xref: /openbmc/linux/security/keys/big_key.c (revision e65e175b) 
1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /* Large capacity key type
3  *
4  * Copyright (C) 2017-2020 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/random.h>
16 #include <keys/user-type.h>
17 #include <keys/big_key-type.h>
18 #include <crypto/chacha20poly1305.h>
19 
20 /*
21  * Layout of key payload words.
22  */
23 struct big_key_payload {
24 	u8 *data;
25 	struct path path;
26 	size_t length;
27 };
28 #define to_big_key_payload(payload)			\
29 	(struct big_key_payload *)((payload).data)
30 
31 /*
32  * If the data is under this limit, there's no point creating a shm file to
33  * hold it as the permanently resident metadata for the shmem fs will be at
34  * least as large as the data.
35  */
36 #define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))
37 
38 /*
39  * big_key defined keys take an arbitrary string as the description and an
40  * arbitrary blob of data as the payload
41  */
42 struct key_type key_type_big_key = {
43 	.name			= "big_key",
44 	.preparse		= big_key_preparse,
45 	.free_preparse		= big_key_free_preparse,
46 	.instantiate		= generic_key_instantiate,
47 	.revoke			= big_key_revoke,
48 	.destroy		= big_key_destroy,
49 	.describe		= big_key_describe,
50 	.read			= big_key_read,
51 	.update			= big_key_update,
52 };
53 
54 /*
55  * Preparse a big key
56  */
57 int big_key_preparse(struct key_preparsed_payload *prep)
58 {
59 	struct big_key_payload *payload = to_big_key_payload(prep->payload);
60 	struct file *file;
61 	u8 *buf, *enckey;
62 	ssize_t written;
63 	size_t datalen = prep->datalen;
64 	size_t enclen = datalen + CHACHA20POLY1305_AUTHTAG_SIZE;
65 	int ret;
66 
67 	BUILD_BUG_ON(sizeof(*payload) != sizeof(prep->payload.data));
68 
69 	if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
70 		return -EINVAL;
71 
72 	/* Set an arbitrary quota */
73 	prep->quotalen = 16;
74 
75 	payload->length = datalen;
76 
77 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
78 		/* Create a shmem file to store the data in.  This will permit the data
79 		 * to be swapped out if needed.
80 		 *
81 		 * File content is stored encrypted with randomly generated key.
82 		 * Since the key is random for each file, we can set the nonce
83 		 * to zero, provided we never define a ->update() call.
84 		 */
85 		loff_t pos = 0;
86 
87 		buf = kvmalloc(enclen, GFP_KERNEL);
88 		if (!buf)
89 			return -ENOMEM;
90 
91 		/* generate random key */
92 		enckey = kmalloc(CHACHA20POLY1305_KEY_SIZE, GFP_KERNEL);
93 		if (!enckey) {
94 			ret = -ENOMEM;
95 			goto error;
96 		}
97 		ret = get_random_bytes_wait(enckey, CHACHA20POLY1305_KEY_SIZE);
98 		if (unlikely(ret))
99 			goto err_enckey;
100 
101 		/* encrypt data */
102 		chacha20poly1305_encrypt(buf, prep->data, datalen, NULL, 0,
103 					 0, enckey);
104 
105 		/* save aligned data to file */
106 		file = shmem_kernel_file_setup("", enclen, 0);
107 		if (IS_ERR(file)) {
108 			ret = PTR_ERR(file);
109 			goto err_enckey;
110 		}
111 
112 		written = kernel_write(file, buf, enclen, &pos);
113 		if (written != enclen) {
114 			ret = written;
115 			if (written >= 0)
116 				ret = -EIO;
117 			goto err_fput;
118 		}
119 
120 		/* Pin the mount and dentry to the key so that we can open it again
121 		 * later
122 		 */
123 		payload->data = enckey;
124 		payload->path = file->f_path;
125 		path_get(&payload->path);
126 		fput(file);
127 		kvfree_sensitive(buf, enclen);
128 	} else {
129 		/* Just store the data in a buffer */
130 		void *data = kmalloc(datalen, GFP_KERNEL);
131 
132 		if (!data)
133 			return -ENOMEM;
134 
135 		payload->data = data;
136 		memcpy(data, prep->data, prep->datalen);
137 	}
138 	return 0;
139 
140 err_fput:
141 	fput(file);
142 err_enckey:
143 	kfree_sensitive(enckey);
144 error:
145 	kvfree_sensitive(buf, enclen);
146 	return ret;
147 }
148 
149 /*
150  * Clear preparsement.
