xref: /openbmc/linux/security/keys/big_key.c (revision 20e2fc42) 
1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /* Large capacity key type
3  *
4  * Copyright (C) 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
5  * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
6  * Written by David Howells (dhowells@redhat.com)
7  */
8 
9 #define pr_fmt(fmt) "big_key: "fmt
10 #include <linux/init.h>
11 #include <linux/seq_file.h>
12 #include <linux/file.h>
13 #include <linux/shmem_fs.h>
14 #include <linux/err.h>
15 #include <linux/scatterlist.h>
16 #include <linux/random.h>
17 #include <linux/vmalloc.h>
18 #include <keys/user-type.h>
19 #include <keys/big_key-type.h>
20 #include <crypto/aead.h>
21 #include <crypto/gcm.h>
22 
23 struct big_key_buf {
24 	unsigned int		nr_pages;
25 	void			*virt;
26 	struct scatterlist	*sg;
27 	struct page		*pages[];
28 };
29 
30 /*
31  * Layout of key payload words.
32  */
33 enum {
34 	big_key_data,
35 	big_key_path,
36 	big_key_path_2nd_part,
37 	big_key_len,
38 };
39 
40 /*
41  * Crypto operation with big_key data
42  */
43 enum big_key_op {
44 	BIG_KEY_ENC,
45 	BIG_KEY_DEC,
46 };
47 
48 /*
49  * If the data is under this limit, there's no point creating a shm file to
50  * hold it as the permanently resident metadata for the shmem fs will be at
51  * least as large as the data.
52  */
53 #define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))
54 
55 /*
56  * Key size for big_key data encryption
57  */
58 #define ENC_KEY_SIZE 32
59 
60 /*
61  * Authentication tag length
62  */
63 #define ENC_AUTHTAG_SIZE 16
64 
65 /*
66  * big_key defined keys take an arbitrary string as the description and an
67  * arbitrary blob of data as the payload
68  */
69 struct key_type key_type_big_key = {
70 	.name			= "big_key",
71 	.preparse		= big_key_preparse,
72 	.free_preparse		= big_key_free_preparse,
73 	.instantiate		= generic_key_instantiate,
74 	.revoke			= big_key_revoke,
75 	.destroy		= big_key_destroy,
76 	.describe		= big_key_describe,
77 	.read			= big_key_read,
78 	/* no ->update(); don't add it without changing big_key_crypt() nonce */
79 };
80 
81 /*
82  * Crypto names for big_key data authenticated encryption
83  */
84 static const char big_key_alg_name[] = "gcm(aes)";
85 #define BIG_KEY_IV_SIZE		GCM_AES_IV_SIZE
86 
87 /*
88  * Crypto algorithms for big_key data authenticated encryption
89  */
90 static struct crypto_aead *big_key_aead;
91 
92 /*
93  * Since changing the key affects the entire object, we need a mutex.
94  */
95 static DEFINE_MUTEX(big_key_aead_lock);
96 
97 /*
98  * Encrypt/decrypt big_key data
99  */
100 static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t datalen, u8 *key)
101 {
102 	int ret;
103 	struct aead_request *aead_req;
104 	/* We always use a zero nonce. The reason we can get away with this is
105 	 * because we're using a different randomly generated key for every
106 	 * different encryption. Notably, too, key_type_big_key doesn't define
107 	 * an .update function, so there's no chance we'll wind up reusing the
108 	 * key to encrypt updated data. Simply put: one key, one encryption.
109 	 */
110 	u8 zero_nonce[BIG_KEY_IV_SIZE];
111 
112 	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
113 	if (!aead_req)
114 		return -ENOMEM;
115 
116 	memset(zero_nonce, 0, sizeof(zero_nonce));
117 	aead_request_set_crypt(aead_req, buf->sg, buf->sg, datalen, zero_nonce);
118 	aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
119 	aead_request_set_ad(aead_req, 0);
120 
121 	mutex_lock(&big_key_aead_lock);
122 	if (crypto_aead_setkey(big_key_aead, key, ENC_KEY_SIZE)) {
123 		ret = -EAGAIN;
124 		goto error;
125 	}
126 	if (op == BIG_KEY_ENC)
127 		ret = crypto_aead_encrypt(aead_req);
128 	else
129 		ret = crypto_aead_decrypt(aead_req);
130 error:
131 	mutex_unlock(&big_key_aead_lock);
132 	aead_request_free(aead_req);
133 	return ret;
134 }
135 
136 /*
137  * Free up the buffer.
138  */
139 static void big_key_free_buffer(struct big_key_buf *buf)
140 {
141 	unsigned int i;
142 
143 	if (buf->virt) {
144 		memset(buf->virt, 0, buf->nr_pages * PAGE_SIZE);
145 		vunmap(buf->virt);
146 	}
147 
148 	for (i = 0; i < buf->nr_pages; i++)
149 		if (buf->pages[i])
150 			__free_page(buf->pages[i]);
151 
152 	kfree(buf);
153 }
154 
155 /*
156  * Allocate a buffer consisting of a set of pages with a virtual mapping
157  * applied over them.
