xref: /openbmc/linux/security/keys/big_key.c (revision 6aa7de05)
1 /* Large capacity key type
2  *
3  * Copyright (C) 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
4  * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
5  * Written by David Howells (dhowells@redhat.com)
6  *
7  * This program is free software; you can redistribute it and/or
8  * modify it under the terms of the GNU General Public Licence
9  * as published by the Free Software Foundation; either version
10  * 2 of the Licence, or (at your option) any later version.
11  */
12 
13 #define pr_fmt(fmt) "big_key: "fmt
14 #include <linux/init.h>
15 #include <linux/seq_file.h>
16 #include <linux/file.h>
17 #include <linux/shmem_fs.h>
18 #include <linux/err.h>
19 #include <linux/scatterlist.h>
20 #include <linux/random.h>
21 #include <keys/user-type.h>
22 #include <keys/big_key-type.h>
23 #include <crypto/aead.h>
24 
25 /*
26  * Layout of key payload words.
27  */
28 enum {
29 	big_key_data,
30 	big_key_path,
31 	big_key_path_2nd_part,
32 	big_key_len,
33 };
34 
35 /*
36  * Crypto operation with big_key data
37  */
38 enum big_key_op {
39 	BIG_KEY_ENC,
40 	BIG_KEY_DEC,
41 };
42 
43 /*
44  * If the data is under this limit, there's no point creating a shm file to
45  * hold it as the permanently resident metadata for the shmem fs will be at
46  * least as large as the data.
47  */
48 #define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))
49 
50 /*
51  * Key size for big_key data encryption
52  */
53 #define ENC_KEY_SIZE 32
54 
55 /*
56  * Authentication tag length
57  */
58 #define ENC_AUTHTAG_SIZE 16
59 
60 /*
61  * big_key defined keys take an arbitrary string as the description and an
62  * arbitrary blob of data as the payload
63  */
64 struct key_type key_type_big_key = {
65 	.name			= "big_key",
66 	.preparse		= big_key_preparse,
67 	.free_preparse		= big_key_free_preparse,
68 	.instantiate		= generic_key_instantiate,
69 	.revoke			= big_key_revoke,
70 	.destroy		= big_key_destroy,
71 	.describe		= big_key_describe,
72 	.read			= big_key_read,
73 	/* no ->update(); don't add it without changing big_key_crypt() nonce */
74 };
75 
76 /*
77  * Crypto names for big_key data authenticated encryption
78  */
79 static const char big_key_alg_name[] = "gcm(aes)";
80 
81 /*
82  * Crypto algorithms for big_key data authenticated encryption
83  */
84 static struct crypto_aead *big_key_aead;
85 
86 /*
87  * Since changing the key affects the entire object, we need a mutex.
88  */
89 static DEFINE_MUTEX(big_key_aead_lock);
90 
91 /*
92  * Encrypt/decrypt big_key data
93  */
94 static int big_key_crypt(enum big_key_op op, u8 *data, size_t datalen, u8 *key)
95 {
96 	int ret;
97 	struct scatterlist sgio;
98 	struct aead_request *aead_req;
99 	/* We always use a zero nonce. The reason we can get away with this is
100 	 * because we're using a different randomly generated key for every
101 	 * different encryption. Notably, too, key_type_big_key doesn't define
102 	 * an .update function, so there's no chance we'll wind up reusing the
103 	 * key to encrypt updated data. Simply put: one key, one encryption.
