr_info("umount pinned fs: refusing further loads\n");
 124                 } else {
 125                         pinned_root = NULL;
 126                 }
 127         }
 128 }
 129
 130 static int loadpin_check(struct file *file, enum kernel_read_file_id id)
 131 {
 132         struct super_block *load_root;
 133         const char *origin = kernel_read_file_id_str(id);
 134         bool first_root_pin = false;
 135         bool load_root_writable;
 136
 137         /* If the file id is excluded, ignore the pinning. */
 138         if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
 139             ignore_read_file_id[id]) {
 140                 report_load(origin, file, "pinning-excluded");
 141                 return 0;
 142         }
 143
 144         /* This handles the older init_module API that has a NULL file. */
 145         if (!file) {
 146                 if (!enforce) {
 147                         report_load(origin, NULL, "old-api-pinning-ignored");
 148                         return 0;
 149                 }
 150
 151                 report_load(origin, NULL, "old-api-denied");
 152                 return -EPERM;
 153         }
 154
 155         load_root = file->f_path.mnt->mnt_sb;
 156         load_root_writable = sb_is_writable(load_root);
 157
 158         /* First loaded module/firmware defines the root for all others. */
 159         spin_lock(&pinned_root_spinlock);
 160         /*
 161          * pinned_root is only NULL at startup or when the pinned root has
 162          * been unmounted while we are not in enforcing mode. Otherwise, it
 163          * is either a valid reference, or an ERR_PTR.
 164          */
 165         if (!pinned_root) {
 166                 pinned_root = load_root;
 167                 first_root_pin = true;
 168         }
 169         spin_unlock(&pinned_root_spinlock);
 170
 171         if (first_root_pin) {
 172                 report_writable(pinned_root, load_root_writable);
 173                 set_sysctl(load_root_writable);
 174                 report_load(origin, file, "pinned");
 175         }
 176
 177         if (IS_ERR_OR_NULL(pinned_root) ||
 178             ((load_root != pinned_root) && !dm_verity_loadpin_is_bdev_trusted(load_root->s_bdev))) {
 179                 if (unlikely(!enforce)) {
 180                         report_load(origin, file, "pinning-ignored");
 181                         return 0;
 182                 }
 183
 184                 report_load(origin, file, "denied");
 185                 return -EPERM;
 186         }
 187
 188         return 0;
 189 }
 190
 191 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
 192                              bool contents)
 193 {
 194         /*
 195          * LoadPin only cares about the _origin_ of a file, not its
 196          * contents, so we can ignore the "are full contents available"
 197          * argument here.
 198          */
 199         return loadpin_check(file, id);
 200 }
 201
 202 static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
 203 {
 204         /*
 205          * LoadPin only cares about the _origin_ of a file, not its
 206          * contents, so a NULL file is passed, and we can ignore the
 207          * state of "contents".
 208          */
 209         return loadpin_check(NULL, (enum kernel_read_file_id) id);
 210 }
 211
 212 static const struct lsm_id loadpin_lsmid = {
 213         .name = "loadpin",
 214         .id = LSM_ID_LOADPIN,
 215 };
 216
 217 static struct security_hook_list loadpin_hooks[] __ro_after_init = {
 218         LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
 219         LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
 220         LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
 221 };
 222
 223 static void __init parse_exclude(void)
 224 {
 225         int i, j;
 226         char *cur;
 227
 228         /*
 229          * Make sure all the arrays stay within expected sizes. This
 230          * is slightly weird because kernel_read_file_str[] includes
 231          * READING_MAX_ID, which isn't actually meaningful here.
 232          */
 233         BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
 234                      ARRAY_SIZE(ignore_read_file_id));
 235         BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
 236                      ARRAY_SIZE(ignore_read_file_id));
 237
 238         for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
 239                 cur = exclude_read_files[i];
 240                 if (!cur)
 241                         break;
 242                 if (*cur == '\0')
 243                         continue;
 244
 245                 for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
 246                         if (strcmp(cur, kernel_read_file_str[j]) == 0) {
 247                                 pr_info("excluding: %s\n",
 248                                         kernel_read_file_str[j]);
 249                                 ignore_read_file_id[j] = 1;
 250                                 /*
 251                                  * Can not break, because one read_file_str
 252                                  * may map to more than on read_file_id.