158  */
159 static void *big_key_alloc_buffer(size_t len)
160 {
161 	struct big_key_buf *buf;
162 	unsigned int npg = (len + PAGE_SIZE - 1) >> PAGE_SHIFT;
163 	unsigned int i, l;
164 
165 	buf = kzalloc(sizeof(struct big_key_buf) +
166 		      sizeof(struct page) * npg +
167 		      sizeof(struct scatterlist) * npg,
168 		      GFP_KERNEL);
169 	if (!buf)
170 		return NULL;
171 
172 	buf->nr_pages = npg;
173 	buf->sg = (void *)(buf->pages + npg);
174 	sg_init_table(buf->sg, npg);
175 
176 	for (i = 0; i < buf->nr_pages; i++) {
177 		buf->pages[i] = alloc_page(GFP_KERNEL);
178 		if (!buf->pages[i])
179 			goto nomem;
180 
181 		l = min_t(size_t, len, PAGE_SIZE);
182 		sg_set_page(&buf->sg[i], buf->pages[i], l, 0);
183 		len -= l;
184 	}
185 
186 	buf->virt = vmap(buf->pages, buf->nr_pages, VM_MAP, PAGE_KERNEL);
187 	if (!buf->virt)
188 		goto nomem;
189 
190 	return buf;
191 
192 nomem:
193 	big_key_free_buffer(buf);
194 	return NULL;
195 }
196 
197 /*
198  * Preparse a big key
199  */
200 int big_key_preparse(struct key_preparsed_payload *prep)
201 {
202 	struct big_key_buf *buf;
203 	struct path *path = (struct path *)&prep->payload.data[big_key_path];
204 	struct file *file;
205 	u8 *enckey;
206 	ssize_t written;
207 	size_t datalen = prep->datalen, enclen = datalen + ENC_AUTHTAG_SIZE;
208 	int ret;
209 
210 	if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
211 		return -EINVAL;
212 
213 	/* Set an arbitrary quota */
214 	prep->quotalen = 16;
215 
216 	prep->payload.data[big_key_len] = (void *)(unsigned long)datalen;
217 
218 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
219 		/* Create a shmem file to store the data in.  This will permit the data
220 		 * to be swapped out if needed.
221 		 *
222 		 * File content is stored encrypted with randomly generated key.
223 		 */
224 		loff_t pos = 0;
225 
226 		buf = big_key_alloc_buffer(enclen);
227 		if (!buf)
228 			return -ENOMEM;
229 		memcpy(buf->virt, prep->data, datalen);
230 
231 		/* generate random key */
232 		enckey = kmalloc(ENC_KEY_SIZE, GFP_KERNEL);
233 		if (!enckey) {
234 			ret = -ENOMEM;
235 			goto error;
236 		}
237 		ret = get_random_bytes_wait(enckey, ENC_KEY_SIZE);
238 		if (unlikely(ret))
239 			goto err_enckey;
240 
241 		/* encrypt aligned data */
242 		ret = big_key_crypt(BIG_KEY_ENC, buf, datalen, enckey);
243 		if (ret)
244 			goto err_enckey;
245 
246 		/* save aligned data to file */
247 		file = shmem_kernel_file_setup("", enclen, 0);
248 		if (IS_ERR(file)) {
249 			ret = PTR_ERR(file);
250 			goto err_enckey;
251 		}
252 
253 		written = kernel_write(file, buf->virt, enclen, &pos);
254 		if (written != enclen) {
255 			ret = written;
256 			if (written >= 0)
257 				ret = -ENOMEM;
258 			goto err_fput;
259 		}
260 
261 		/* Pin the mount and dentry to the key so that we can open it again
262 		 * later
263 		 */
264 		prep->payload.data[big_key_data] = enckey;
265 		*path = file->f_path;
266 		path_get(path);
267 		fput(file);
268 		big_key_free_buffer(buf);
269 	} else {
270 		/* Just store the data in a buffer */
271 		void *data = kmalloc(datalen, GFP_KERNEL);
272 
273 		if (!data)
274 			return -ENOMEM;
275 
276 		prep->payload.data[big_key_data] = data;
277 		memcpy(data, prep->data, prep->datalen);
278 	}
279 	return 0;
280 
281 err_fput:
282 	fput(file);
283 err_enckey:
284 	kzfree(enckey);
285 error:
286 	big_key_free_buffer(buf);
287 	return ret;
288 }
289 
290 /*
291  * Clear preparsement.
