1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * Key setup for v1 encryption policies 4 * 5 * Copyright 2015, 2019 Google LLC 6 */ 7 8 /* 9 * This file implements compatibility functions for the original encryption 10 * policy version ("v1"), including: 11 * 12 * - Deriving per-file encryption keys using the AES-128-ECB based KDF 13 * (rather than the new method of using HKDF-SHA512) 14 * 15 * - Retrieving fscrypt master keys from process-subscribed keyrings 16 * (rather than the new method of using a filesystem-level keyring) 17 * 18 * - Handling policies with the DIRECT_KEY flag set using a master key table 19 * (rather than the new method of implementing DIRECT_KEY with per-mode keys 20 * managed alongside the master keys in the filesystem-level keyring) 21 */ 22 23 #include <crypto/algapi.h> 24 #include <crypto/skcipher.h> 25 #include <keys/user-type.h> 26 #include <linux/hashtable.h> 27 #include <linux/scatterlist.h> 28 29 #include "fscrypt_private.h" 30 31 /* Table of keys referenced by DIRECT_KEY policies */ 32 static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */ 33 static DEFINE_SPINLOCK(fscrypt_direct_keys_lock); 34 35 /* 36 * v1 key derivation function. This generates the derived key by encrypting the 37 * master key with AES-128-ECB using the nonce as the AES key. This provides a 38 * unique derived key with sufficient entropy for each inode. However, it's 39 * nonstandard, non-extensible, doesn't evenly distribute the entropy from the 40 * master key, and is trivially reversible: an attacker who compromises a 41 * derived key can "decrypt" it to get back to the master key, then derive any 42 * other key. For all new code, use HKDF instead. 43 * 44 * The master key must be at least as long as the derived key. If the master 45 * key is longer, then only the first 'derived_keysize' bytes are used. 46 */ 47 static int derive_key_aes(const u8 *master_key, 48 const u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE], 49 u8 *derived_key, unsigned int derived_keysize) 50 { 51 int res = 0; 52 struct skcipher_request *req = NULL; 53 DECLARE_CRYPTO_WAIT(wait); 54 struct scatterlist src_sg, dst_sg; 55 struct crypto_skcipher *tfm = crypto_alloc_skcipher("ecb(aes)", 0, 0); 56 57 if (IS_ERR(tfm)) { 58 res = PTR_ERR(tfm); 59 tfm = NULL; 60 goto out; 61 } 62 crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_FORBID_WEAK_KEYS); 63 req = skcipher_request_alloc(tfm, GFP_NOFS); 64 if (!req) { 65 res = -ENOMEM; 66 goto out; 67 } 68 skcipher_request_set_callback(req, 69 CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP, 70 crypto_req_done, &wait); 71 res = crypto_skcipher_setkey(tfm, nonce, FS_KEY_DERIVATION_NONCE_SIZE); 72 if (res < 0) 73 goto out; 74 75 sg_init_one(&src_sg, master_key, derived_keysize); 76 sg_init_one(&dst_sg, derived_key, derived_keysize); 77 skcipher_request_set_crypt(req, &src_sg, &dst_sg, derived_keysize, 78 NULL); 79 res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait); 80 out: 81 skcipher_request_free(req); 82 crypto_free_skcipher(tfm); 83 return res; 84 } 85 86 /* 87 * Search the current task's subscribed keyrings for a "logon" key with 88 * description prefix:descriptor, and if found acquire a read lock on it and 89 * return a pointer to its validated payload in *payload_ret. 90 */ 91 static struct key * 92 find_and_lock_process_key(const char *prefix, 93 const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE], 94 unsigned int min_keysize, 95 const struct fscrypt_key **payload_ret) 96 { 97 char *description; 98 struct key *key; 99 const struct user_key_payload *ukp; 100 const struct fscrypt_key *payload; 101 102 description = kasprintf(GFP_NOFS, "%s%*phN", prefix, 103 FSCRYPT_KEY_DESCRIPTOR_SIZE, descriptor); 104 if (!description) 105 return ERR_PTR(-ENOMEM); 106 107 key = request_key(&key_type_logon, description, NULL); 108 kfree(description); 109 if (IS_ERR(key)) 110 return key; 111 112 down_read(&key->sem); 113 ukp = user_key_payload_locked(key); 114 115 if (!ukp) /* was the key revoked before we acquired its semaphore? */ 116 goto invalid; 117 118 payload = (const struct fscrypt_key *)ukp->data; 119 120 if (ukp->datalen != sizeof(struct fscrypt_key) || 121 payload->size < 1 || payload->size > FSCRYPT_MAX_KEY_SIZE) { 122 fscrypt_warn(NULL, 123 "key with description '%s' has invalid payload", 124 key->description); 125 goto invalid; 126 } 127 128 if (payload->size < min_keysize) { 129 fscrypt_warn(NULL, 130 "key with description '%s' is too short (got %u bytes, need %u+ bytes)", 131 key->description, payload->size, min_keysize); 132 goto invalid; 133 } 134 135 *payload_ret = payload; 136 return key; 137 138 invalid: 139 up_read(&key->sem); 140 key_put(key); 141 return ERR_PTR(-ENOKEY); 142 } 143 144 /* Master key referenced by DIRECT_KEY policy */ 145 struct fscrypt_direct_key { 146 struct hlist_node dk_node; 147 refcount_t dk_refcount; 148 const struct fscrypt_mode *dk_mode; 149 struct crypto_skcipher *dk_ctfm; 150 u8 dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 151 u8 dk_raw[FSCRYPT_MAX_KEY_SIZE]; 152 }; 153 154 static void free_direct_key(struct fscrypt_direct_key *dk) 155 { 156 if (dk) { 157 crypto_free_skcipher(dk->dk_ctfm); 158 kzfree(dk); 159 } 160 } 161 162 void fscrypt_put_direct_key(struct fscrypt_direct_key *dk) 163 { 164 if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock)) 165 return; 166 hash_del(&dk->dk_node); 167 spin_unlock(&fscrypt_direct_keys_lock); 168 169 free_direct_key(dk); 170 } 171 172 /* 173 * Find/insert the given key into the fscrypt_direct_keys table. If found, it 174 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL. If 175 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise 176 * NULL is returned. 