fs/crypto/keysetup.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Key setup facility for FS encryption support.
   4  *
   5  * Copyright (C) 2015, Google, Inc.
   6  *
   7  * Originally written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar.
   8  * Heavily modified since then.
   9  */
  10
  11 #include <crypto/skcipher.h>
  12 #include <linux/key.h>
  13
  14 #include "fscrypt_private.h"
  15
  16 static struct fscrypt_mode available_modes[] = {
  17         [FSCRYPT_MODE_AES_256_XTS] = {
  18                 .friendly_name = "AES-256-XTS",
  19                 .cipher_str = "xts(aes)",
  20                 .keysize = 64,
  21                 .ivsize = 16,
  22         },
  23         [FSCRYPT_MODE_AES_256_CTS] = {
  24                 .friendly_name = "AES-256-CTS-CBC",
  25                 .cipher_str = "cts(cbc(aes))",
  26                 .keysize = 32,
  27                 .ivsize = 16,
  28         },
  29         [FSCRYPT_MODE_AES_128_CBC] = {
  30                 .friendly_name = "AES-128-CBC-ESSIV",
  31                 .cipher_str = "essiv(cbc(aes),sha256)",
  32                 .keysize = 16,
  33                 .ivsize = 16,
  34         },
  35         [FSCRYPT_MODE_AES_128_CTS] = {
  36                 .friendly_name = "AES-128-CTS-CBC",
  37                 .cipher_str = "cts(cbc(aes))",
  38                 .keysize = 16,
  39                 .ivsize = 16,
  40         },
  41         [FSCRYPT_MODE_ADIANTUM] = {
  42                 .friendly_name = "Adiantum",
  43                 .cipher_str = "adiantum(xchacha12,aes)",
  44                 .keysize = 32,
  45                 .ivsize = 32,
  46         },
  47 };
  48
  49 static struct fscrypt_mode *
  50 select_encryption_mode(const union fscrypt_policy *policy,
  51                        const struct inode *inode)
  52 {
  53         if (S_ISREG(inode->i_mode))
  54                 return &available_modes[fscrypt_policy_contents_mode(policy)];
  55
  56         if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
  57                 return &available_modes[fscrypt_policy_fnames_mode(policy)];
  58
  59         WARN_ONCE(1, "fscrypt: filesystem tried to load encryption info for inode %lu, which is not encryptable (file type %d)\n",
  60                   inode->i_ino, (inode->i_mode & S_IFMT));
  61         return ERR_PTR(-EINVAL);
  62 }
  63
  64 /* Create a symmetric cipher object for the given encryption mode and key */
  65 struct crypto_skcipher *fscrypt_allocate_skcipher(struct fscrypt_mode *mode,
  66                                                   const u8 *raw_key,
  67                                                   const struct inode *inode)
  68 {
  69         struct crypto_skcipher *tfm;
  70         int err;
  71
  72         tfm = crypto_alloc_skcipher(mode->cipher_str, 0, 0);
  73         if (IS_ERR(tfm)) {
  74                 if (PTR_ERR(tfm) == -ENOENT) {
  75                         fscrypt_warn(inode,
  76                                      "Missing crypto API support for %s (API name: \"%s\")",
  77                                      mode->friendly_name, mode->cipher_str);
  78                         return ERR_PTR(-ENOPKG);
  79                 }
  80                 fscrypt_err(inode, "Error allocating '%s' transform: %ld",
  81                             mode->cipher_str, PTR_ERR(tfm));
  82                 return tfm;
  83         }
  84         if (!xchg(&mode->logged_impl_name, 1)) {
  85                 /*
  86                  * fscrypt performance can vary greatly depending on which
  87                  * crypto algorithm implementation is used.  Help people debug
  88                  * performance problems by logging the ->cra_driver_name the
  89                  * first time a mode is used.