104 	 */
105 	u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
106 
107 	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
108 	if (!aead_req)
109 		return -ENOMEM;
110 
111 	memset(zero_nonce, 0, sizeof(zero_nonce));
112 	sg_init_one(&sgio, data, datalen + (op == BIG_KEY_ENC ? ENC_AUTHTAG_SIZE : 0));
113 	aead_request_set_crypt(aead_req, &sgio, &sgio, datalen, zero_nonce);
114 	aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
115 	aead_request_set_ad(aead_req, 0);
116 
117 	mutex_lock(&big_key_aead_lock);
118 	if (crypto_aead_setkey(big_key_aead, key, ENC_KEY_SIZE)) {
119 		ret = -EAGAIN;
120 		goto error;
121 	}
122 	if (op == BIG_KEY_ENC)
123 		ret = crypto_aead_encrypt(aead_req);
124 	else
125 		ret = crypto_aead_decrypt(aead_req);
126 error:
127 	mutex_unlock(&big_key_aead_lock);
128 	aead_request_free(aead_req);
129 	return ret;
130 }
131 
132 /*
133  * Preparse a big key
134  */
135 int big_key_preparse(struct key_preparsed_payload *prep)
136 {
137 	struct path *path = (struct path *)&prep->payload.data[big_key_path];
138 	struct file *file;
139 	u8 *enckey;
140 	u8 *data = NULL;
141 	ssize_t written;
142 	size_t datalen = prep->datalen;
143 	int ret;
144 
145 	ret = -EINVAL;
146 	if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
147 		goto error;
148 
149 	/* Set an arbitrary quota */
150 	prep->quotalen = 16;
151 
152 	prep->payload.data[big_key_len] = (void *)(unsigned long)datalen;
153 
154 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
155 		/* Create a shmem file to store the data in.  This will permit the data
156 		 * to be swapped out if needed.
157 		 *
158 		 * File content is stored encrypted with randomly generated key.
159 		 */
160 		size_t enclen = datalen + ENC_AUTHTAG_SIZE;
161 		loff_t pos = 0;
162 
163 		data = kmalloc(enclen, GFP_KERNEL);
164 		if (!data)
165 			return -ENOMEM;
166 		memcpy(data, prep->data, datalen);
167 
168 		/* generate random key */
169 		enckey = kmalloc(ENC_KEY_SIZE, GFP_KERNEL);
170 		if (!enckey) {
171 			ret = -ENOMEM;
172 			goto error;
173 		}
174 		ret = get_random_bytes_wait(enckey, ENC_KEY_SIZE);
175 		if (unlikely(ret))
176 			goto err_enckey;
177 
178 		/* encrypt aligned data */
179 		ret = big_key_crypt(BIG_KEY_ENC, data, datalen, enckey);
180 		if (ret)
181 			goto err_enckey;
182 
183 		/* save aligned data to file */
184 		file = shmem_kernel_file_setup("", enclen, 0);
185 		if (IS_ERR(file)) {
186 			ret = PTR_ERR(file);
187 			goto err_enckey;
188 		}
189 
190 		written = kernel_write(file, data, enclen, &pos);
191 		if (written != enclen) {
192 			ret = written;
193 			if (written >= 0)
194 				ret = -ENOMEM;
195 			goto err_fput;
196 		}
197 
198 		/* Pin the mount and dentry to the key so that we can open it again
199 		 * later
200 		 */
201 		prep->payload.data[big_key_data] = enckey;
202 		*path = file->f_path;
203 		path_get(path);
204 		fput(file);
205 		kzfree(data);
206 	} else {
207 		/* Just store the data in a buffer */
208 		void *data = kmalloc(datalen, GFP_KERNEL);
209 
210 		if (!data)
211 			return -ENOMEM;
212 
213 		prep->payload.data[big_key_data] = data;
214 		memcpy(data, prep->data, prep->datalen);
215 	}
216 	return 0;
217 
218 err_fput:
219 	fput(file);
220 err_enckey:
221 	kzfree(enckey);
222 error:
223 	kzfree(data);
224 	return ret;
225 }
226 
227 /*
228  * Clear preparsement.
