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/aes.h> 24 #include <crypto/utils.h> 25 #include <keys/user-type.h> 26 #include <linux/hashtable.h> 27 28 #include "fscrypt_private.h" 29 30 /* Table of keys referenced by DIRECT_KEY policies */ 31 static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */ 32 static DEFINE_SPINLOCK(fscrypt_direct_keys_lock); 33 34 /* 35 * Search the current task's subscribed keyrings for a "logon" key with 36 * description prefix:descriptor, and if found acquire a read lock on it and 37 * return a pointer to its validated payload in *payload_ret. 38 */ 39 static struct key * 40 find_and_lock_process_key(const char *prefix, 41 const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE], 42 unsigned int min_keysize, 43 const struct fscrypt_key **payload_ret) 44 { 45 char *description; 46 struct key *key; 47 const struct user_key_payload *ukp; 48 const struct fscrypt_key *payload; 49 50 description = kasprintf(GFP_KERNEL, "%s%*phN", prefix, 51 FSCRYPT_KEY_DESCRIPTOR_SIZE, descriptor); 52 if (!description) 53 return ERR_PTR(-ENOMEM); 54 55 key = request_key(&key_type_logon, description, NULL); 56 kfree(description); 57 if (IS_ERR(key)) 58 return key; 59 60 down_read(&key->sem); 61 ukp = user_key_payload_locked(key); 62 63 if (!ukp) /* was the key revoked before we acquired its semaphore? */ 64 goto invalid; 65 66 payload = (const struct fscrypt_key *)ukp->data; 67 68 if (ukp->datalen != sizeof(struct fscrypt_key) || 69 payload->size < 1 || payload->size > sizeof(payload->raw)) { 70 fscrypt_warn(NULL, 71 "key with description '%s' has invalid payload", 72 key->description); 73 goto invalid; 74 } 75 76 if (payload->size < min_keysize) { 77 fscrypt_warn(NULL, 78 "key with description '%s' is too short (got %u bytes, need %u+ bytes)", 79 key->description, payload->size, min_keysize); 80 goto invalid; 81 } 82 83 *payload_ret = payload; 84 return key; 85 86 invalid: 87 up_read(&key->sem); 88 key_put(key); 89 return ERR_PTR(-ENOKEY); 90 } 91 92 /* Master key referenced by DIRECT_KEY policy */ 93 struct fscrypt_direct_key { 94 struct super_block *dk_sb; 95 struct hlist_node dk_node; 96 refcount_t dk_refcount; 97 const struct fscrypt_mode *dk_mode; 98 struct fscrypt_prepared_key dk_key; 99 u8 dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 100 u8 dk_raw[FSCRYPT_MAX_RAW_KEY_SIZE]; 101 }; 102 103 static void free_direct_key(struct fscrypt_direct_key *dk) 104 { 105 if (dk) { 106 fscrypt_destroy_prepared_key(dk->dk_sb, &dk->dk_key); 107 kfree_sensitive(dk); 108 } 109 } 110 111 void fscrypt_put_direct_key(struct fscrypt_direct_key *dk) 112 { 113 if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock)) 114 return; 115 hash_del(&dk->dk_node); 116 spin_unlock(&fscrypt_direct_keys_lock); 117 118 free_direct_key(dk); 119 } 120 121 /* 122 * Find/insert the given key into the fscrypt_direct_keys table. If found, it 123 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL. If 124 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise 125 * NULL is returned. 126 */ 127 static struct fscrypt_direct_key * 128 find_or_insert_direct_key(struct fscrypt_direct_key *to_insert, 129 const u8 *raw_key, 130 const struct fscrypt_inode_info *ci) 131 { 132 unsigned long hash_key; 133 struct fscrypt_direct_key *dk; 134 135 /* 136 * Careful: to avoid potentially leaking secret key bytes via timing 137 * information, we must key the hash table by descriptor rather than by 138 * raw key, and use crypto_memneq() when comparing raw keys. 139 */ 140 141 BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE); 142 memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor, 143 sizeof(hash_key)); 144 145 spin_lock(&fscrypt_direct_keys_lock); 146 hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) { 147 if (memcmp(ci->ci_policy.v1.master_key_descriptor, 148 dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0) 149 continue; 150 if (ci->ci_mode != dk->dk_mode) 151 continue; 152 if (!fscrypt_is_key_prepared(&dk->dk_key, ci)) 153 continue; 154 if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize)) 155 continue; 156 /* using existing tfm with same (descriptor, mode, raw_key) */ 157 refcount_inc(&dk->dk_refcount); 158 spin_unlock(&fscrypt_direct_keys_lock); 159 free_direct_key(to_insert); 