  90                  */
  91                 pr_info("fscrypt: %s using implementation \"%s\"\n",
  92                         mode->friendly_name,
  93                         crypto_skcipher_alg(tfm)->base.cra_driver_name);
  94         }
  95         crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_FORBID_WEAK_KEYS);
  96         err = crypto_skcipher_setkey(tfm, raw_key, mode->keysize);
  97         if (err)
  98                 goto err_free_tfm;
  99
 100         return tfm;
 101
 102 err_free_tfm:
 103         crypto_free_skcipher(tfm);
 104         return ERR_PTR(err);
 105 }
 106
 107 /* Given the per-file key, set up the file's crypto transform object */
 108 int fscrypt_set_derived_key(struct fscrypt_info *ci, const u8 *derived_key)
 109 {
 110         struct crypto_skcipher *tfm;
 111
 112         tfm = fscrypt_allocate_skcipher(ci->ci_mode, derived_key, ci->ci_inode);
 113         if (IS_ERR(tfm))
 114                 return PTR_ERR(tfm);
 115
 116         ci->ci_ctfm = tfm;
 117         ci->ci_owns_key = true;
 118         return 0;
 119 }
 120
 121 static int setup_per_mode_key(struct fscrypt_info *ci,
 122                               struct fscrypt_master_key *mk,
 123                               struct crypto_skcipher **tfms,
 124                               u8 hkdf_context, bool include_fs_uuid)
 125 {
 126         const struct inode *inode = ci->ci_inode;
 127         const struct super_block *sb = inode->i_sb;
 128         struct fscrypt_mode *mode = ci->ci_mode;
 129         u8 mode_num = mode - available_modes;
 130         struct crypto_skcipher *tfm, *prev_tfm;
 131         u8 mode_key[FSCRYPT_MAX_KEY_SIZE];
 132         u8 hkdf_info[sizeof(mode_num) + sizeof(sb->s_uuid)];
 133         unsigned int hkdf_infolen = 0;
 134         int err;
 135
 136         if (WARN_ON(mode_num > __FSCRYPT_MODE_MAX))
 137                 return -EINVAL;
 138
 139         /* pairs with cmpxchg() below */
 140         tfm = READ_ONCE(tfms[mode_num]);
 141         if (likely(tfm != NULL))
 142                 goto done;
 143
 144         BUILD_BUG_ON(sizeof(mode_num) != 1);
 145         BUILD_BUG_ON(sizeof(sb->s_uuid) != 16);
 146         BUILD_BUG_ON(sizeof(hkdf_info) != 17);
 147         hkdf_info[hkdf_infolen++] = mode_num;
 148         if (include_fs_uuid) {
 149                 memcpy(&hkdf_info[hkdf_infolen], &sb->s_uuid,
 150                        sizeof(sb->s_uuid));
 151                 hkdf_infolen += sizeof(sb->s_uuid);
 152         }
 153         err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
 154                                   hkdf_context, hkdf_info, hkdf_infolen,
 155                                   mode_key, mode->keysize);
 156         if (err)
 157                 return err;
 158         tfm = fscrypt_allocate_skcipher(mode, mode_key, inode);
 159         memzero_explicit(mode_key, mode->keysize);
 160         if (IS_ERR(tfm))
 161                 return PTR_ERR(tfm);
 162
 163         /* pairs with READ_ONCE() above */
 164         prev_tfm = cmpxchg(&tfms[mode_num], NULL, tfm);
 165         if (prev_tfm != NULL) {
 166                 crypto_free_skcipher(tfm);
 167                 tfm = prev_tfm;
 168         }
 169 done:
 170         ci->ci_ctfm = tfm;
 171         return 0;
 172 }
 173
 174 static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
 175                                      struct fscrypt_master_key *mk)
 176 {
 177         u8 derived_key[FSCRYPT_MAX_KEY_SIZE];
 178         int err;
 179
 180         if (ci->ci_policy.v2.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) {
 181                 /*
 182                  * DIRECT_KEY: instead of deriving per-file keys, the per-file
 183                  * nonce will be included in all the IVs.  But unlike v1
 184                  * policies, for v2 policies in this case we don't encrypt with
 185                  * the master key directly but rather derive a per-mode key.
 186                  * This ensures that the master key is consistently used only
 187                  * for HKDF, avoiding key reuse issues.
 188                  */
 189                 if (!fscrypt_mode_supports_direct_key(ci->ci_mode)) {
 190                         fscrypt_warn(ci->ci_inode,
 191                                      "Direct key flag not allowed with %s",
 192                                      ci->ci_mode->friendly_name);
 193                         return -EINVAL;
 194                 }
 195                 return setup_per_mode_key(ci, mk, mk->mk_direct_tfms,
 196                                           HKDF_CONTEXT_DIRECT_KEY, false);
 197         } else if (ci->ci_policy.v2.flags &
 198                    FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64) {
 199                 /*
 200                  * IV_INO_LBLK_64: encryption keys are derived from (master_key,
 201                  * mode_num, filesystem_uuid), and inode number is included in
 202                  * the IVs.  This format is optimized for use with inline
 203                  * encryption hardware compliant with the UFS or eMMC standards.
