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 */
derive_key_aes(const u8 * master_key,const u8 nonce[FSCRYPT_FILE_NONCE_SIZE],u8 * derived_key,unsigned int derived_keysize)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 =
56 crypto_alloc_skcipher("ecb(aes)", 0, FSCRYPT_CRYPTOAPI_MASK);
57
58 if (IS_ERR(tfm)) {
59 res = PTR_ERR(tfm);
60 tfm = NULL;
61 goto out;
62 }
63 crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_FORBID_WEAK_KEYS);
64 req = skcipher_request_alloc(tfm, GFP_KERNEL);
65 if (!req) {
66 res = -ENOMEM;
67 goto out;
68 }
69 skcipher_request_set_callback(req,
70 CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
71 crypto_req_done, &wait);
72 res = crypto_skcipher_setkey(tfm, nonce, FSCRYPT_FILE_NONCE_SIZE);
73 if (res < 0)
74 goto out;
75
76 sg_init_one(&src_sg, master_key, derived_keysize);
77 sg_init_one(&dst_sg, derived_key, derived_keysize);
78 skcipher_request_set_crypt(req, &src_sg, &dst_sg, derived_keysize,
79 NULL);
80 res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
81 out:
82 skcipher_request_free(req);
83 crypto_free_skcipher(tfm);
84 return res;
85 }
86
87 /*
88 * Search the current task's subscribed keyrings for a "logon" key with
89 * description prefix:descriptor, and if found acquire a read lock on it and
90 * return a pointer to its validated payload in *payload_ret.
91 */
92 static struct key *
find_and_lock_process_key(const char * prefix,const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE],unsigned int min_keysize,const struct fscrypt_key ** payload_ret)93 find_and_lock_process_key(const char *prefix,
94 const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE],
95 unsigned int min_keysize,
96 const struct fscrypt_key **payload_ret)
97 {
98 char *description;
99 struct key *key;
100 const struct user_key_payload *ukp;
101 const struct fscrypt_key *payload;
102
103 description = kasprintf(GFP_KERNEL, "%s%*phN", prefix,
104 FSCRYPT_KEY_DESCRIPTOR_SIZE, descriptor);
105 if (!description)
106 return ERR_PTR(-ENOMEM);
107
108 key = request_key(&key_type_logon, description, NULL);
109 kfree(description);
110 if (IS_ERR(key))
111 return key;
112
113 down_read(&key->sem);
114 ukp = user_key_payload_locked(key);
115
116 if (!ukp) /* was the key revoked before we acquired its semaphore? */
117 goto invalid;
118
119 payload = (const struct fscrypt_key *)ukp->data;
120
121 if (ukp->datalen != sizeof(struct fscrypt_key) ||
122 payload->size < 1 ||
123 payload->size > FSCRYPT_MAX_STANDARD_KEY_SIZE) {
124 fscrypt_warn(NULL,
125 "key with description '%s' has invalid payload",
126 key->description);
127 goto invalid;
128 }
129
130 if (payload->size < min_keysize) {
131 fscrypt_warn(NULL,
132 "key with description '%s' is too short (got %u bytes, need %u+ bytes)",
133 key->description, payload->size, min_keysize);
134 goto invalid;
135 }
136
137 *payload_ret = payload;
138 return key;
139
140 invalid:
141 up_read(&key->sem);
142 key_put(key);
143 return ERR_PTR(-ENOKEY);
144 }
145
146 /* Master key referenced by DIRECT_KEY policy */
147 struct fscrypt_direct_key {
148 struct super_block *dk_sb;
149 struct hlist_node dk_node;
150 refcount_t dk_refcount;
151 const struct fscrypt_mode *dk_mode;
152 struct fscrypt_prepared_key dk_key;
153 u8 dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
154 u8 dk_raw[FSCRYPT_MAX_STANDARD_KEY_SIZE];
155 };
156
free_direct_key(struct fscrypt_direct_key * dk)157 static void free_direct_key(struct fscrypt_direct_key *dk)
158 {
159 if (dk) {
160 fscrypt_destroy_prepared_key(dk->dk_sb, &dk->dk_key);
161 kfree_sensitive(dk);
162 }
163 }
164
fscrypt_put_direct_key(struct fscrypt_direct_key * dk)165 void fscrypt_put_direct_key(struct fscrypt_direct_key *dk)
166 {
167 if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock))
168 return;
169 hash_del(&dk->dk_node);
170 spin_unlock(&fscrypt_direct_keys_lock);
171
172 free_direct_key(dk);
173 }
174
175 /*
176 * Find/insert the given key into the fscrypt_direct_keys table. If found, it
177 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL. If
178 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise
179 * NULL is returned.
180 */
181 static struct fscrypt_direct_key *
find_or_insert_direct_key(struct fscrypt_direct_key * to_insert,const u8 * raw_key,const struct fscrypt_inode_info * ci)182 find_or_insert_direct_key(struct fscrypt_direct_key *to_insert,
183 const u8 *raw_key,
184 const struct fscrypt_inode_info *ci)
185 {
186 unsigned long hash_key;
187 struct fscrypt_direct_key *dk;
188
189 /*
190 * Careful: to avoid potentially leaking secret key bytes via timing
191 * information, we must key the hash table by descriptor rather than by
192 * raw key, and use crypto_memneq() when comparing raw keys.