151  */
152 void big_key_free_preparse(struct key_preparsed_payload *prep)
153 {
154 	struct big_key_payload *payload = to_big_key_payload(prep->payload);
155 
156 	if (prep->datalen > BIG_KEY_FILE_THRESHOLD)
157 		path_put(&payload->path);
158 	kfree_sensitive(payload->data);
159 }
160 
161 /*
162  * dispose of the links from a revoked keyring
163  * - called with the key sem write-locked
164  */
165 void big_key_revoke(struct key *key)
166 {
167 	struct big_key_payload *payload = to_big_key_payload(key->payload);
168 
169 	/* clear the quota */
170 	key_payload_reserve(key, 0);
171 	if (key_is_positive(key) && payload->length > BIG_KEY_FILE_THRESHOLD)
172 		vfs_truncate(&payload->path, 0);
173 }
174 
175 /*
176  * dispose of the data dangling from the corpse of a big_key key
177  */
178 void big_key_destroy(struct key *key)
179 {
180 	struct big_key_payload *payload = to_big_key_payload(key->payload);
181 
182 	if (payload->length > BIG_KEY_FILE_THRESHOLD) {
183 		path_put(&payload->path);
184 		payload->path.mnt = NULL;
185 		payload->path.dentry = NULL;
186 	}
187 	kfree_sensitive(payload->data);
188 	payload->data = NULL;
189 }
190 
191 /*
192  * Update a big key
193  */
194 int big_key_update(struct key *key, struct key_preparsed_payload *prep)
195 {
196 	int ret;
197 
198 	ret = key_payload_reserve(key, prep->datalen);
199 	if (ret < 0)
200 		return ret;
201 
202 	if (key_is_positive(key))
203 		big_key_destroy(key);
204 
205 	return generic_key_instantiate(key, prep);
206 }
207 
208 /*
209  * describe the big_key key
210  */
211 void big_key_describe(const struct key *key, struct seq_file *m)
212 {
213 	struct big_key_payload *payload = to_big_key_payload(key->payload);
214 
215 	seq_puts(m, key->description);
216 
217 	if (key_is_positive(key))
218 		seq_printf(m, ": %zu [%s]",
219 			   payload->length,
220 			   payload->length > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
221 }
222 
223 /*
224  * read the key data
225  * - the key's semaphore is read-locked
226  */
227 long big_key_read(const struct key *key, char *buffer, size_t buflen)
228 {
229 	struct big_key_payload *payload = to_big_key_payload(key->payload);
230 	size_t datalen = payload->length;
231 	long ret;
232 
233 	if (!buffer || buflen < datalen)
234 		return datalen;
235 
236 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
237 		struct file *file;
238 		u8 *buf, *enckey = payload->data;
239 		size_t enclen = datalen + CHACHA20POLY1305_AUTHTAG_SIZE;
240 		loff_t pos = 0;
241 
242 		buf = kvmalloc(enclen, GFP_KERNEL);
243 		if (!buf)
244 			return -ENOMEM;
245 
246 		file = dentry_open(&payload->path, O_RDONLY, current_cred());
247 		if (IS_ERR(file)) {
248 			ret = PTR_ERR(file);
249 			goto error;
250 		}
251 
252 		/* read file to kernel and decrypt */
253 		ret = kernel_read(file, buf, enclen, &pos);
254 		if (ret != enclen) {
255 			if (ret >= 0)
256 				ret = -EIO;
257 			goto err_fput;
258 		}
259 
260 		ret = chacha20poly1305_decrypt(buf, buf, enclen, NULL, 0, 0,
261 					       enckey) ? 0 : -EBADMSG;
262 		if (unlikely(ret))
263 			goto err_fput;
264 
265 		ret = datalen;
266 
267 		/* copy out decrypted data */
268 		memcpy(buffer, buf, datalen);
269 
270 err_fput:
271 		fput(file);
272 error:
273 		kvfree_sensitive(buf, enclen);
274 	} else {
275 		ret = datalen;
276 		memcpy(buffer, payload->data, datalen);
277 	}
278 
279 	return ret;
280 }
281 
282 /*
283  * Register key type
284  */
285 static int __init big_key_init(void)
286 {
287 	return register_key_type(&key_type_big_key);
288 }
289 
290 late_initcall(big_key_init);
291