 204                  */
 205                 return setup_per_mode_key(ci, mk, mk->mk_iv_ino_lblk_64_tfms,
 206                                           HKDF_CONTEXT_IV_INO_LBLK_64_KEY,
 207                                           true);
 208         }
 209
 210         err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
 211                                   HKDF_CONTEXT_PER_FILE_KEY,
 212                                   ci->ci_nonce, FS_KEY_DERIVATION_NONCE_SIZE,
 213                                   derived_key, ci->ci_mode->keysize);
 214         if (err)
 215                 return err;
 216
 217         err = fscrypt_set_derived_key(ci, derived_key);
 218         memzero_explicit(derived_key, ci->ci_mode->keysize);
 219         return err;
 220 }
 221
 222 /*
 223  * Find the master key, then set up the inode's actual encryption key.
 224  *
 225  * If the master key is found in the filesystem-level keyring, then the
 226  * corresponding 'struct key' is returned in *master_key_ret with
 227  * ->mk_secret_sem read-locked.  This is needed to ensure that only one task
 228  * links the fscrypt_info into ->mk_decrypted_inodes (as multiple tasks may race
 229  * to create an fscrypt_info for the same inode), and to synchronize the master
 230  * key being removed with a new inode starting to use it.
 231  */
 232 static int setup_file_encryption_key(struct fscrypt_info *ci,
 233                                      struct key **master_key_ret)
 234 {
 235         struct key *key;
 236         struct fscrypt_master_key *mk = NULL;
 237         struct fscrypt_key_specifier mk_spec;
 238         int err;
 239
 240         switch (ci->ci_policy.version) {
 241         case FSCRYPT_POLICY_V1:
 242                 mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR;
 243                 memcpy(mk_spec.u.descriptor,
 244                        ci->ci_policy.v1.master_key_descriptor,
 245                        FSCRYPT_KEY_DESCRIPTOR_SIZE);
 246                 break;
 247         case FSCRYPT_POLICY_V2:
 248                 mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
 249                 memcpy(mk_spec.u.identifier,
 250                        ci->ci_policy.v2.master_key_identifier,
 251                        FSCRYPT_KEY_IDENTIFIER_SIZE);
 252                 break;
 253         default:
 254                 WARN_ON(1);
 255                 return -EINVAL;
 256         }
 257
 258         key = fscrypt_find_master_key(ci->ci_inode->i_sb, &mk_spec);
 259         if (IS_ERR(key)) {
 260                 if (key != ERR_PTR(-ENOKEY) ||
 261                     ci->ci_policy.version != FSCRYPT_POLICY_V1)
 262                         return PTR_ERR(key);
 263
 264                 /*
 265                  * As a legacy fallback for v1 policies, search for the key in
 266                  * the current task's subscribed keyrings too.  Don't move this
 267                  * to before the search of ->s_master_keys, since users
 268                  * shouldn't be able to override filesystem-level keys.
 269                  */
 270                 return fscrypt_setup_v1_file_key_via_subscribed_keyrings(ci);
 271         }
 272
 273         mk = key->payload.data[0];
 274         down_read(&mk->mk_secret_sem);
 275
 276         /* Has the secret been removed (via FS_IOC_REMOVE_ENCRYPTION_KEY)? */
 277         if (!is_master_key_secret_present(&mk->mk_secret)) {
 278                 err = -ENOKEY;
 279                 goto out_release_key;
 280         }
 281
 282         /*
 283          * Require that the master key be at least as long as the derived key.
 284          * Otherwise, the derived key cannot possibly contain as much entropy as
 285          * that required by the encryption mode it will be used for.  For v1
 286          * policies it's also required for the KDF to work at all.
