xref: /linux/security/keys/big_key.c (revision 3503d56cc7233ced602e38a4c13caa64f00ab2aa) 
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 enum {
24 	big_key_data,
25 	big_key_path,
26 	big_key_path_2nd_part,
27 	big_key_len,
28 };
29 
30 /*
31  * If the data is under this limit, there's no point creating a shm file to
32  * hold it as the permanently resident metadata for the shmem fs will be at
33  * least as large as the data.
34  */
35 #define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))
36 
37 /*
38  * big_key defined keys take an arbitrary string as the description and an
39  * arbitrary blob of data as the payload
40  */
41 struct key_type key_type_big_key = {
42 	.name			= "big_key",
43 	.preparse		= big_key_preparse,
44 	.free_preparse		= big_key_free_preparse,
45 	.instantiate		= generic_key_instantiate,
46 	.revoke			= big_key_revoke,
47 	.destroy		= big_key_destroy,
48 	.describe		= big_key_describe,
49 	.read			= big_key_read,
50 	.update			= big_key_update,
51 };
52 
53 /*
54  * Preparse a big key
55  */
56 int big_key_preparse(struct key_preparsed_payload *prep)
57 {
58 	struct path *path = (struct path *)&prep->payload.data[big_key_path];
59 	struct file *file;
60 	u8 *buf, *enckey;
61 	ssize_t written;
62 	size_t datalen = prep->datalen;
63 	size_t enclen = datalen + CHACHA20POLY1305_AUTHTAG_SIZE;
64 	int ret;
65 
66 	if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
67 		return -EINVAL;
68 
69 	/* Set an arbitrary quota */
70 	prep->quotalen = 16;
71 
72 	prep->payload.data[big_key_len] = (void *)(unsigned long)datalen;
73 
74 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
75 		/* Create a shmem file to store the data in.  This will permit the data
76 		 * to be swapped out if needed.
77 		 *
78 		 * File content is stored encrypted with randomly generated key.
79 		 * Since the key is random for each file, we can set the nonce
80 		 * to zero, provided we never define a ->update() call.
81 		 */
82 		loff_t pos = 0;
83 
84 		buf = kvmalloc(enclen, GFP_KERNEL);
85 		if (!buf)
86 			return -ENOMEM;
87 
88 		/* generate random key */
89 		enckey = kmalloc(CHACHA20POLY1305_KEY_SIZE, GFP_KERNEL);
90 		if (!enckey) {
91 			ret = -ENOMEM;
92 			goto error;
93 		}
94 		ret = get_random_bytes_wait(enckey, CHACHA20POLY1305_KEY_SIZE);
95 		if (unlikely(ret))
96 			goto err_enckey;
97 
98 		/* encrypt data */
99 		chacha20poly1305_encrypt(buf, prep->data, datalen, NULL, 0,
100 					 0, enckey);
101 
102 		/* save aligned data to file */
103 		file = shmem_kernel_file_setup("", enclen, 0);
104 		if (IS_ERR(file)) {
105 			ret = PTR_ERR(file);
106 			goto err_enckey;
107 		}
108 
109 		written = kernel_write(file, buf, enclen, &pos);
110 		if (written != enclen) {
111 			ret = written;
112 			if (written >= 0)
113 				ret = -EIO;
114 			goto err_fput;
115 		}
116 
117 		/* Pin the mount and dentry to the key so that we can open it again
118 		 * later
119 		 */
120 		prep->payload.data[big_key_data] = enckey;
121 		*path = file->f_path;
122 		path_get(path);
123 		fput(file);
124 		memzero_explicit(buf, enclen);
125 		kvfree(buf);
126 	} else {
127 		/* Just store the data in a buffer */
128 		void *data = kmalloc(datalen, GFP_KERNEL);
129 
130 		if (!data)
131 			return -ENOMEM;
132 
133 		prep->payload.data[big_key_data] = data;
134 		memcpy(data, prep->data, prep->datalen);
135 	}
136 	return 0;
137 
138 err_fput:
139 	fput(file);
140 err_enckey:
141 	kzfree(enckey);
142 error:
143 	memzero_explicit(buf, enclen);
144 	kvfree(buf);
145 	return ret;
146 }
147 
148 /*
149  * Clear preparsement.
