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 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[FS_KEY_DERIVATION_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_NOFS);
 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, FS_KEY_DERIVATION_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_NOFS, "%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 crypto_skcipher		*dk_ctfm;
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		crypto_free_skcipher(dk->dk_ctfm);
158		kzfree(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 (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize))
203			continue;
204		/* using existing tfm with same (descriptor, mode, raw_key) */
205		refcount_inc(&dk->dk_refcount);
206		spin_unlock(&fscrypt_direct_keys_lock);
207		free_direct_key(to_insert);
208		return dk;
209	}
210	if (to_insert)
211		hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key);
212	spin_unlock(&fscrypt_direct_keys_lock);
213	return to_insert;
214}
215
216/* Prepare to encrypt directly using the master key in the given mode */
217static struct fscrypt_direct_key *
218fscrypt_get_direct_key(const struct fscrypt_info *ci, const u8 *raw_key)
219{
220	struct fscrypt_direct_key *dk;
221	int err;
222
223	/* Is there already a tfm for this key? */
224	dk = find_or_insert_direct_key(NULL, raw_key, ci);
225	if (dk)
226		return dk;
227
228	/* Nope, allocate one. */
229	dk = kzalloc(sizeof(*dk), GFP_NOFS);
230	if (!dk)
231		return ERR_PTR(-ENOMEM);
232	refcount_set(&dk->dk_refcount, 1);
233	dk->dk_mode = ci->ci_mode;
234	dk->dk_ctfm = fscrypt_allocate_skcipher(ci->ci_mode, raw_key,
235						ci->ci_inode);
236	if (IS_ERR(dk->dk_ctfm)) {
237		err = PTR_ERR(dk->dk_ctfm);
238		dk->dk_ctfm = NULL;
239		goto err_free_dk;
240	}
241	memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor,
242	       FSCRYPT_KEY_DESCRIPTOR_SIZE);
243	memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize);
244
245	return find_or_insert_direct_key(dk, raw_key, ci);
246
247err_free_dk:
248	free_direct_key(dk);
249	return ERR_PTR(err);
250}
251
252/* v1 policy, DIRECT_KEY: use the master key directly */
253static int setup_v1_file_key_direct(struct fscrypt_info *ci,
254				    const u8 *raw_master_key)
255{
256	const struct fscrypt_mode *mode = ci->ci_mode;
257	struct fscrypt_direct_key *dk;
258
259	if (!fscrypt_mode_supports_direct_key(mode)) {
260		fscrypt_warn(ci->ci_inode,
261			     "Direct key mode not allowed with %s",
262			     mode->friendly_name);
263		return -EINVAL;
264	}
265
266	if (ci->ci_policy.v1.contents_encryption_mode !=
267	    ci->ci_policy.v1.filenames_encryption_mode) {
268		fscrypt_warn(ci->ci_inode,
269			     "Direct key mode not allowed with different contents and filenames modes");
270		return -EINVAL;
271	}
272
273	/* ESSIV implies 16-byte IVs which implies !DIRECT_KEY */
274	if (WARN_ON(mode->needs_essiv))
275		return -EINVAL;
276
277	dk = fscrypt_get_direct_key(ci, raw_master_key);
278	if (IS_ERR(dk))
279		return PTR_ERR(dk);
280	ci->ci_direct_key = dk;
281	ci->ci_ctfm = dk->dk_ctfm;
282	return 0;
283}
284
285/* v1 policy, !DIRECT_KEY: derive the file's encryption key */
286static int setup_v1_file_key_derived(struct fscrypt_info *ci,
287				     const u8 *raw_master_key)
288{
289	u8 *derived_key;
290	int err;
291
292	/*
293	 * This cannot be a stack buffer because it will be passed to the
294	 * scatterlist crypto API during derive_key_aes().
295	 */
296	derived_key = kmalloc(ci->ci_mode->keysize, GFP_NOFS);
297	if (!derived_key)
298		return -ENOMEM;
299
300	err = derive_key_aes(raw_master_key, ci->ci_nonce,
301			     derived_key, ci->ci_mode->keysize);
302	if (err)
303		goto out;
304
305	err = fscrypt_set_derived_key(ci, derived_key);
306out:
307	kzfree(derived_key);
308	return err;
309}
310
311int fscrypt_setup_v1_file_key(struct fscrypt_info *ci, const u8 *raw_master_key)
312{
313	if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY)
314		return setup_v1_file_key_direct(ci, raw_master_key);
315	else
316		return setup_v1_file_key_derived(ci, raw_master_key);
317}
318
319int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci)
320{
321	struct key *key;
322	const struct fscrypt_key *payload;
323	int err;
324
325	key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
326					ci->ci_policy.v1.master_key_descriptor,
327					ci->ci_mode->keysize, &payload);
328	if (key == ERR_PTR(-ENOKEY) && ci->ci_inode->i_sb->s_cop->key_prefix) {
329		key = find_and_lock_process_key(ci->ci_inode->i_sb->s_cop->key_prefix,
330						ci->ci_policy.v1.master_key_descriptor,
331						ci->ci_mode->keysize, &payload);
332	}
333	if (IS_ERR(key))
334		return PTR_ERR(key);
335
336	err = fscrypt_setup_v1_file_key(ci, payload->raw);
337	up_read(&key->sem);
338	key_put(key);
339	return err;
340}