fs/crypto/keyinfo.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * key management facility for FS encryption support.
   4  *
   5  * Copyright (C) 2015, Google, Inc.
   6  *
   7  * This contains encryption key functions.
   8  *
   9  * Written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar, 2015.
  10  */
  11
  12 #include <keys/user-type.h>
  13 #include <linux/scatterlist.h>
  14 #include <linux/ratelimit.h>
  15 #include <crypto/aes.h>
  16 #include <crypto/sha.h>
  17 #include <crypto/skcipher.h>
  18 #include "fscrypt_private.h"
  19
  20 static struct crypto_shash *essiv_hash_tfm;
  21
  22 /*
  23  * Key derivation function.  This generates the derived key by encrypting the
  24  * master key with AES-128-ECB using the inode's nonce as the AES key.
  25  *
  26  * The master key must be at least as long as the derived key.  If the master
  27  * key is longer, then only the first 'derived_keysize' bytes are used.
  28  */
  29 static int derive_key_aes(const u8 *master_key,
  30                           const struct fscrypt_context *ctx,
  31                           u8 *derived_key, unsigned int derived_keysize)
  32 {
  33         int res = 0;
  34         struct skcipher_request *req = NULL;
  35         DECLARE_CRYPTO_WAIT(wait);
  36         struct scatterlist src_sg, dst_sg;
  37         struct crypto_skcipher *tfm = crypto_alloc_skcipher("ecb(aes)", 0, 0);
  38
  39         if (IS_ERR(tfm)) {
  40                 res = PTR_ERR(tfm);
  41                 tfm = NULL;
  42                 goto out;
  43         }
  44         crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_WEAK_KEY);
  45         req = skcipher_request_alloc(tfm, GFP_NOFS);
  46         if (!req) {
  47                 res = -ENOMEM;
  48                 goto out;
  49         }
  50         skcipher_request_set_callback(req,
  51                         CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
  52                         crypto_req_done, &wait);
  53         res = crypto_skcipher_setkey(tfm, ctx->nonce, sizeof(ctx->nonce));
  54         if (res < 0)
  55                 goto out;
  56
  57         sg_init_one(&src_sg, master_key, derived_keysize);
  58         sg_init_one(&dst_sg, derived_key, derived_keysize);
  59         skcipher_request_set_crypt(req, &src_sg, &dst_sg, derived_keysize,
  60                                    NULL);
  61         res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
  62 out:
  63         skcipher_request_free(req);
  64         crypto_free_skcipher(tfm);
  65         return res;
  66 }
  67
  68 /*
  69  * Search the current task's subscribed keyrings for a "logon" key with
  70  * description prefix:descriptor, and if found acquire a read lock on it and
  71  * return a pointer to its validated payload in *payload_ret.
  72  */
  73 static struct key *
  74 find_and_lock_process_key(const char *prefix,
  75                           const u8 descriptor[FS_KEY_DESCRIPTOR_SIZE],
  76                           unsigned int min_keysize,
  77                           const struct fscrypt_key **payload_ret)
  78 {
  79         char *description;
  80         struct key *key;
  81         const struct user_key_payload *ukp;
  82         const struct fscrypt_key *payload;
  83
  84         description = kasprintf(GFP_NOFS, "%s%*phN", prefix,
  85                                 FS_KEY_DESCRIPTOR_SIZE, descriptor);
  86         if (!description)
  87                 return ERR_PTR(-ENOMEM);
  88
  89         key = request_key(&key_type_logon, description, NULL);
  90         kfree(description);
  91         if (IS_ERR(key))
  92                 return key;
  93
  94         down_read(&key->sem);
  95         ukp = user_key_payload_locked(key);
  96
  97         if (!ukp) /* was the key revoked before we acquired its semaphore? */
  98                 goto invalid;
  99
 100         payload = (const struct fscrypt_key *)ukp->data;
 101
 102         if (ukp->datalen != sizeof(struct fscrypt_key) ||
 103             payload->size < 1 || payload->size > FS_MAX_KEY_SIZE) {
 104                 fscrypt_warn(NULL,
 105                              "key with description '%s' has invalid payload",
 106                              key->description);
 107                 goto invalid;
 108         }
 109
 110         if (payload->size < min_keysize) {
 111                 fscrypt_warn(NULL,
