1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * key management facility for FS encryption support.
4 *
5 * Copyright (C) 2015, Google, Inc.
6 *
7 * This contains encryption key functions.
8 *
9 * Written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar, 2015.
10 */
11
12 #include <keys/user-type.h>
13 #include <linux/scatterlist.h>
14 #include <linux/ratelimit.h>
15 #include <crypto/aes.h>
16 #include <crypto/sha.h>
17 #include <crypto/skcipher.h>
18 #include "fscrypt_private.h"
19
20 static struct crypto_shash *essiv_hash_tfm;
21
22 /*
23 * Key derivation function. This generates the derived key by encrypting the
24 * master key with AES-128-ECB using the inode's nonce as the AES key.
25 *
26 * The master key must be at least as long as the derived key. If the master
27 * key is longer, then only the first 'derived_keysize' bytes are used.
28 */
derive_key_aes(const u8 * master_key,const struct fscrypt_context * ctx,u8 * derived_key,unsigned int derived_keysize)29 static int derive_key_aes(const u8 *master_key,
30 const struct fscrypt_context *ctx,
31 u8 *derived_key, unsigned int derived_keysize)
32 {
33 int res = 0;
34 struct skcipher_request *req = NULL;
35 DECLARE_CRYPTO_WAIT(wait);
36 struct scatterlist src_sg, dst_sg;
37 struct crypto_skcipher *tfm = crypto_alloc_skcipher("ecb(aes)", 0, 0);
38
39 if (IS_ERR(tfm)) {
40 res = PTR_ERR(tfm);
41 tfm = NULL;
42 goto out;
43 }
44 crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_WEAK_KEY);
45 req = skcipher_request_alloc(tfm, GFP_NOFS);
46 if (!req) {
47 res = -ENOMEM;
48 goto out;
49 }
50 skcipher_request_set_callback(req,
51 CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
52 crypto_req_done, &wait);
53 res = crypto_skcipher_setkey(tfm, ctx->nonce, sizeof(ctx->nonce));
54 if (res < 0)
55 goto out;
56
57 sg_init_one(&src_sg, master_key, derived_keysize);
58 sg_init_one(&dst_sg, derived_key, derived_keysize);
59 skcipher_request_set_crypt(req, &src_sg, &dst_sg, derived_keysize,
60 NULL);
61 res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
62 out:
63 skcipher_request_free(req);
64 crypto_free_skcipher(tfm);
65 return res;
66 }
67
68 /*
69 * Search the current task's subscribed keyrings for a "logon" key with
70 * description prefix:descriptor, and if found acquire a read lock on it and
71 * return a pointer to its validated payload in *payload_ret.
72 */
73 static struct key *
find_and_lock_process_key(const char * prefix,const u8 descriptor[FS_KEY_DESCRIPTOR_SIZE],unsigned int min_keysize,const struct fscrypt_key ** payload_ret)74 find_and_lock_process_key(const char *prefix,
75 const u8 descriptor[FS_KEY_DESCRIPTOR_SIZE],
76 unsigned int min_keysize,
77 const struct fscrypt_key **payload_ret)
78 {
79 char *description;
80 struct key *key;
81 const struct user_key_payload *ukp;
82 const struct fscrypt_key *payload;
83
84 description = kasprintf(GFP_NOFS, "%s%*phN", prefix,
85 FS_KEY_DESCRIPTOR_SIZE, descriptor);
86 if (!description)
87 return ERR_PTR(-ENOMEM);
88
89 key = request_key(&key_type_logon, description, NULL);
90 kfree(description);
91 if (IS_ERR(key))
92 return key;
93