193 */
194
195 BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE);
196 memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor,
197 sizeof(hash_key));
198
199 spin_lock(&fscrypt_direct_keys_lock);
200 hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) {
201 if (memcmp(ci->ci_policy.v1.master_key_descriptor,
202 dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0)
203 continue;
204 if (ci->ci_mode != dk->dk_mode)
205 continue;
206 if (!fscrypt_is_key_prepared(&dk->dk_key, ci))
207 continue;
208 if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize))
209 continue;
210 /* using existing tfm with same (descriptor, mode, raw_key) */
211 refcount_inc(&dk->dk_refcount);
212 spin_unlock(&fscrypt_direct_keys_lock);
213 free_direct_key(to_insert);
214 return dk;
215 }
216 if (to_insert)
217 hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key);
218 spin_unlock(&fscrypt_direct_keys_lock);
219 return to_insert;
220 }
221
222 /* Prepare to encrypt directly using the master key in the given mode */
223 static struct fscrypt_direct_key *
fscrypt_get_direct_key(const struct fscrypt_inode_info * ci,const u8 * raw_key)224 fscrypt_get_direct_key(const struct fscrypt_inode_info *ci, const u8 *raw_key)
225 {
226 struct fscrypt_direct_key *dk;
227 int err;
228
229 /* Is there already a tfm for this key? */
230 dk = find_or_insert_direct_key(NULL, raw_key, ci);
231 if (dk)
232 return dk;
233
234 /* Nope, allocate one. */
235 dk = kzalloc(sizeof(*dk), GFP_KERNEL);
236 if (!dk)
237 return ERR_PTR(-ENOMEM);
238 dk->dk_sb = ci->ci_inode->i_sb;
239 refcount_set(&dk->dk_refcount, 1);
240 dk->dk_mode = ci->ci_mode;
241 err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci);
242 if (err)
243 goto err_free_dk;
244 memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor,
245 FSCRYPT_KEY_DESCRIPTOR_SIZE);
246 memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize);
247
248 return find_or_insert_direct_key(dk, raw_key, ci);
249
250 err_free_dk:
251 free_direct_key(dk);
252 return ERR_PTR(err);
253 }
254
255 /* v1 policy, DIRECT_KEY: use the master key directly */
setup_v1_file_key_direct(struct fscrypt_inode_info * ci,const u8 * raw_master_key)256 static int setup_v1_file_key_direct(struct fscrypt_inode_info *ci,
257 const u8 *raw_master_key)
258 {
259 struct fscrypt_direct_key *dk;
260
261 dk = fscrypt_get_direct_key(ci, raw_master_key);
262 if (IS_ERR(dk))
263 return PTR_ERR(dk);
264 ci->ci_direct_key = dk;
265 ci->ci_enc_key = dk->dk_key;
266 return 0;
267 }
268
269 /* v1 policy, !DIRECT_KEY: derive the file's encryption key */
setup_v1_file_key_derived(struct fscrypt_inode_info * ci,const u8 * raw_master_key)270 static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci,
271 const u8 *raw_master_key)
272 {
273 u8 *derived_key;
274 int err;
275
276 /*
277 * This cannot be a stack buffer because it will be passed to the
278 * scatterlist crypto API during derive_key_aes().
279 */
280 derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL);
281 if (!derived_key)
282 return -ENOMEM;
283
284 err = derive_key_aes(raw_master_key, ci->ci_nonce,
285 derived_key, ci->ci_mode->keysize);
286 if (err)
287 goto out;
288
289 err = fscrypt_set_per_file_enc_key(ci, derived_key);
290 out:
291 kfree_sensitive(derived_key);
292 return err;
293 }
294
fscrypt_setup_v1_file_key(struct fscrypt_inode_info * ci,const u8 * raw_master_key)295 int fscrypt_setup_v1_file_key(struct fscrypt_inode_info *ci,
296 const u8 *raw_master_key)
297 {
298 if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY)
299 return setup_v1_file_key_direct(ci, raw_master_key);
300 else
301 return setup_v1_file_key_derived(ci, raw_master_key);
302 }
303
304 int
fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info * ci)305 fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info *ci)
306 {
307 const struct super_block *sb = ci->ci_inode->i_sb;
308 struct key *key;
309 const struct fscrypt_key *payload;
310 int err;
311
312 key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
313 ci->ci_policy.v1.master_key_descriptor,
314 ci->ci_mode->keysize, &payload);
315 if (key == ERR_PTR(-ENOKEY) && sb->s_cop->legacy_key_prefix) {
316 key = find_and_lock_process_key(sb->s_cop->legacy_key_prefix,
317 ci->ci_policy.v1.master_key_descriptor,
318 ci->ci_mode->keysize, &payload);
319 }
320 if (IS_ERR(key))
321 return PTR_ERR(key);
322
323 err = fscrypt_setup_v1_file_key(ci, payload->raw);
324 up_read(&key->sem);
325 key_put(key);
326 return err;
327 }
328