 112                              "key with description '%s' is too short (got %u bytes, need %u+ bytes)",
 113                              key->description, payload->size, min_keysize);
 114                 goto invalid;
 115         }
 116
 117         *payload_ret = payload;
 118         return key;
 119
 120 invalid:
 121         up_read(&key->sem);
 122         key_put(key);
 123         return ERR_PTR(-ENOKEY);
 124 }
 125
 126 /* Find the master key, then derive the inode's actual encryption key */
 127 static int find_and_derive_key(const struct inode *inode,
 128                                const struct fscrypt_context *ctx,
 129                                u8 *derived_key, unsigned int derived_keysize)
 130 {
 131         struct key *key;
 132         const struct fscrypt_key *payload;
 133         int err;
 134
 135         key = find_and_lock_process_key(FS_KEY_DESC_PREFIX,
 136                                         ctx->master_key_descriptor,
 137                                         derived_keysize, &payload);
 138         if (key == ERR_PTR(-ENOKEY) && inode->i_sb->s_cop->key_prefix) {
 139                 key = find_and_lock_process_key(inode->i_sb->s_cop->key_prefix,
 140                                                 ctx->master_key_descriptor,
 141                                                 derived_keysize, &payload);
 142         }
 143         if (IS_ERR(key))
 144                 return PTR_ERR(key);
 145         err = derive_key_aes(payload->raw, ctx, derived_key, derived_keysize);
 146         up_read(&key->sem);
 147         key_put(key);
 148         return err;
 149 }
 150
 151 static struct fscrypt_mode {
 152         const char *friendly_name;
 153         const char *cipher_str;
 154         int keysize;
 155         bool logged_impl_name;
 156 } available_modes[] = {
 157         [FS_ENCRYPTION_MODE_AES_256_XTS] = {
 158                 .friendly_name = "AES-256-XTS",
 159                 .cipher_str = "xts(aes)",
 160                 .keysize = 64,
 161         },
 162         [FS_ENCRYPTION_MODE_AES_256_CTS] = {
 163                 .friendly_name = "AES-256-CTS-CBC",
 164                 .cipher_str = "cts(cbc(aes))",
 165                 .keysize = 32,
 166         },
 167         [FS_ENCRYPTION_MODE_AES_128_CBC] = {
 168                 .friendly_name = "AES-128-CBC",
 169                 .cipher_str = "cbc(aes)",
 170                 .keysize = 16,
 171         },
 172         [FS_ENCRYPTION_MODE_AES_128_CTS] = {
 173                 .friendly_name = "AES-128-CTS-CBC",
 174                 .cipher_str = "cts(cbc(aes))",
 175                 .keysize = 16,
 176         },
 177 };
 178
 179 static struct fscrypt_mode *
 180 select_encryption_mode(const struct fscrypt_info *ci, const struct inode *inode)
 181 {
 182         if (!fscrypt_valid_enc_modes(ci->ci_data_mode, ci->ci_filename_mode)) {
 183                 fscrypt_warn(inode->i_sb,
 184                              "inode %lu uses unsupported encryption modes (contents mode %d, filenames mode %d)",
 185                              inode->i_ino, ci->ci_data_mode,
 186                              ci->ci_filename_mode);
 187                 return ERR_PTR(-EINVAL);
 188         }
 189
 190         if (S_ISREG(inode->i_mode))
 191                 return &available_modes[ci->ci_data_mode];
 192
 193         if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
 194                 return &available_modes[ci->ci_filename_mode];
 195
 196         WARN_ONCE(1, "fscrypt: filesystem tried to load encryption info for inode %lu, which is not encryptable (file type %d)\n",
 197                   inode->i_ino, (inode->i_mode & S_IFMT));
 198         return ERR_PTR(-EINVAL);
 199 }
 200
 201 static void put_crypt_info(struct fscrypt_info *ci)
 202 {
 203         if (!ci)
 204                 return;
 205
 206         crypto_free_skcipher(ci->ci_ctfm);
 207         crypto_free_cipher(ci->ci_essiv_tfm);
 208         kmem_cache_free(fscrypt_info_cachep, ci);
 209 }
 210
 211 static int derive_essiv_salt(const u8 *key, int keysize, u8 *salt)
 212 {
 213         struct crypto_shash *tfm = READ_ONCE(essiv_hash_tfm);
 214
 215         /* init hash transform on demand */
 216         if (unlikely(!tfm)) {
 217                 struct crypto_shash *prev_tfm;
 218
 219                 tfm = crypto_alloc_shash("sha256", 0, 0);