177 */ 178 static struct fscrypt_direct_key * 179 find_or_insert_direct_key(struct fscrypt_direct_key *to_insert, 180 const u8 *raw_key, const struct fscrypt_info *ci) 181 { 182 unsigned long hash_key; 183 struct fscrypt_direct_key *dk; 184 185 /* 186 * Careful: to avoid potentially leaking secret key bytes via timing 187 * information, we must key the hash table by descriptor rather than by 188 * raw key, and use crypto_memneq() when comparing raw keys. 189 */ 190 191 BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE); 192 memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor, 193 sizeof(hash_key)); 194 195 spin_lock(&fscrypt_direct_keys_lock); 196 hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) { 197 if (memcmp(ci->ci_policy.v1.master_key_descriptor, 198 dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0) 199 continue; 200 if (ci->ci_mode != dk->dk_mode) 201 continue; 202 if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize)) 203 continue; 204 /* using existing tfm with same (descriptor, mode, raw_key) */ 205 refcount_inc(&dk->dk_refcount); 206 spin_unlock(&fscrypt_direct_keys_lock); 207 free_direct_key(to_insert); 208 return dk; 209 } 210 if (to_insert) 211 hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key); 212 spin_unlock(&fscrypt_direct_keys_lock); 213 return to_insert; 214 } 215 216 /* Prepare to encrypt directly using the master key in the given mode */ 217 static struct fscrypt_direct_key * 218 fscrypt_get_direct_key(const struct fscrypt_info *ci, const u8 *raw_key) 219 { 220 struct fscrypt_direct_key *dk; 221 int err; 222 223 /* Is there already a tfm for this key? */ 224 dk = find_or_insert_direct_key(NULL, raw_key, ci); 225 if (dk) 226 return dk; 227 228 /* Nope, allocate one. */ 229 dk = kzalloc(sizeof(*dk), GFP_NOFS); 230 if (!dk) 231 return ERR_PTR(-ENOMEM); 232 refcount_set(&dk->dk_refcount, 1); 233 dk->dk_mode = ci->ci_mode; 234 dk->dk_ctfm = fscrypt_allocate_skcipher(ci->ci_mode, raw_key, 235 ci->ci_inode); 236 if (IS_ERR(dk->dk_ctfm)) { 237 err = PTR_ERR(dk->dk_ctfm); 238 dk->dk_ctfm = NULL; 239 goto err_free_dk; 240 } 241 memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor, 242 FSCRYPT_KEY_DESCRIPTOR_SIZE); 243 memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize); 244 245 return find_or_insert_direct_key(dk, raw_key, ci); 246 247 err_free_dk: 248 free_direct_key(dk); 249 return ERR_PTR(err); 250 } 251 252 /* v1 policy, DIRECT_KEY: use the master key directly */ 253 static int setup_v1_file_key_direct(struct fscrypt_info *ci, 254 const u8 *raw_master_key) 255 { 256 struct fscrypt_direct_key *dk; 257 258 dk = fscrypt_get_direct_key(ci, raw_master_key); 259 if (IS_ERR(dk)) 260 return PTR_ERR(dk); 261 ci->ci_direct_key = dk; 262 ci->ci_ctfm = dk->dk_ctfm; 263 return 0; 264 } 265 266 /* v1 policy, !DIRECT_KEY: derive the file's encryption key */ 267 static int setup_v1_file_key_derived(struct fscrypt_info *ci, 268 const u8 *raw_master_key) 269 { 270 u8 *derived_key; 271 int err; 272 273 /* 274 * This cannot be a stack buffer because it will be passed to the 275 * scatterlist crypto API during derive_key_aes(). 276 */ 277 derived_key = kmalloc(ci->ci_mode->keysize, GFP_NOFS); 278 if (!derived_key) 279 return -ENOMEM; 280 281 err = derive_key_aes(raw_master_key, ci->ci_nonce, 282 derived_key, ci->ci_mode->keysize); 283 if (err) 284 goto out; 285 286 err = fscrypt_set_per_file_enc_key(ci, derived_key); 287 out: 288 kzfree(derived_key); 289 return err; 290 } 291 292 int fscrypt_setup_v1_file_key(struct fscrypt_info *ci, const u8 *raw_master_key) 293 { 294 if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) 295 return setup_v1_file_key_direct(ci, raw_master_key); 296 else 297 return setup_v1_file_key_derived(ci, raw_master_key); 298 } 299 300 int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci) 301 { 302 struct key *key; 303 const struct fscrypt_key *payload; 304 int err; 305 306 key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX, 307 ci->ci_policy.v1.master_key_descriptor, 308 ci->ci_mode->keysize, &payload); 309 if (key == ERR_PTR(-ENOKEY) && ci->ci_inode->i_sb->s_cop->key_prefix) { 310 key = find_and_lock_process_key(ci->ci_inode->i_sb->s_cop->key_prefix, 311 ci->ci_policy.v1.master_key_descriptor, 312 ci->ci_mode->keysize, &payload); 313 } 314 if (IS_ERR(key)) 315 return PTR_ERR(key); 316 317 err = fscrypt_setup_v1_file_key(ci, payload->raw); 318 up_read(&key->sem); 319 key_put(key); 320 return err; 321 } 322