150  */
151 void big_key_free_preparse(struct key_preparsed_payload *prep)
152 {
153 	if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
154 		struct path *path = (struct path *)&prep->payload.data[big_key_path];
155 
156 		path_put(path);
157 	}
158 	kzfree(prep->payload.data[big_key_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 path *path = (struct path *)&key->payload.data[big_key_path];
168 
169 	/* clear the quota */
170 	key_payload_reserve(key, 0);
171 	if (key_is_positive(key) &&
172 	    (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
173 		vfs_truncate(path, 0);
174 }
175 
176 /*
177  * dispose of the data dangling from the corpse of a big_key key
178  */
179 void big_key_destroy(struct key *key)
180 {
181 	size_t datalen = (size_t)key->payload.data[big_key_len];
182 
183 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
184 		struct path *path = (struct path *)&key->payload.data[big_key_path];
185 
186 		path_put(path);
187 		path->mnt = NULL;
188 		path->dentry = NULL;
189 	}
190 	kzfree(key->payload.data[big_key_data]);
191 	key->payload.data[big_key_data] = NULL;
192 }
193 
194 /*
195  * Update a big key
196  */
197 int big_key_update(struct key *key, struct key_preparsed_payload *prep)
198 {
199 	int ret;
200 
201 	ret = key_payload_reserve(key, prep->datalen);
202 	if (ret < 0)
203 		return ret;
204 
205 	if (key_is_positive(key))
206 		big_key_destroy(key);
207 
208 	return generic_key_instantiate(key, prep);
209 }
210 
211 /*
212  * describe the big_key key
213  */
214 void big_key_describe(const struct key *key, struct seq_file *m)
215 {
216 	size_t datalen = (size_t)key->payload.data[big_key_len];
217 
218 	seq_puts(m, key->description);
219 
220 	if (key_is_positive(key))
221 		seq_printf(m, ": %zu [%s]",
222 			   datalen,
223 			   datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
224 }
225 
226 /*
227  * read the key data
228  * - the key's semaphore is read-locked
229  */
230 long big_key_read(const struct key *key, char *buffer, size_t buflen)
231 {
232 	size_t datalen = (size_t)key->payload.data[big_key_len];
233 	long ret;
234 
235 	if (!buffer || buflen < datalen)
236 		return datalen;
237 
238 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
239 		struct path *path = (struct path *)&key->payload.data[big_key_path];
240 		struct file *file;
241 		u8 *buf, *enckey = (u8 *)key->payload.data[big_key_data];
242 		size_t enclen = datalen + CHACHA20POLY1305_AUTHTAG_SIZE;
243 		loff_t pos = 0;
244 
245 		buf = kvmalloc(enclen, GFP_KERNEL);
246 		if (!buf)
247 			return -ENOMEM;
248 
249 		file = dentry_open(path, O_RDONLY, current_cred());
250 		if (IS_ERR(file)) {
251 			ret = PTR_ERR(file);
252 			goto error;
253 		}
254 
255 		/* read file to kernel and decrypt */
256 		ret = kernel_read(file, buf, enclen, &pos);
257 		if (ret != enclen) {
258 			if (ret >= 0)
259 				ret = -EIO;
260 			goto err_fput;
261 		}
262 
263 		ret = chacha20poly1305_decrypt(buf, buf, enclen, NULL, 0, 0,
264 					       enckey) ? 0 : -EBADMSG;
265 		if (unlikely(ret))
266 			goto err_fput;
267 
268 		ret = datalen;
269 
270 		/* copy out decrypted data */
271 		memcpy(buffer, buf, datalen);
272 
273 err_fput:
274 		fput(file);
275 error:
276 		memzero_explicit(buf, enclen);
277 		kvfree(buf);
278 	} else {
279 		ret = datalen;
280 		memcpy(buffer, key->payload.data[big_key_data], datalen);
281 	}
282 
283 	return ret;
284 }
285 
286 /*
287  * Register key type
288  */
289 static int __init big_key_init(void)
290 {
291 	return register_key_type(&key_type_big_key);
292 }
293 
294 late_initcall(big_key_init);
295