 220                 if (IS_ERR(tfm)) {
 221                         fscrypt_warn(NULL,
 222                                      "error allocating SHA-256 transform: %ld",
 223                                      PTR_ERR(tfm));
 224                         return PTR_ERR(tfm);
 225                 }
 226                 prev_tfm = cmpxchg(&essiv_hash_tfm, NULL, tfm);
 227                 if (prev_tfm) {
 228                         crypto_free_shash(tfm);
 229                         tfm = prev_tfm;
 230                 }
 231         }
 232
 233         {
 234                 SHASH_DESC_ON_STACK(desc, tfm);
 235                 desc->tfm = tfm;
 236                 desc->flags = 0;
 237
 238                 return crypto_shash_digest(desc, key, keysize, salt);
 239         }
 240 }
 241
 242 static int init_essiv_generator(struct fscrypt_info *ci, const u8 *raw_key,
 243                                 int keysize)
 244 {
 245         int err;
 246         struct crypto_cipher *essiv_tfm;
 247         u8 salt[SHA256_DIGEST_SIZE];
 248
 249         essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
 250         if (IS_ERR(essiv_tfm))
 251                 return PTR_ERR(essiv_tfm);
 252
 253         ci->ci_essiv_tfm = essiv_tfm;
 254
 255         err = derive_essiv_salt(raw_key, keysize, salt);
 256         if (err)
 257                 goto out;
 258
 259         /*
 260          * Using SHA256 to derive the salt/key will result in AES-256 being
 261          * used for IV generation. File contents encryption will still use the
 262          * configured keysize (AES-128) nevertheless.
 263          */
 264         err = crypto_cipher_setkey(essiv_tfm, salt, sizeof(salt));
 265         if (err)
 266                 goto out;
 267
 268 out:
 269         memzero_explicit(salt, sizeof(salt));
 270         return err;
 271 }
 272
 273 void __exit fscrypt_essiv_cleanup(void)
 274 {
 275         crypto_free_shash(essiv_hash_tfm);
 276 }
 277
 278 int fscrypt_get_encryption_info(struct inode *inode)
 279 {
 280         struct fscrypt_info *crypt_info;
 281         struct fscrypt_context ctx;
 282         struct crypto_skcipher *ctfm;
 283         struct fscrypt_mode *mode;
 284         u8 *raw_key = NULL;
 285         int res;
 286
 287         if (inode->i_crypt_info)
 288                 return 0;
 289
 290         res = fscrypt_initialize(inode->i_sb->s_cop->flags);
 291         if (res)
 292                 return res;
 293
 294         res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx));
 295         if (res < 0) {
 296                 if (!fscrypt_dummy_context_enabled(inode) ||
 297                     IS_ENCRYPTED(inode))
 298                         return res;
 299                 /* Fake up a context for an unencrypted directory */
 300                 memset(&ctx, 0, sizeof(ctx));
 301                 ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1;
 302                 ctx.contents_encryption_mode = FS_ENCRYPTION_MODE_AES_256_XTS;
 303                 ctx.filenames_encryption_mode = FS_ENCRYPTION_MODE_AES_256_CTS;
 304                 memset(ctx.master_key_descriptor, 0x42, FS_KEY_DESCRIPTOR_SIZE);
 305         } else if (res != sizeof(ctx)) {
 306                 return -EINVAL;
 307         }
 308
 309         if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1)
 310                 return -EINVAL;
 311
 312         if (ctx.flags & ~FS_POLICY_FLAGS_VALID)
 313                 return -EINVAL;
 314
 315         crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS);
 316         if (!crypt_info)
 317                 return -ENOMEM;
 318
 319         crypt_info->ci_flags = ctx.flags;
 320         crypt_info->ci_data_mode = ctx.contents_encryption_mode;
 321         crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
 322         crypt_info->ci_ctfm = NULL;
 323         crypt_info->ci_essiv_tfm = NULL;
 324         memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
 325                                 sizeof(crypt_info->ci_master_key));
 326
 327         mode = select_encryption_mode(crypt_info, inode);
 328         if (IS_ERR(mode)) {
 329                 res = PTR_ERR(mode);
 330                 goto out;
 331         }
 332
 333         /*
 334          * This cannot be a stack buffer because it is passed to the scatterlist
 335          * crypto API as part of key derivation.
