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 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 *
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)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
free_direct_key(struct fscrypt_direct_key * dk)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
fscrypt_put_direct_key(struct fscrypt_direct_key * dk)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 *
find_or_insert_direct_key(struct fscrypt_direct_key * to_insert,const u8 * raw_key,const struct fscrypt_inode_info * ci)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 *
fscrypt_get_direct_key(const struct fscrypt_inode_info * ci,const u8 * raw_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 */
setup_v1_file_key_direct(struct fscrypt_inode_info * ci,const u8 * raw_master_key)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 */
setup_v1_file_key_derived(struct fscrypt_inode_info * ci,const u8 * raw_master_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
fscrypt_setup_v1_file_key(struct fscrypt_inode_info * ci,const u8 * raw_master_key)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
fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info * ci)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