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/skcipher.h> 24 #include <crypto/utils.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[FSCRYPT_FILE_NONCE_SIZE], 49 u8 *derived_key, unsigned int derived_keysize) 50 { 51 struct crypto_sync_skcipher *tfm; 52 int err; 53 54 tfm = crypto_alloc_sync_skcipher("ecb(aes)", 0, FSCRYPT_CRYPTOAPI_MASK); 55 if (IS_ERR(tfm)) 56 return PTR_ERR(tfm); 57 58 err = crypto_sync_skcipher_setkey(tfm, nonce, FSCRYPT_FILE_NONCE_SIZE); 59 if (err == 0) { 60 SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm); 61 struct scatterlist src_sg, dst_sg; 62 63 skcipher_request_set_callback(req, 64 CRYPTO_TFM_REQ_MAY_BACKLOG | 65 CRYPTO_TFM_REQ_MAY_SLEEP, 66 NULL, NULL); 67 sg_init_one(&src_sg, master_key, derived_keysize); 68 sg_init_one(&dst_sg, derived_key, derived_keysize); 69 skcipher_request_set_crypt(req, &src_sg, &dst_sg, 70 derived_keysize, NULL); 71 err = crypto_skcipher_encrypt(req); 72 } 73 crypto_free_sync_skcipher(tfm); 74 return err; 75 } 76 77 /* 78 * Search the current task's subscribed keyrings for a "logon" key with 79 * description prefix:descriptor, and if found acquire a read lock on it and 80 * return a pointer to its validated payload in *payload_ret. 81 */ 82 static struct key * 83 find_and_lock_process_key(const char *prefix, 84 const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE], 85 unsigned int min_keysize, 86 const struct fscrypt_key **payload_ret) 87 { 88 char *description; 89 struct key *key; 90 const struct user_key_payload *ukp; 91 const struct fscrypt_key *payload; 92 93 description = kasprintf(GFP_KERNEL, "%s%*phN", prefix, 94 FSCRYPT_KEY_DESCRIPTOR_SIZE, descriptor); 95 if (!description) 96 return ERR_PTR(-ENOMEM); 97 98 key = request_key(&key_type_logon, description, NULL); 99 kfree(description); 100 if (IS_ERR(key)) 101 return key; 102 103 down_read(&key->sem); 104 ukp = user_key_payload_locked(key); 105 106 if (!ukp) /* was the key revoked before we acquired its semaphore? */ 107 goto invalid; 108 109 payload = (const struct fscrypt_key *)ukp->data; 110 111 if (ukp->datalen != sizeof(struct fscrypt_key) || 112 payload->size < 1 || payload->size > sizeof(payload->raw)) { 113 fscrypt_warn(NULL, 114 "key with description '%s' has invalid payload", 115 key->description); 116 goto invalid; 117 } 118 119 if (payload->size < min_keysize) { 120 fscrypt_warn(NULL, 121 "key with description '%s' is too short (got %u bytes, need %u+ bytes)", 122 key->description, payload->size, min_keysize); 123 goto invalid; 124 } 125 126 *payload_ret = payload; 127 return key; 128 129 invalid: 130 up_read(&key->sem); 131 key_put(key); 132 return ERR_PTR(-ENOKEY); 133 } 134 135 /* Master key referenced by DIRECT_KEY policy */ 136 struct fscrypt_direct_key { 137 struct super_block *dk_sb; 138 struct hlist_node dk_node; 139 refcount_t dk_refcount; 140 const struct fscrypt_mode *dk_mode; 141 struct fscrypt_prepared_key dk_key; 142 u8 dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 143 u8 dk_raw[FSCRYPT_MAX_RAW_KEY_SIZE]; 144 }; 145 146 static void free_direct_key(struct fscrypt_direct_key *dk) 147 { 148 if (dk) { 149 fscrypt_destroy_prepared_key(dk->dk_sb, &dk->dk_key); 150 kfree_sensitive(dk); 151 } 152 } 153 154 void fscrypt_put_direct_key(struct fscrypt_direct_key *dk) 155 { 156 if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock)) 157 return; 158 hash_del(&dk->dk_node); 159 spin_unlock(&fscrypt_direct_keys_lock); 160 161 free_direct_key(dk); 162 } 163 164 /* 165 * Find/insert the given key into the fscrypt_direct_keys table. If found, it 166 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL. If 167 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise 168 * NULL is returned. 