292  */
293 void big_key_free_preparse(struct key_preparsed_payload *prep)
294 {
295 	if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
296 		struct path *path = (struct path *)&prep->payload.data[big_key_path];
297 
298 		path_put(path);
299 	}
300 	kzfree(prep->payload.data[big_key_data]);
301 }
302 
303 /*
304  * dispose of the links from a revoked keyring
305  * - called with the key sem write-locked
306  */
307 void big_key_revoke(struct key *key)
308 {
309 	struct path *path = (struct path *)&key->payload.data[big_key_path];
310 
311 	/* clear the quota */
312 	key_payload_reserve(key, 0);
313 	if (key_is_positive(key) &&
314 	    (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
315 		vfs_truncate(path, 0);
316 }
317 
318 /*
319  * dispose of the data dangling from the corpse of a big_key key
320  */
321 void big_key_destroy(struct key *key)
322 {
323 	size_t datalen = (size_t)key->payload.data[big_key_len];
324 
325 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
326 		struct path *path = (struct path *)&key->payload.data[big_key_path];
327 
328 		path_put(path);
329 		path->mnt = NULL;
330 		path->dentry = NULL;
331 	}
332 	kzfree(key->payload.data[big_key_data]);
333 	key->payload.data[big_key_data] = NULL;
334 }
335 
336 /*
337  * describe the big_key key
338  */
339 void big_key_describe(const struct key *key, struct seq_file *m)
340 {
341 	size_t datalen = (size_t)key->payload.data[big_key_len];
342 
343 	seq_puts(m, key->description);
344 
345 	if (key_is_positive(key))
346 		seq_printf(m, ": %zu [%s]",
347 			   datalen,
348 			   datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
349 }
350 
351 /*
352  * read the key data
353  * - the key's semaphore is read-locked
354  */
355 long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
356 {
357 	size_t datalen = (size_t)key->payload.data[big_key_len];
358 	long ret;
359 
360 	if (!buffer || buflen < datalen)
361 		return datalen;
362 
363 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
364 		struct big_key_buf *buf;
365 		struct path *path = (struct path *)&key->payload.data[big_key_path];
366 		struct file *file;
367 		u8 *enckey = (u8 *)key->payload.data[big_key_data];
368 		size_t enclen = datalen + ENC_AUTHTAG_SIZE;
369 		loff_t pos = 0;
370 
371 		buf = big_key_alloc_buffer(enclen);
372 		if (!buf)
373 			return -ENOMEM;
374 
375 		file = dentry_open(path, O_RDONLY, current_cred());
376 		if (IS_ERR(file)) {
377 			ret = PTR_ERR(file);
378 			goto error;
379 		}
380 
381 		/* read file to kernel and decrypt */
382 		ret = kernel_read(file, buf->virt, enclen, &pos);
383 		if (ret >= 0 && ret != enclen) {
384 			ret = -EIO;
385 			goto err_fput;
386 		}
387 
388 		ret = big_key_crypt(BIG_KEY_DEC, buf, enclen, enckey);
389 		if (ret)
390 			goto err_fput;
391 
392 		ret = datalen;
393 
394 		/* copy decrypted data to user */
395 		if (copy_to_user(buffer, buf->virt, datalen) != 0)
396 			ret = -EFAULT;
397 
398 err_fput:
399 		fput(file);
400 error:
401 		big_key_free_buffer(buf);
402 	} else {
403 		ret = datalen;
404 		if (copy_to_user(buffer, key->payload.data[big_key_data],
405 				 datalen) != 0)
406 			ret = -EFAULT;
407 	}
408 
409 	return ret;
410 }
411 
412 /*
413  * Register key type
414  */
415 static int __init big_key_init(void)
416 {
417 	int ret;
418 
419 	/* init block cipher */
420 	big_key_aead = crypto_alloc_aead(big_key_alg_name, 0, CRYPTO_ALG_ASYNC);
421 	if (IS_ERR(big_key_aead)) {
422 		ret = PTR_ERR(big_key_aead);
423 		pr_err("Can't alloc crypto: %d\n", ret);
424 		return ret;
425 	}
426 
427 	if (unlikely(crypto_aead_ivsize(big_key_aead) != BIG_KEY_IV_SIZE)) {
428 		WARN(1, "big key algorithm changed?");
429 		ret = -EINVAL;
430 		goto free_aead;
431 	}
432 
433 	ret = crypto_aead_setauthsize(big_key_aead, ENC_AUTHTAG_SIZE);
434 	if (ret < 0) {
435 		pr_err("Can't set crypto auth tag len: %d\n", ret);
436 		goto free_aead;
437 	}
438 
439 	ret = register_key_type(&key_type_big_key);
440 	if (ret < 0) {
441 		pr_err("Can't register type: %d\n", ret);
442 		goto free_aead;
443 	}
444 
445 	return 0;
446 
447 free_aead:
448 	crypto_free_aead(big_key_aead);
449 	return ret;
450 }
451 
452 late_initcall(big_key_init);
453