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 = 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 *
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)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
free_direct_key(struct fscrypt_direct_key * dk)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
fscrypt_put_direct_key(struct fscrypt_direct_key * dk)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 *
find_or_insert_direct_key(struct fscrypt_direct_key * to_insert,const u8 * raw_key,const struct fscrypt_inode_info * ci)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 *
fscrypt_get_direct_key(const struct fscrypt_inode_info * ci,const u8 * raw_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 */
setup_v1_file_key_direct(struct fscrypt_inode_info * ci,const u8 * raw_master_key)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 */
setup_v1_file_key_derived(struct fscrypt_inode_info * ci,const u8 * raw_master_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
fscrypt_setup_v1_file_key(struct fscrypt_inode_info * ci,const u8 * raw_master_key)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
fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info * ci)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 }
326