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 */ 32static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */ 33static 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 */ 47static 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 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_KERNEL); 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, FSCRYPT_FILE_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); 80out: 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 */ 91static struct key * 92find_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_KERNEL, "%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 138invalid: 139 up_read(&key->sem); 140 key_put(key); 141 return ERR_PTR(-ENOKEY); 142} 143 144/* Master key referenced by DIRECT_KEY policy */ 145struct fscrypt_direct_key { 146 struct super_block *dk_sb; 147 struct hlist_node dk_node; 148 refcount_t dk_refcount; 149 const struct fscrypt_mode *dk_mode; 150 struct fscrypt_prepared_key dk_key; 151 u8 dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 152 u8 dk_raw[FSCRYPT_MAX_KEY_SIZE]; 153}; 154 155static void free_direct_key(struct fscrypt_direct_key *dk) 156{ 157 if (dk) { 158 fscrypt_destroy_prepared_key(dk->dk_sb, &dk->dk_key); 159 kfree_sensitive(dk); 160 } 161} 162 163void fscrypt_put_direct_key(struct fscrypt_direct_key *dk) 164{ 165 if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock)) 166 return; 167 hash_del(&dk->dk_node); 168 spin_unlock(&fscrypt_direct_keys_lock); 169 170 free_direct_key(dk); 171} 172 173/* 174 * Find/insert the given key into the fscrypt_direct_keys table. If found, it 175 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL. If 176 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise 177 * NULL is returned. 178 */ 179static struct fscrypt_direct_key * 180find_or_insert_direct_key(struct fscrypt_direct_key *to_insert, 181 const u8 *raw_key, const struct fscrypt_info *ci) 182{ 183 unsigned long hash_key; 184 struct fscrypt_direct_key *dk; 185 186 /* 187 * Careful: to avoid potentially leaking secret key bytes via timing 188 * information, we must key the hash table by descriptor rather than by 189 * raw key, and use crypto_memneq() when comparing raw keys. 190 */ 191 192 BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE); 193 memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor, 194 sizeof(hash_key)); 195 196 spin_lock(&fscrypt_direct_keys_lock); 197 hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) { 198 if (memcmp(ci->ci_policy.v1.master_key_descriptor, 199 dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0) 200 continue; 201 if (ci->ci_mode != dk->dk_mode) 202 continue; 203 if (!fscrypt_is_key_prepared(&dk->dk_key, ci)) 204 continue; 205 if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize)) 206 continue; 207 /* using existing tfm with same (descriptor, mode, raw_key) */ 208 refcount_inc(&dk->dk_refcount); 209 spin_unlock(&fscrypt_direct_keys_lock); 210 free_direct_key(to_insert); 211 return dk; 212 } 213 if (to_insert) 214 hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key); 215 spin_unlock(&fscrypt_direct_keys_lock); 216 return to_insert; 217} 218 219/* Prepare to encrypt directly using the master key in the given mode */ 220static struct fscrypt_direct_key * 221fscrypt_get_direct_key(const struct fscrypt_info *ci, const u8 *raw_key) 222{ 223 struct fscrypt_direct_key *dk; 224 int err; 225 226 /* Is there already a tfm for this key? */ 227 dk = find_or_insert_direct_key(NULL, raw_key, ci); 228 if (dk) 229 return dk; 230 231 /* Nope, allocate one. */ 232 dk = kzalloc(sizeof(*dk), GFP_KERNEL); 233 if (!dk) 234 return ERR_PTR(-ENOMEM); 235 dk->dk_sb = ci->ci_inode->i_sb; 236 refcount_set(&dk->dk_refcount, 1); 237 dk->dk_mode = ci->ci_mode; 238 err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci); 239 if (err) 240 goto err_free_dk; 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 247err_free_dk: 248 free_direct_key(dk); 249 return ERR_PTR(err); 250} 251 252/* v1 policy, DIRECT_KEY: use the master key directly */ 253static 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_enc_key = dk->dk_key; 263 return 0; 264} 265 266/* v1 policy, !DIRECT_KEY: derive the file's encryption key */ 267static 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_KERNEL); 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); 287out: 288 kfree_sensitive(derived_key); 289 return err; 290} 291 292int 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 300int 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