94 down_read(&key->sem);
95 ukp = user_key_payload_locked(key);
96
97 if (!ukp) /* was the key revoked before we acquired its semaphore? */
98 goto invalid;
99
100 payload = (const struct fscrypt_key *)ukp->data;
101
102 if (ukp->datalen != sizeof(struct fscrypt_key) ||
103 payload->size < 1 || payload->size > FS_MAX_KEY_SIZE) {
104 fscrypt_warn(NULL,
105 "key with description '%s' has invalid payload",
106 key->description);
107 goto invalid;
108 }
109
110 if (payload->size < min_keysize) {
111 fscrypt_warn(NULL,
112 "key with description '%s' is too short (got %u bytes, need %u+ bytes)",
113 key->description, payload->size, min_keysize);
114 goto invalid;
115 }
116
117 *payload_ret = payload;
118 return key;
119
120 invalid:
121 up_read(&key->sem);
122 key_put(key);
123 return ERR_PTR(-ENOKEY);
124 }
125
126 /* Find the master key, then derive the inode's actual encryption key */
find_and_derive_key(const struct inode * inode,const struct fscrypt_context * ctx,u8 * derived_key,unsigned int derived_keysize)127 static int find_and_derive_key(const struct inode *inode,
128 const struct fscrypt_context *ctx,
129 u8 *derived_key, unsigned int derived_keysize)
130 {
131 struct key *key;
132 const struct fscrypt_key *payload;
133 int err;
134
135 key = find_and_lock_process_key(FS_KEY_DESC_PREFIX,
136 ctx->master_key_descriptor,
137 derived_keysize, &payload);
138 if (key == ERR_PTR(-ENOKEY) && inode->i_sb->s_cop->key_prefix) {
139 key = find_and_lock_process_key(inode->i_sb->s_cop->key_prefix,
140 ctx->master_key_descriptor,
141 derived_keysize, &payload);
142 }
143 if (IS_ERR(key))
144 return PTR_ERR(key);
145 err = derive_key_aes(payload->raw, ctx, derived_key, derived_keysize);
146 up_read(&key->sem);
147 key_put(key);
148 return err;
149 }
150
151 static struct fscrypt_mode {
152 const char *friendly_name;
153 const char *cipher_str;
154 int keysize;
155 bool logged_impl_name;
156 } available_modes[] = {
157 [FS_ENCRYPTION_MODE_AES_256_XTS] = {
158 .friendly_name = "AES-256-XTS",
159 .cipher_str = "xts(aes)",
160 .keysize = 64,
161 },
162 [FS_ENCRYPTION_MODE_AES_256_CTS] = {
163 .friendly_name = "AES-256-CTS-CBC",
164 .cipher_str = "cts(cbc(aes))",
165 .keysize = 32,
166 },
167 [FS_ENCRYPTION_MODE_AES_128_CBC] = {
168 .friendly_name = "AES-128-CBC",
169 .cipher_str = "cbc(aes)",
170 .keysize = 16,
171 },
172 [FS_ENCRYPTION_MODE_AES_128_CTS] = {
173 .friendly_name = "AES-128-CTS-CBC",
174 .cipher_str = "cts(cbc(aes))",
175 .keysize = 16,
176 },
177 };
178
179 static struct fscrypt_mode *
select_encryption_mode(const struct fscrypt_info * ci,const struct inode * inode)180 select_encryption_mode(const struct fscrypt_info *ci, const struct inode *inode)
181 {
182 if (!fscrypt_valid_enc_modes(ci->ci_data_mode, ci->ci_filename_mode)) {
183 fscrypt_warn(inode->i_sb,
184 "inode %lu uses unsupported encryption modes (contents mode %d, filenames mode %d)",
185 inode->i_ino, ci->ci_data_mode,
186 ci->ci_filename_mode);
187 return ERR_PTR(-EINVAL);
188 }
189
190 if (S_ISREG(inode->i_mode))
191 return &available_modes[ci->ci_data_mode];
192
193 if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