 336          */
 337         res = -ENOMEM;
 338         raw_key = kmalloc(mode->keysize, GFP_NOFS);
 339         if (!raw_key)
 340                 goto out;
 341
 342         res = find_and_derive_key(inode, &ctx, raw_key, mode->keysize);
 343         if (res)
 344                 goto out;
 345
 346         ctfm = crypto_alloc_skcipher(mode->cipher_str, 0, 0);
 347         if (IS_ERR(ctfm)) {
 348                 res = PTR_ERR(ctfm);
 349                 fscrypt_warn(inode->i_sb,
 350                              "error allocating '%s' transform for inode %lu: %d",
 351                              mode->cipher_str, inode->i_ino, res);
 352                 goto out;
 353         }
 354         if (unlikely(!mode->logged_impl_name)) {
 355                 /*
 356                  * fscrypt performance can vary greatly depending on which
 357                  * crypto algorithm implementation is used.  Help people debug
 358                  * performance problems by logging the ->cra_driver_name the
 359                  * first time a mode is used.  Note that multiple threads can
 360                  * race here, but it doesn't really matter.
 361                  */
 362                 mode->logged_impl_name = true;
 363                 pr_info("fscrypt: %s using implementation \"%s\"\n",
 364                         mode->friendly_name,
 365                         crypto_skcipher_alg(ctfm)->base.cra_driver_name);
 366         }
 367         crypt_info->ci_ctfm = ctfm;
 368         crypto_skcipher_set_flags(ctfm, CRYPTO_TFM_REQ_WEAK_KEY);
 369         res = crypto_skcipher_setkey(ctfm, raw_key, mode->keysize);
 370         if (res)
 371                 goto out;
 372
 373         if (S_ISREG(inode->i_mode) &&
 374             crypt_info->ci_data_mode == FS_ENCRYPTION_MODE_AES_128_CBC) {
 375                 res = init_essiv_generator(crypt_info, raw_key, mode->keysize);
 376                 if (res) {
 377                         fscrypt_warn(inode->i_sb,
 378                                      "error initializing ESSIV generator for inode %lu: %d",
 379                                      inode->i_ino, res);
 380                         goto out;
 381                 }
 382         }
 383         if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) == NULL)
 384                 crypt_info = NULL;
 385 out:
 386         if (res == -ENOKEY)
 387                 res = 0;
 388         put_crypt_info(crypt_info);
 389         kzfree(raw_key);
 390         return res;
 391 }
 392 EXPORT_SYMBOL(fscrypt_get_encryption_info);
 393
 394 void fscrypt_put_encryption_info(struct inode *inode)
 395 {
 396         put_crypt_info(inode->i_crypt_info);
 397         inode->i_crypt_info = NULL;
 398 }
 399 EXPORT_SYMBOL(fscrypt_put_encryption_info);