160 return dk; 161 } 162 if (to_insert) 163 hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key); 164 spin_unlock(&fscrypt_direct_keys_lock); 165 return to_insert; 166 } 167 168 /* Prepare to encrypt directly using the master key in the given mode */ 169 static struct fscrypt_direct_key * 170 fscrypt_get_direct_key(const struct fscrypt_inode_info *ci, const u8 *raw_key) 171 { 172 struct fscrypt_direct_key *dk; 173 int err; 174 175 /* Is there already a tfm for this key? */ 176 dk = find_or_insert_direct_key(NULL, raw_key, ci); 177 if (dk) 178 return dk; 179 180 /* Nope, allocate one. */ 181 dk = kzalloc_obj(*dk); 182 if (!dk) 183 return ERR_PTR(-ENOMEM); 184 dk->dk_sb = ci->ci_inode->i_sb; 185 refcount_set(&dk->dk_refcount, 1); 186 dk->dk_mode = ci->ci_mode; 187 err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci); 188 if (err) 189 goto err_free_dk; 190 memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor, 191 FSCRYPT_KEY_DESCRIPTOR_SIZE); 192 memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize); 193 194 return find_or_insert_direct_key(dk, raw_key, ci); 195 196 err_free_dk: 197 free_direct_key(dk); 198 return ERR_PTR(err); 199 } 200 201 /* v1 policy, DIRECT_KEY: use the master key directly */ 202 static int setup_v1_file_key_direct(struct fscrypt_inode_info *ci, 203 const u8 *raw_master_key) 204 { 205 struct fscrypt_direct_key *dk; 206 207 dk = fscrypt_get_direct_key(ci, raw_master_key); 208 if (IS_ERR(dk)) 209 return PTR_ERR(dk); 210 ci->ci_direct_key = dk; 211 ci->ci_enc_key = dk->dk_key; 212 return 0; 213 } 214 215 /* 216 * v1 policy, !DIRECT_KEY: derive the file's encryption key. 217 * 218 * The v1 key derivation function generates the derived key by encrypting the 219 * master key with AES-128-ECB using the file's nonce as the AES key. This 220 * provides a unique derived key with sufficient entropy for each inode. 221 * However, it's nonstandard, non-extensible, doesn't evenly distribute the 222 * entropy from the master key, and is trivially reversible: an attacker who 223 * compromises a derived key can "decrypt" it to get back to the master key, 224 * then derive any other key. For all new code, use HKDF instead. 225 * 226 * The master key must be at least as long as the derived key. If the master 227 * key is longer, then only the first ci->ci_mode->keysize bytes are used. 228 */ 229 static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci, 230 const u8 *raw_master_key) 231 { 232 const unsigned int derived_keysize = ci->ci_mode->keysize; 233 u8 derived_key[FSCRYPT_MAX_RAW_KEY_SIZE]; 234 struct aes_enckey aes; 235 int err; 236 237 if (WARN_ON_ONCE(derived_keysize > FSCRYPT_MAX_RAW_KEY_SIZE || 238 derived_keysize % AES_BLOCK_SIZE != 0)) 239 return -EINVAL; 240 241 static_assert(FSCRYPT_FILE_NONCE_SIZE == AES_KEYSIZE_128); 242 aes_prepareenckey(&aes, ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE); 243 for (unsigned int i = 0; i < derived_keysize; i += AES_BLOCK_SIZE) 244 aes_encrypt(&aes, &derived_key[i], &raw_master_key[i]); 245 246 err = fscrypt_set_per_file_enc_key(ci, derived_key); 247 248 memzero_explicit(derived_key, derived_keysize); 249 /* No need to zeroize 'aes', as its key is not secret. */ 250 return err; 251 } 252 253 int fscrypt_setup_v1_file_key(struct fscrypt_inode_info *ci, 254 const u8 *raw_master_key) 255 { 256 if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) 257 return setup_v1_file_key_direct(ci, raw_master_key); 258 else 259 return setup_v1_file_key_derived(ci, raw_master_key); 260 } 261 262 int 263 fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info *ci) 264 { 265 const struct super_block *sb = ci->ci_inode->i_sb; 266 struct key *key; 267 const struct fscrypt_key *payload; 268 int err; 269 270 key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX, 271 ci->ci_policy.v1.master_key_descriptor, 272 ci->ci_mode->keysize, &payload); 273 if (key == ERR_PTR(-ENOKEY) && sb->s_cop->legacy_key_prefix) { 274 key = find_and_lock_process_key(sb->s_cop->legacy_key_prefix, 275 ci->ci_policy.v1.master_key_descriptor, 276 ci->ci_mode->keysize, &payload); 277 } 278 if (IS_ERR(key)) 279 return PTR_ERR(key); 280 281 err = fscrypt_setup_v1_file_key(ci, payload->raw); 282 up_read(&key->sem); 283 key_put(key); 284 return err; 285 } 286