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/algapi.h>
24#include <crypto/skcipher.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 */
32static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */
33static 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 */
47static 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);
80out:
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 */
91static struct key *
92find_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
138invalid:
139	up_read(&key->sem);
140	key_put(key);
141	return ERR_PTR(-ENOKEY);
142}
143
144/* Master key referenced by DIRECT_KEY policy */
145struct fscrypt_direct_key {
146	struct hlist_node		dk_node;
147	refcount_t			dk_refcount;
148	const struct fscrypt_mode	*dk_mode;
149	struct fscrypt_prepared_key	dk_key;
150	u8				dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
151	u8				dk_raw[FSCRYPT_MAX_KEY_SIZE];
152};
153
154static void free_direct_key(struct fscrypt_direct_key *dk)
155{
156	if (dk) {
157		fscrypt_destroy_prepared_key(&dk->dk_key);
158		kfree_sensitive(dk);
159	}
160}
161
162void fscrypt_put_direct_key(struct fscrypt_direct_key *dk)
163{
164	if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock))
165		return;
166	hash_del(&dk->dk_node);
167	spin_unlock(&fscrypt_direct_keys_lock);
168
169	free_direct_key(dk);
170}
171
172/*
173 * Find/insert the given key into the fscrypt_direct_keys table.  If found, it
174 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL.  If
175 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise
176 * NULL is returned.
177 */
178static struct fscrypt_direct_key *
179find_or_insert_direct_key(struct fscrypt_direct_key *to_insert,
180			  const u8 *raw_key, const struct fscrypt_info *ci)
181{
182	unsigned long hash_key;
183	struct fscrypt_direct_key *dk;
184
185	/*
186	 * Careful: to avoid potentially leaking secret key bytes via timing
187	 * information, we must key the hash table by descriptor rather than by
188	 * raw key, and use crypto_memneq() when comparing raw keys.
189	 */
190
191	BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE);
192	memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor,
193	       sizeof(hash_key));
194
195	spin_lock(&fscrypt_direct_keys_lock);
196	hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) {
197		if (memcmp(ci->ci_policy.v1.master_key_descriptor,
198			   dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0)
199			continue;
200		if (ci->ci_mode != dk->dk_mode)
201			continue;
202		if (!fscrypt_is_key_prepared(&dk->dk_key, ci))
203			continue;
204		if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize))
205			continue;
206		/* using existing tfm with same (descriptor, mode, raw_key) */
207		refcount_inc(&dk->dk_refcount);
208		spin_unlock(&fscrypt_direct_keys_lock);
209		free_direct_key(to_insert);
210		return dk;
211	}
212	if (to_insert)
213		hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key);
214	spin_unlock(&fscrypt_direct_keys_lock);
215	return to_insert;
216}
217
218/* Prepare to encrypt directly using the master key in the given mode */
219static struct fscrypt_direct_key *
220fscrypt_get_direct_key(const struct fscrypt_info *ci, const u8 *raw_key)
221{
222	struct fscrypt_direct_key *dk;
223	int err;
224
225	/* Is there already a tfm for this key? */
226	dk = find_or_insert_direct_key(NULL, raw_key, ci);
227	if (dk)
228		return dk;
229
230	/* Nope, allocate one. */
231	dk = kzalloc(sizeof(*dk), GFP_KERNEL);
232	if (!dk)
233		return ERR_PTR(-ENOMEM);
234	refcount_set(&dk->dk_refcount, 1);
235	dk->dk_mode = ci->ci_mode;
236	err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci);
237	if (err)
238		goto err_free_dk;
239	memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor,
240	       FSCRYPT_KEY_DESCRIPTOR_SIZE);
241	memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize);
242
243	return find_or_insert_direct_key(dk, raw_key, ci);
244
245err_free_dk:
246	free_direct_key(dk);
247	return ERR_PTR(err);
248}
249
250/* v1 policy, DIRECT_KEY: use the master key directly */
251static int setup_v1_file_key_direct(struct fscrypt_info *ci,
252				    const u8 *raw_master_key)
253{
254	struct fscrypt_direct_key *dk;
255
256	dk = fscrypt_get_direct_key(ci, raw_master_key);
257	if (IS_ERR(dk))
258		return PTR_ERR(dk);
259	ci->ci_direct_key = dk;
260	ci->ci_enc_key = dk->dk_key;
261	return 0;
262}
263
264/* v1 policy, !DIRECT_KEY: derive the file's encryption key */
265static int setup_v1_file_key_derived(struct fscrypt_info *ci,
266				     const u8 *raw_master_key)
267{
268	u8 *derived_key;
269	int err;
270
271	/*
272	 * This cannot be a stack buffer because it will be passed to the
273	 * scatterlist crypto API during derive_key_aes().
274	 */
275	derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL);
276	if (!derived_key)
277		return -ENOMEM;
278
279	err = derive_key_aes(raw_master_key, ci->ci_nonce,
280			     derived_key, ci->ci_mode->keysize);
281	if (err)
282		goto out;
283
284	err = fscrypt_set_per_file_enc_key(ci, derived_key);
285out:
286	kfree_sensitive(derived_key);
287	return err;
288}
289
290int fscrypt_setup_v1_file_key(struct fscrypt_info *ci, const u8 *raw_master_key)
291{
292	if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY)
293		return setup_v1_file_key_direct(ci, raw_master_key);
294	else
295		return setup_v1_file_key_derived(ci, raw_master_key);
296}
297
298int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci)
299{
300	struct key *key;
301	const struct fscrypt_key *payload;
302	int err;
303
304	key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
305					ci->ci_policy.v1.master_key_descriptor,
306					ci->ci_mode->keysize, &payload);
307	if (key == ERR_PTR(-ENOKEY) && ci->ci_inode->i_sb->s_cop->key_prefix) {
308		key = find_and_lock_process_key(ci->ci_inode->i_sb->s_cop->key_prefix,
309						ci->ci_policy.v1.master_key_descriptor,
310						ci->ci_mode->keysize, &payload);
311	}
312	if (IS_ERR(key))
313		return PTR_ERR(key);
314
315	err = fscrypt_setup_v1_file_key(ci, payload->raw);
316	up_read(&key->sem);
317	key_put(key);
318	return err;
319}
320