keysetup_v1.c

  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 */
 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	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 */
 82static struct key *
 83find_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
129invalid:
130	up_read(&key->sem);
131	key_put(key);
132	return ERR_PTR(-ENOKEY);
133}
134
135/* Master key referenced by DIRECT_KEY policy */
136struct 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
146static 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
154void 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 */
170static struct fscrypt_direct_key *
171find_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 */
212static struct fscrypt_direct_key *
213fscrypt_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
239err_free_dk:
240	free_direct_key(dk);
241	return ERR_PTR(err);
242}
243
244/* v1 policy, DIRECT_KEY: use the master key directly */
245static 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 */
259static 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);
279out:
280	kfree_sensitive(derived_key);
281	return err;
282}
283
284int 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
293int
294fscrypt_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}