194 return &available_modes[ci->ci_filename_mode];
195
196 WARN_ONCE(1, "fscrypt: filesystem tried to load encryption info for inode %lu, which is not encryptable (file type %d)\n",
197 inode->i_ino, (inode->i_mode & S_IFMT));
198 return ERR_PTR(-EINVAL);
199 }
200
put_crypt_info(struct fscrypt_info * ci)201 static void put_crypt_info(struct fscrypt_info *ci)
202 {
203 if (!ci)
204 return;
205
206 crypto_free_skcipher(ci->ci_ctfm);
207 crypto_free_cipher(ci->ci_essiv_tfm);
208 kmem_cache_free(fscrypt_info_cachep, ci);
209 }
210
derive_essiv_salt(const u8 * key,int keysize,u8 * salt)211 static int derive_essiv_salt(const u8 *key, int keysize, u8 *salt)
212 {
213 struct crypto_shash *tfm = READ_ONCE(essiv_hash_tfm);
214
215 /* init hash transform on demand */
216 if (unlikely(!tfm)) {
217 struct crypto_shash *prev_tfm;
218
219 tfm = crypto_alloc_shash("sha256", 0, 0);
220 if (IS_ERR(tfm)) {
221 fscrypt_warn(NULL,
222 "error allocating SHA-256 transform: %ld",
223 PTR_ERR(tfm));
224 return PTR_ERR(tfm);
225 }
226 prev_tfm = cmpxchg(&essiv_hash_tfm, NULL, tfm);
227 if (prev_tfm) {
228 crypto_free_shash(tfm);
229 tfm = prev_tfm;
230 }
231 }
232
233 {
234 SHASH_DESC_ON_STACK(desc, tfm);
235 desc->tfm = tfm;
236 desc->flags = 0;
237
238 return crypto_shash_digest(desc, key, keysize, salt);
239 }
240 }
241
init_essiv_generator(struct fscrypt_info * ci,const u8 * raw_key,int keysize)242 static int init_essiv_generator(struct fscrypt_info *ci, const u8 *raw_key,
243 int keysize)
244 {
245 int err;
246 struct crypto_cipher *essiv_tfm;
247 u8 salt[SHA256_DIGEST_SIZE];
248
249 essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
250 if (IS_ERR(essiv_tfm))
251 return PTR_ERR(essiv_tfm);
252
253 ci->ci_essiv_tfm = essiv_tfm;
254
255 err = derive_essiv_salt(raw_key, keysize, salt);
256 if (err)
257 goto out;
258
259 /*
260 * Using SHA256 to derive the salt/key will result in AES-256 being
261 * used for IV generation. File contents encryption will still use the
262 * configured keysize (AES-128) nevertheless.
263 */
264 err = crypto_cipher_setkey(essiv_tfm, salt, sizeof(salt));
265 if (err)
266 goto out;
267
268 out:
269 memzero_explicit(salt, sizeof(salt));
270 return err;
271 }
272
fscrypt_essiv_cleanup(void)273 void __exit fscrypt_essiv_cleanup(void)
274 {
275 crypto_free_shash(essiv_hash_tfm);
276 }
277
fscrypt_get_encryption_info(struct inode * inode)278 int fscrypt_get_encryption_info(struct inode *inode)
279 {
280 struct fscrypt_info *crypt_info;
281 struct fscrypt_context ctx;
282 struct crypto_skcipher *ctfm;
283 struct fscrypt_mode *mode;
284 u8 *raw_key = NULL;
285 int res;
286
287 if (inode->i_crypt_info)
288 return 0;
289
290 res = fscrypt_initialize(inode->i_sb->s_cop->flags);
291 if (res)
292 return res;
293
294 res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx));
295 if (res < 0) {
296 if (!fscrypt_dummy_context_enabled(inode) ||
297 IS_ENCRYPTED(inode))
298 return res;
299 /* Fake up a context for an unencrypted directory */
300 memset(&ctx, 0, sizeof(ctx));
301 ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1;