 287          */
 288         if (mk->mk_secret.size < ci->ci_mode->keysize) {
 289                 fscrypt_warn(NULL,
 290                              "key with %s %*phN is too short (got %u bytes, need %u+ bytes)",
 291                              master_key_spec_type(&mk_spec),
 292                              master_key_spec_len(&mk_spec), (u8 *)&mk_spec.u,
 293                              mk->mk_secret.size, ci->ci_mode->keysize);
 294                 err = -ENOKEY;
 295                 goto out_release_key;
 296         }
 297
 298         switch (ci->ci_policy.version) {
 299         case FSCRYPT_POLICY_V1:
 300                 err = fscrypt_setup_v1_file_key(ci, mk->mk_secret.raw);
 301                 break;
 302         case FSCRYPT_POLICY_V2:
 303                 err = fscrypt_setup_v2_file_key(ci, mk);
 304                 break;
 305         default:
 306                 WARN_ON(1);
 307                 err = -EINVAL;
 308                 break;
 309         }
 310         if (err)
 311                 goto out_release_key;
 312
 313         *master_key_ret = key;
 314         return 0;
 315
 316 out_release_key:
 317         up_read(&mk->mk_secret_sem);
 318         key_put(key);
 319         return err;
 320 }
 321
 322 static void put_crypt_info(struct fscrypt_info *ci)
 323 {
 324         struct key *key;
 325
 326         if (!ci)
 327                 return;
 328
 329         if (ci->ci_direct_key)
 330                 fscrypt_put_direct_key(ci->ci_direct_key);
 331         else if (ci->ci_owns_key)
 332                 crypto_free_skcipher(ci->ci_ctfm);
 333
 334         key = ci->ci_master_key;
 335         if (key) {
 336                 struct fscrypt_master_key *mk = key->payload.data[0];
 337
 338                 /*
 339                  * Remove this inode from the list of inodes that were unlocked
 340                  * with the master key.
 341                  *
 342                  * In addition, if we're removing the last inode from a key that
 343                  * already had its secret removed, invalidate the key so that it
 344                  * gets removed from ->s_master_keys.
 345                  */
 346                 spin_lock(&mk->mk_decrypted_inodes_lock);
 347                 list_del(&ci->ci_master_key_link);
 348                 spin_unlock(&mk->mk_decrypted_inodes_lock);
 349                 if (refcount_dec_and_test(&mk->mk_refcount))
 350                         key_invalidate(key);
 351                 key_put(key);
 352         }
 353         memzero_explicit(ci, sizeof(*ci));
 354         kmem_cache_free(fscrypt_info_cachep, ci);
 355 }
 356
 357 int fscrypt_get_encryption_info(struct inode *inode)
 358 {
 359         struct fscrypt_info *crypt_info;
 360         union fscrypt_context ctx;
 361         struct fscrypt_mode *mode;
 362         struct key *master_key = NULL;
 363         int res;
 364
 365         if (fscrypt_has_encryption_key(inode))
 366                 return 0;
 367
 368         res = fscrypt_initialize(inode->i_sb->s_cop->flags);
 369         if (res)
 370                 return res;
 371
 372         res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx));
 373         if (res < 0) {
 374                 if (!fscrypt_dummy_context_enabled(inode) ||
 375                     IS_ENCRYPTED(inode)) {
 376                         fscrypt_warn(inode,
 377                                      "Error %d getting encryption context",
 378                                      res);
 379                         return res;
 380                 }
 381                 /* Fake up a context for an unencrypted directory */
 382                 memset(&ctx, 0, sizeof(ctx));
 383                 ctx.version = FSCRYPT_CONTEXT_V1;
 384                 ctx.v1.contents_encryption_mode = FSCRYPT_MODE_AES_256_XTS;
 385                 ctx.v1.filenames_encryption_mode = FSCRYPT_MODE_AES_256_CTS;
 386                 memset(ctx.v1.master_key_descriptor, 0x42,
 387                        FSCRYPT_KEY_DESCRIPTOR_SIZE);
 388                 res = sizeof(ctx.v1);
 389         }
 390
 391         crypt_info = kmem_cache_zalloc(fscrypt_info_cachep, GFP_NOFS);
 392         if (!crypt_info)
 393                 return -ENOMEM;
 394
 395         crypt_info->ci_inode = inode;
 396
 397         res = fscrypt_policy_from_context(&crypt_info->ci_policy, &ctx, res);
 398         if (res) {
 399                 fscrypt_warn(inode,
 400                              "Unrecognized or corrupt encryption context");
 401                 goto out;
 402         }
 403
 404         switch (ctx.version) {
 405         case FSCRYPT_CONTEXT_V1:
 406                 memcpy(crypt_info->ci_nonce, ctx.v1.nonce,
 407                        FS_KEY_DERIVATION_NONCE_SIZE);
 408                 break;
 409         case FSCRYPT_CONTEXT_V2:
 410                 memcpy(crypt_info->ci_nonce, ctx.v2.nonce,
 411                        FS_KEY_DERIVATION_NONCE_SIZE);
 412                 break;
 413         default:
 414                 WARN_ON(1);
 415                 res = -EINVAL;
 416                 goto out;
 417         }
 418
 419         if (!fscrypt_supported_policy(&crypt_info->ci_policy, inode)) {
 420                 res = -EINVAL;
 421                 goto out;
 422         }
 423
 424         mode = select_encryption_mode(&crypt_info->ci_policy, inode);
 425         if (IS_ERR(mode)) {
 426                 res = PTR_ERR(mode);
 427                 goto out;
 428         }
 429         WARN_ON(mode->ivsize > FSCRYPT_MAX_IV_SIZE);
 430         crypt_info->ci_mode = mode;
 431
 432         res = setup_file_encryption_key(crypt_info, &master_key);
 433         if (res)
 434                 goto out;
 435
 436         if (cmpxchg_release(&inode->i_crypt_info, NULL, crypt_info) == NULL) {
 437                 if (master_key) {
 438                         struct fscrypt_master_key *mk =
 439                                 master_key->payload.data[0];
 440
 441                         refcount_inc(&mk->mk_refcount);
 442                         crypt_info->ci_master_key = key_get(master_key);
 443                         spin_lock(&mk->mk_decrypted_inodes_lock);
 444                         list_add(&crypt_info->ci_master_key_link,
 445                                  &mk->mk_decrypted_inodes);
 446                         spin_unlock(&mk->mk_decrypted_inodes_lock);
 447                 }
 448                 crypt_info = NULL;
 449         }
 450         res = 0;
 451 out:
 452         if (master_key) {
 453                 struct fscrypt_master_key *mk = master_key->payload.data[0];
 454
 455                 up_read(&mk->mk_secret_sem);
 456                 key_put(master_key);
 457         }
 458         if (res == -ENOKEY)
 459                 res = 0;
 460         put_crypt_info(crypt_info);
 461         return res;
 462 }
 463 EXPORT_SYMBOL(fscrypt_get_encryption_info);
 464
 465 /**
 466  * fscrypt_put_encryption_info - free most of an inode's fscrypt data
 467  *
 468  * Free the inode's fscrypt_info.  Filesystems must call this when the inode is
 469  * being evicted.  An RCU grace period need not have elapsed yet.
 470  */
 471 void fscrypt_put_encryption_info(struct inode *inode)
 472 {
 473         put_crypt_info(inode->i_crypt_info);
 474         inode->i_crypt_info = NULL;
 475 }
 476 EXPORT_SYMBOL(fscrypt_put_encryption_info);
 477
 478 /**
 479  * fscrypt_free_inode - free an inode's fscrypt data requiring RCU delay
 480  *
 481  * Free the inode's cached decrypted symlink target, if any.  Filesystems must
 482  * call this after an RCU grace period, just before they free the inode.
 483  */
 484 void fscrypt_free_inode(struct inode *inode)
 485 {
 486         if (IS_ENCRYPTED(inode) && S_ISLNK(inode->i_mode)) {
 487                 kfree(inode->i_link);
 488                 inode->i_link = NULL;
 489         }
 490 }
 491 EXPORT_SYMBOL(fscrypt_free_inode);
 492
 493 /**
 494  * fscrypt_drop_inode - check whether the inode's master key has been removed
 495  *
 496  * Filesystems supporting fscrypt must call this from their ->drop_inode()
 497  * method so that encrypted inodes are evicted as soon as they're no longer in
 498  * use and their master key has been removed.
 499  *
 500  * Return: 1 if fscrypt wants the inode to be evicted now, otherwise 0
 501  */
 502 int fscrypt_drop_inode(struct inode *inode)
 503 {
 504         const struct fscrypt_info *ci = READ_ONCE(inode->i_crypt_info);
 505         const struct fscrypt_master_key *mk;
 506
 507         /*
 508          * If ci is NULL, then the inode doesn't have an encryption key set up
 509          * so it's irrelevant.  If ci_master_key is NULL, then the master key
 510          * was provided via the legacy mechanism of the process-subscribed
 511          * keyrings, so we don't know whether it's been removed or not.
 512          */
 513         if (!ci || !ci->ci_master_key)
 514                 return 0;
 515         mk = ci->ci_master_key->payload.data[0];
 516
 517         /*
 518          * Note: since we aren't holding ->mk_secret_sem, the result here can
 519          * immediately become outdated.  But there's no correctness problem with
 520          * unnecessarily evicting.  Nor is there a correctness problem with not
 521          * evicting while iput() is racing with the key being removed, since
 522          * then the thread removing the key will either evict the inode itself
 523          * or will correctly detect that it wasn't evicted due to the race.
 524          */
 525         return !is_master_key_secret_present(&mk->mk_secret);
 526 }
 527 EXPORT_SYMBOL_GPL(fscrypt_drop_inode);