229  */
230 void big_key_free_preparse(struct key_preparsed_payload *prep)
231 {
232 	if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
233 		struct path *path = (struct path *)&prep->payload.data[big_key_path];
234 
235 		path_put(path);
236 	}
237 	kzfree(prep->payload.data[big_key_data]);
238 }
239 
240 /*
241  * dispose of the links from a revoked keyring
242  * - called with the key sem write-locked
243  */
244 void big_key_revoke(struct key *key)
245 {
246 	struct path *path = (struct path *)&key->payload.data[big_key_path];
247 
248 	/* clear the quota */
249 	key_payload_reserve(key, 0);
250 	if (key_is_positive(key) &&
251 	    (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
252 		vfs_truncate(path, 0);
253 }
254 
255 /*
256  * dispose of the data dangling from the corpse of a big_key key
257  */
258 void big_key_destroy(struct key *key)
259 {
260 	size_t datalen = (size_t)key->payload.data[big_key_len];
261 
262 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
263 		struct path *path = (struct path *)&key->payload.data[big_key_path];
264 
265 		path_put(path);
266 		path->mnt = NULL;
267 		path->dentry = NULL;
268 	}
269 	kzfree(key->payload.data[big_key_data]);
270 	key->payload.data[big_key_data] = NULL;
271 }
272 
273 /*
274  * describe the big_key key
275  */
276 void big_key_describe(const struct key *key, struct seq_file *m)
277 {
278 	size_t datalen = (size_t)key->payload.data[big_key_len];
279 
280 	seq_puts(m, key->description);
281 
282 	if (key_is_positive(key))
283 		seq_printf(m, ": %zu [%s]",
284 			   datalen,
285 			   datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
286 }
287 
288 /*
289  * read the key data
290  * - the key's semaphore is read-locked
291  */
292 long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
293 {
294 	size_t datalen = (size_t)key->payload.data[big_key_len];
295 	long ret;
296 
297 	if (!buffer || buflen < datalen)
298 		return datalen;
299 
300 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
301 		struct path *path = (struct path *)&key->payload.data[big_key_path];
302 		struct file *file;
303 		u8 *data;
304 		u8 *enckey = (u8 *)key->payload.data[big_key_data];
305 		size_t enclen = datalen + ENC_AUTHTAG_SIZE;
306 		loff_t pos = 0;
307 
308 		data = kmalloc(enclen, GFP_KERNEL);
309 		if (!data)
310 			return -ENOMEM;
311 
312 		file = dentry_open(path, O_RDONLY, current_cred());
313 		if (IS_ERR(file)) {
314 			ret = PTR_ERR(file);
315 			goto error;
316 		}
317 
318 		/* read file to kernel and decrypt */
319 		ret = kernel_read(file, data, enclen, &pos);
320 		if (ret >= 0 && ret != enclen) {
321 			ret = -EIO;
322 			goto err_fput;
323 		}
324 
325 		ret = big_key_crypt(BIG_KEY_DEC, data, enclen, enckey);
326 		if (ret)
327 			goto err_fput;
328 
329 		ret = datalen;
330 
331 		/* copy decrypted data to user */
332 		if (copy_to_user(buffer, data, datalen) != 0)
333 			ret = -EFAULT;
334 
335 err_fput:
336 		fput(file);
337 error:
338 		kzfree(data);
339 	} else {
340 		ret = datalen;
341 		if (copy_to_user(buffer, key->payload.data[big_key_data],
342 				 datalen) != 0)
343 			ret = -EFAULT;
344 	}
345 
346 	return ret;
347 }
348 
349 /*
350  * Register key type
351  */
352 static int __init big_key_init(void)
353 {
354 	int ret;
355 
356 	/* init block cipher */
357 	big_key_aead = crypto_alloc_aead(big_key_alg_name, 0, CRYPTO_ALG_ASYNC);
358 	if (IS_ERR(big_key_aead)) {
359 		ret = PTR_ERR(big_key_aead);
360 		pr_err("Can't alloc crypto: %d\n", ret);
361 		return ret;
362 	}
363 	ret = crypto_aead_setauthsize(big_key_aead, ENC_AUTHTAG_SIZE);
364 	if (ret < 0) {
365 		pr_err("Can't set crypto auth tag len: %d\n", ret);
366 		goto free_aead;
367 	}
368 
369 	ret = register_key_type(&key_type_big_key);
370 	if (ret < 0) {
371 		pr_err("Can't register type: %d\n", ret);
372 		goto free_aead;
373 	}
374 
375 	return 0;
376 
377 free_aead:
378 	crypto_free_aead(big_key_aead);
379 	return ret;
380 }
381 
382 late_initcall(big_key_init);
383