302 ctx.contents_encryption_mode = FS_ENCRYPTION_MODE_AES_256_XTS;
303 ctx.filenames_encryption_mode = FS_ENCRYPTION_MODE_AES_256_CTS;
304 memset(ctx.master_key_descriptor, 0x42, FS_KEY_DESCRIPTOR_SIZE);
305 } else if (res != sizeof(ctx)) {
306 return -EINVAL;
307 }
308
309 if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1)
310 return -EINVAL;
311
312 if (ctx.flags & ~FS_POLICY_FLAGS_VALID)
313 return -EINVAL;
314
315 crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS);
316 if (!crypt_info)
317 return -ENOMEM;
318
319 crypt_info->ci_flags = ctx.flags;
320 crypt_info->ci_data_mode = ctx.contents_encryption_mode;
321 crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
322 crypt_info->ci_ctfm = NULL;
323 crypt_info->ci_essiv_tfm = NULL;
324 memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
325 sizeof(crypt_info->ci_master_key));
326
327 mode = select_encryption_mode(crypt_info, inode);
328 if (IS_ERR(mode)) {
329 res = PTR_ERR(mode);
330 goto out;
331 }
332
333 /*
334 * This cannot be a stack buffer because it is passed to the scatterlist
335 * crypto API as part of key derivation.
336 */
337 res = -ENOMEM;
338 raw_key = kmalloc(mode->keysize, GFP_NOFS);
339 if (!raw_key)
340 goto out;
341
342 res = find_and_derive_key(inode, &ctx, raw_key, mode->keysize);
343 if (res)
344 goto out;
345
346 ctfm = crypto_alloc_skcipher(mode->cipher_str, 0, 0);
347 if (IS_ERR(ctfm)) {
348 res = PTR_ERR(ctfm);
349 fscrypt_warn(inode->i_sb,
350 "error allocating '%s' transform for inode %lu: %d",
351 mode->cipher_str, inode->i_ino, res);
352 goto out;
353 }
354 if (unlikely(!mode->logged_impl_name)) {
355 /*
356 * fscrypt performance can vary greatly depending on which
357 * crypto algorithm implementation is used. Help people debug
358 * performance problems by logging the ->cra_driver_name the
359 * first time a mode is used. Note that multiple threads can
360 * race here, but it doesn't really matter.
361 */
362 mode->logged_impl_name = true;
363 pr_info("fscrypt: %s using implementation \"%s\"\n",
364 mode->friendly_name,
365 crypto_skcipher_alg(ctfm)->base.cra_driver_name);
366 }
367 crypt_info->ci_ctfm = ctfm;
368 crypto_skcipher_set_flags(ctfm, CRYPTO_TFM_REQ_WEAK_KEY);
369 res = crypto_skcipher_setkey(ctfm, raw_key, mode->keysize);
370 if (res)
371 goto out;
372
373 if (S_ISREG(inode->i_mode) &&
374 crypt_info->ci_data_mode == FS_ENCRYPTION_MODE_AES_128_CBC) {
375 res = init_essiv_generator(crypt_info, raw_key, mode->keysize);
376 if (res) {
377 fscrypt_warn(inode->i_sb,
378 "error initializing ESSIV generator for inode %lu: %d",
379 inode->i_ino, res);
380 goto out;
381 }
382 }
383 if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) == NULL)
384 crypt_info = NULL;
385 out:
386 if (res == -ENOKEY)
387 res = 0;
388 put_crypt_info(crypt_info);
389 kzfree(raw_key);
390 return res;
391 }
392 EXPORT_SYMBOL(fscrypt_get_encryption_info);
393
fscrypt_put_encryption_info(struct inode * inode)394 void fscrypt_put_encryption_info(struct inode *inode)
395 {
396 put_crypt_info(inode->i_crypt_info);
397 inode->i_crypt_info = NULL;
398 }
399 EXPORT_SYMBOL(fscrypt_put_encryption_info);
400