 253                                  */
 254                         }
 255                 }
 256         }
 257 }
 258
 259 static int __init loadpin_init(void)
 260 {
 261         pr_info("ready to pin (currently %senforcing)\n",
 262                 enforce ? "" : "not ");
 263         parse_exclude();
 264 #ifdef CONFIG_SYSCTL
 265         if (!register_sysctl("kernel/loadpin", loadpin_sysctl_table))
 266                 pr_notice("sysctl registration failed!\n");
 267 #endif
 268         security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks),
 269                            &loadpin_lsmid);
 270
 271         return 0;
 272 }
 273
 274 DEFINE_LSM(loadpin) = {
 275         .name = "loadpin",
 276         .init = loadpin_init,
 277 };
 278
 279 #ifdef CONFIG_SECURITY_LOADPIN_VERITY
 280
 281 enum loadpin_securityfs_interface_index {
 282         LOADPIN_DM_VERITY,
 283 };
 284
 285 static int read_trusted_verity_root_digests(unsigned int fd)
 286 {
 287         struct fd f;
 288         void *data;
 289         int rc;
 290         char *p, *d;
 291
 292         if (deny_reading_verity_digests)
 293                 return -EPERM;
 294
 295         /* The list of trusted root digests can only be set up once */
 296         if (!list_empty(&dm_verity_loadpin_trusted_root_digests))
 297                 return -EPERM;
 298
 299         f = fdget(fd);
 300         if (!f.file)
 301                 return -EINVAL;
 302
 303         data = kzalloc(SZ_4K, GFP_KERNEL);
 304         if (!data) {
 305                 rc = -ENOMEM;
 306                 goto err;
 307         }
 308
 309         rc = kernel_read_file(f.file, 0, (void **)&data, SZ_4K - 1, NULL, READING_POLICY);
 310         if (rc < 0)
 311                 goto err;
 312
 313         p = data;
 314         p[rc] = '\0';
 315         p = strim(p);
 316
 317         p = strim(data);
 318         while ((d = strsep(&p, "\n")) != NULL) {
 319                 int len;
 320                 struct dm_verity_loadpin_trusted_root_digest *trd;
 321
 322                 if (d == data) {
 323                         /* first line, validate header */
 324                         if (strcmp(d, VERITY_DIGEST_FILE_HEADER)) {
 325                                 rc = -EPROTO;
 326                                 goto err;
 327                         }
 328
 329                         continue;
 330                 }
 331
 332                 len = strlen(d);
 333
 334                 if (len % 2) {
 335                         rc = -EPROTO;
 336                         goto err;
 337                 }
 338
 339                 len /= 2;
 340
 341                 trd = kzalloc(struct_size(trd, data, len), GFP_KERNEL);
 342                 if (!trd) {
 343                         rc = -ENOMEM;
 344                         goto err;
 345                 }
 346                 trd->len = len;
 347
 348                 if (hex2bin(trd->data, d, len)) {
 349                         kfree(trd);
 350                         rc = -EPROTO;
 351                         goto err;
 352                 }
 353
 354                 list_add_tail(&trd->node, &dm_verity_loadpin_trusted_root_digests);
 355         }
 356
 357         if (list_empty(&dm_verity_loadpin_trusted_root_digests)) {
 358                 rc = -EPROTO;
 359                 goto err;
 360         }
 361
 362         kfree(data);
 363         fdput(f);
 364
 365         return 0;
 366
 367 err:
 368         kfree(data);
 369
 370         /* any failure in loading/parsing invalidates the entire list */
 371         {
 372                 struct dm_verity_loadpin_trusted_root_digest *trd, *tmp;
 373
 374                 list_for_each_entry_safe(trd, tmp, &dm_verity_loadpin_trusted_root_digests, node) {
 375                         list_del(&trd->node);
 376                         kfree(trd);
 377                 }
 378         }
 379
 380         /* disallow further attempts after reading a corrupt/invalid file */
 381         deny_reading_verity_digests = true;
 382
 383         fdput(f);
 384
 385         return rc;
 386 }
 387
 388 /******************************** securityfs ********************************/
 389
 390 static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 391 {
 392         void __user *uarg = (void __user *)arg;
 393         unsigned int fd;
 394
 395         switch (cmd) {
 396         case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
 397                 if (copy_from_user(&fd, uarg, sizeof(fd)))
 398                         return -EFAULT;
 399
 400                 return read_trusted_verity_root_digests(fd);
 401
 402         default:
 403                 return -EINVAL;
 404         }
 405 }
 406
 407 static const struct file_operations loadpin_dm_verity_ops = {
 408         .unlocked_ioctl = dm_verity_ioctl,
 409         .compat_ioctl = compat_ptr_ioctl,
 410 };
 411
 412 /**
 413  * init_loadpin_securityfs - create the securityfs directory for LoadPin
 414  *
 415  * We can not put this method normally under the loadpin_init() code path since
 416  * the security subsystem gets initialized before the vfs caches.
 417  *
 418  * Returns 0 if the securityfs directory creation was successful.
 419  */
 420 static int __init init_loadpin_securityfs(void)
 421 {
 422         struct dentry *loadpin_dir, *dentry;
 423
 424         loadpin_dir = securityfs_create_dir("loadpin", NULL);
 425         if (IS_ERR(loadpin_dir)) {
 426                 pr_err("LoadPin: could not create securityfs dir: %ld\n",
 427                        PTR_ERR(loadpin_dir));
 428                 return PTR_ERR(loadpin_dir);
 429         }
 430
 431         dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
 432                                         (void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
 433         if (IS_ERR(dentry)) {
 434                 pr_err("LoadPin: could not create securityfs entry 'dm-verity': %ld\n",
 435                        PTR_ERR(dentry));
 436                 return PTR_ERR(dentry);
 437         }
 438
 439         return 0;
 440 }
 441
 442 fs_initcall(init_loadpin_securityfs);
 443
 444 #endif /* CONFIG_SECURITY_LOADPIN_VERITY */
 445
 446 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
 447 module_param(enforce, int, 0);
 448 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
 449 module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
 450 MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");