fs/crypto/keysetup_v1.c

   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         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);
  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_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
 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 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
 155 static 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
 163 void 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  */
 179 static struct fscrypt_direct_key *
 180 find_or_insert_direct_key(struct fscrypt_direct_key *to_insert,
 181                           const u8 *raw_key,
 182                           const struct fscrypt_inode_info *ci)
 183 {
 184         unsigned long hash_key;
 185         struct fscrypt_direct_key *dk;
 186
 187         /*
 188          * Careful: to avoid potentially leaking secret key bytes via timing
 189          * information, we must key the hash table by descriptor rather than by
 190          * raw key, and use crypto_memneq() when comparing raw keys.
 191          */
 192
 193         BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE);
 194         memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor,
 195                sizeof(hash_key));
 196
 197         spin_lock(&fscrypt_direct_keys_lock);
 198         hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) {
 199                 if (memcmp(ci->ci_policy.v1.master_key_descriptor,
 200                            dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0)
 201                         continue;
 202                 if (ci->ci_mode != dk->dk_mode)
 203                         continue;
 204                 if (!fscrypt_is_key_prepared(&dk->dk_key, ci))
 205                         continue;
 206                 if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize))
 207                         continue;
 208                 /* using existing tfm with same (descriptor, mode, raw_key) */
 209                 refcount_inc(&dk->dk_refcount);
 210                 spin_unlock(&fscrypt_direct_keys_lock);
 211                 free_direct_key(to_insert);
 212                 return dk;
 213         }
 214         if (to_insert)
 215                 hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key);
 216         spin_unlock(&fscrypt_direct_keys_lock);
 217         return to_insert;
 218 }
 219
 220 /* Prepare to encrypt directly using the master key in the given mode */
 221 static struct fscrypt_direct_key *
 222 fscrypt_get_direct_key(const struct fscrypt_inode_info *ci, const u8 *raw_key)
 223 {
 224         struct fscrypt_direct_key *dk;
 225         int err;
 226
 227         /* Is there already a tfm for this key? */
 228         dk = find_or_insert_direct_key(NULL, raw_key, ci);
 229         if (dk)
 230                 return dk;
 231
 232         /* Nope, allocate one. */
 233         dk = kzalloc(sizeof(*dk), GFP_KERNEL);
 234         if (!dk)
 235                 return ERR_PTR(-ENOMEM);
 236         dk->dk_sb = ci->ci_inode->i_sb;
 237         refcount_set(&dk->dk_refcount, 1);
 238         dk->dk_mode = ci->ci_mode;
 239         err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci);
 240         if (err)
 241                 goto err_free_dk;
 242         memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor,
 243                FSCRYPT_KEY_DESCRIPTOR_SIZE);
 244         memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize);
 245
 246         return find_or_insert_direct_key(dk, raw_key, ci);
 247
 248 err_free_dk:
 249         free_direct_key(dk);
 250         return ERR_PTR(err);
 251 }
 252
 253 /* v1 policy, DIRECT_KEY: use the master key directly */
 254 static int setup_v1_file_key_direct(struct fscrypt_inode_info *ci,
 255                                     const u8 *raw_master_key)
 256 {
 257         struct fscrypt_direct_key *dk;
 258
 259         dk = fscrypt_get_direct_key(ci, raw_master_key);
 260         if (IS_ERR(dk))
 261                 return PTR_ERR(dk);
 262         ci->ci_direct_key = dk;
 263         ci->ci_enc_key = dk->dk_key;
 264         return 0;
 265 }
 266
 267 /* v1 policy, !DIRECT_KEY: derive the file's encryption key */
 268 static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci,
 269                                      const u8 *raw_master_key)
 270 {
 271         u8 *derived_key;
 272         int err;
 273
 274         /*
 275          * This cannot be a stack buffer because it will be passed to the
 276          * scatterlist crypto API during derive_key_aes().
 277          */
 278         derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL);
 279         if (!derived_key)
 280                 return -ENOMEM;
 281
 282         err = derive_key_aes(raw_master_key, ci->ci_nonce,
 283                              derived_key, ci->ci_mode->keysize);
 284         if (err)
 285                 goto out;
 286
 287         err = fscrypt_set_per_file_enc_key(ci, derived_key);
 288 out:
 289         kfree_sensitive(derived_key);
 290         return err;
 291 }
 292
 293 int fscrypt_setup_v1_file_key(struct fscrypt_inode_info *ci,
 294                               const u8 *raw_master_key)
 295 {
 296         if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY)
 297                 return setup_v1_file_key_direct(ci, raw_master_key);
 298         else
 299                 return setup_v1_file_key_derived(ci, raw_master_key);
 300 }
 301
 302 int
 303 fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info *ci)
 304 {
 305         const struct super_block *sb = ci->ci_inode->i_sb;
 306         struct key *key;
 307         const struct fscrypt_key *payload;
 308         int err;
 309
 310         key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
 311                                         ci->ci_policy.v1.master_key_descriptor,
 312                                         ci->ci_mode->keysize, &payload);
 313         if (key == ERR_PTR(-ENOKEY) && sb->s_cop->legacy_key_prefix) {
 314                 key = find_and_lock_process_key(sb->s_cop->legacy_key_prefix,
 315                                                 ci->ci_policy.v1.master_key_descriptor,
 316                                                 ci->ci_mode->keysize, &payload);
 317         }
 318         if (IS_ERR(key))
 319                 return PTR_ERR(key);
 320
 321         err = fscrypt_setup_v1_file_key(ci, payload->raw);
 322         up_read(&key->sem);
 323         key_put(key);
 324         return err;
 325 }