xref: /linux/security/keys/big_key.c (revision c79c3c34f75d72a066e292b10aa50fc758c97c89) 
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 		kvfree_sensitive(buf, enclen);
125 	} else {
126 		/* Just store the data in a buffer */
127 		void *data = kmalloc(datalen, GFP_KERNEL);
128 
129 		if (!data)
130 			return -ENOMEM;
131 
132 		prep->payload.data[big_key_data] = data;
133 		memcpy(data, prep->data, prep->datalen);
134 	}
135 	return 0;
136 
137 err_fput:
138 	fput(file);
139 err_enckey:
140 	kfree_sensitive(enckey);
141 error:
142 	kvfree_sensitive(buf, enclen);
143 	return ret;
144 }
145 
146 /*
147  * Clear preparsement.
148  */
149 void big_key_free_preparse(struct key_preparsed_payload *prep)
150 {
151 	if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
152 		struct path *path = (struct path *)&prep->payload.data[big_key_path];
153 
154 		path_put(path);
155 	}
156 	kfree_sensitive(prep->payload.data[big_key_data]);
157 }
158 
159 /*
160  * dispose of the links from a revoked keyring
161  * - called with the key sem write-locked
162  */
163 void big_key_revoke(struct key *key)
164 {
165 	struct path *path = (struct path *)&key->payload.data[big_key_path];
166 
167 	/* clear the quota */
168 	key_payload_reserve(key, 0);
169 	if (key_is_positive(key) &&
170 	    (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
171 		vfs_truncate(path, 0);
172 }
173 
174 /*
175  * dispose of the data dangling from the corpse of a big_key key
176  */
177 void big_key_destroy(struct key *key)
178 {
179 	size_t datalen = (size_t)key->payload.data[big_key_len];
180 
181 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
182 		struct path *path = (struct path *)&key->payload.data[big_key_path];
183 
184 		path_put(path);
185 		path->mnt = NULL;
186 		path->dentry = NULL;
187 	}
188 	kfree_sensitive(key->payload.data[big_key_data]);
189 	key->payload.data[big_key_data] = NULL;
190 }
191 
192 /*
193  * Update a big key
194  */
195 int big_key_update(struct key *key, struct key_preparsed_payload *prep)
196 {
197 	int ret;
198 
199 	ret = key_payload_reserve(key, prep->datalen);
200 	if (ret < 0)
201 		return ret;
202 
203 	if (key_is_positive(key))
204 		big_key_destroy(key);
205 
206 	return generic_key_instantiate(key, prep);
207 }
208 
209 /*
210  * describe the big_key key
211  */
212 void big_key_describe(const struct key *key, struct seq_file *m)
213 {
214 	size_t datalen = (size_t)key->payload.data[big_key_len];
215 
216 	seq_puts(m, key->description);
217 
218 	if (key_is_positive(key))
219 		seq_printf(m, ": %zu [%s]",
220 			   datalen,
221 			   datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
222 }
223 
224 /*
225  * read the key data
226  * - the key's semaphore is read-locked
227  */
228 long big_key_read(const struct key *key, char *buffer, size_t buflen)
229 {
230 	size_t datalen = (size_t)key->payload.data[big_key_len];
231 	long ret;
232 
233 	if (!buffer || buflen < datalen)
234 		return datalen;
235 
236 	if (datalen > BIG_KEY_FILE_THRESHOLD) {
237 		struct path *path = (struct path *)&key->payload.data[big_key_path];
238 		struct file *file;
239 		u8 *buf, *enckey = (u8 *)key->payload.data[big_key_data];
240 		size_t enclen = datalen + CHACHA20POLY1305_AUTHTAG_SIZE;
241 		loff_t pos = 0;
242 
243 		buf = kvmalloc(enclen, GFP_KERNEL);
244 		if (!buf)
245 			return -ENOMEM;
246 
247 		file = dentry_open(path, O_RDONLY, current_cred());
248 		if (IS_ERR(file)) {
249 			ret = PTR_ERR(file);
250 			goto error;
251 		}
252 
253 		/* read file to kernel and decrypt */
254 		ret = kernel_read(file, buf, enclen, &pos);
255 		if (ret != enclen) {
256 			if (ret >= 0)
257 				ret = -EIO;
258 			goto err_fput;
259 		}
260 
261 		ret = chacha20poly1305_decrypt(buf, buf, enclen, NULL, 0, 0,
262 					       enckey) ? 0 : -EBADMSG;
263 		if (unlikely(ret))
264 			goto err_fput;
265 
266 		ret = datalen;
267 
268 		/* copy out decrypted data */
269 		memcpy(buffer, buf, datalen);
270 
271 err_fput:
272 		fput(file);
273 error:
274 		kvfree_sensitive(buf, enclen);
275 	} else {
276 		ret = datalen;
277 		memcpy(buffer, key->payload.data[big_key_data], datalen);
278 	}
279 
280 	return ret;
281 }
282 
283 /*
284  * Register key type
285  */
286 static int __init big_key_init(void)
287 {
288 	return register_key_type(&key_type_big_key);
289 }
290 
291 late_initcall(big_key_init);
292