169 */ 170 static struct fscrypt_direct_key * 171 find_or_insert_direct_key(struct fscrypt_direct_key *to_insert, 172 const u8 *raw_key, 173 const struct fscrypt_inode_info *ci) 174 { 175 unsigned long hash_key; 176 struct fscrypt_direct_key *dk; 177 178 /* 179 * Careful: to avoid potentially leaking secret key bytes via timing 180 * information, we must key the hash table by descriptor rather than by 181 * raw key, and use crypto_memneq() when comparing raw keys. 182 */ 183 184 BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE); 185 memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor, 186 sizeof(hash_key)); 187 188 spin_lock(&fscrypt_direct_keys_lock); 189 hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) { 190 if (memcmp(ci->ci_policy.v1.master_key_descriptor, 191 dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0) 192 continue; 193 if (ci->ci_mode != dk->dk_mode) 194 continue; 195 if (!fscrypt_is_key_prepared(&dk->dk_key, ci)) 196 continue; 197 if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize)) 198 continue; 199 /* using existing tfm with same (descriptor, mode, raw_key) */ 200 refcount_inc(&dk->dk_refcount); 201 spin_unlock(&fscrypt_direct_keys_lock); 202 free_direct_key(to_insert); 203 return dk; 204 } 205 if (to_insert) 206 hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key); 207 spin_unlock(&fscrypt_direct_keys_lock); 208 return to_insert; 209 } 210 211 /* Prepare to encrypt directly using the master key in the given mode */ 212 static struct fscrypt_direct_key * 213 fscrypt_get_direct_key(const struct fscrypt_inode_info *ci, const u8 *raw_key) 214 { 215 struct fscrypt_direct_key *dk; 216 int err; 217 218 /* Is there already a tfm for this key? */ 219 dk = find_or_insert_direct_key(NULL, raw_key, ci); 220 if (dk) 221 return dk; 222 223 /* Nope, allocate one. */ 224 dk = kzalloc(sizeof(*dk), GFP_KERNEL); 225 if (!dk) 226 return ERR_PTR(-ENOMEM); 227 dk->dk_sb = ci->ci_inode->i_sb; 228 refcount_set(&dk->dk_refcount, 1); 229 dk->dk_mode = ci->ci_mode; 230 err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci); 231 if (err) 232 goto err_free_dk; 233 memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor, 234 FSCRYPT_KEY_DESCRIPTOR_SIZE); 235 memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize); 236 237 return find_or_insert_direct_key(dk, raw_key, ci); 238 239 err_free_dk: 240 free_direct_key(dk); 241 return ERR_PTR(err); 242 } 243 244 /* v1 policy, DIRECT_KEY: use the master key directly */ 245 static int setup_v1_file_key_direct(struct fscrypt_inode_info *ci, 246 const u8 *raw_master_key) 247 { 248 struct fscrypt_direct_key *dk; 249 250 dk = fscrypt_get_direct_key(ci, raw_master_key); 251 if (IS_ERR(dk)) 252 return PTR_ERR(dk); 253 ci->ci_direct_key = dk; 254 ci->ci_enc_key = dk->dk_key; 255 return 0; 256 } 257 258 /* v1 policy, !DIRECT_KEY: derive the file's encryption key */ 259 static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci, 260 const u8 *raw_master_key) 261 { 262 u8 *derived_key; 263 int err; 264 265 /* 266 * This cannot be a stack buffer because it will be passed to the 267 * scatterlist crypto API during derive_key_aes(). 268 */ 269 derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL); 270 if (!derived_key) 271 return -ENOMEM; 272 273 err = derive_key_aes(raw_master_key, ci->ci_nonce, 274 derived_key, ci->ci_mode->keysize); 275 if (err) 276 goto out; 277 278 err = fscrypt_set_per_file_enc_key(ci, derived_key); 279 out: 280 kfree_sensitive(derived_key); 281 return err; 282 } 283 284 int fscrypt_setup_v1_file_key(struct fscrypt_inode_info *ci, 285 const u8 *raw_master_key) 286 { 287 if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) 288 return setup_v1_file_key_direct(ci, raw_master_key); 289 else 290 return setup_v1_file_key_derived(ci, raw_master_key); 291 } 292 293 int 294 fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info *ci) 295 { 296 const struct super_block *sb = ci->ci_inode->i_sb; 297 struct key *key; 298 const struct fscrypt_key *payload; 299 int err; 300 301 key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX, 302 ci->ci_policy.v1.master_key_descriptor, 303 ci->ci_mode->keysize, &payload); 304 if (key == ERR_PTR(-ENOKEY) && sb->s_cop->legacy_key_prefix) { 305 key = find_and_lock_process_key(sb->s_cop->legacy_key_prefix, 306 ci->ci_policy.v1.master_key_descriptor, 307 ci->ci_mode->keysize, &payload); 308 } 309 if (IS_ERR(key)) 310 return PTR_ERR(key); 311 312 err = fscrypt_setup_v1_file_key(ci, payload->raw); 313 up_read(&key->sem); 314 key_put(key); 315 return err; 316 } 317