1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * fs/crypto/hooks.c
4 *
5 * Encryption hooks for higher-level filesystem operations.
6 */
7
8 #include "fscrypt_private.h"
9
10 /**
11 * fscrypt_file_open() - prepare to open a possibly-encrypted regular file
12 * @inode: the inode being opened
13 * @filp: the struct file being set up
14 *
15 * Currently, an encrypted regular file can only be opened if its encryption key
16 * is available; access to the raw encrypted contents is not supported.
17 * Therefore, we first set up the inode's encryption key (if not already done)
18 * and return an error if it's unavailable.
19 *
20 * We also verify that if the parent directory (from the path via which the file
21 * is being opened) is encrypted, then the inode being opened uses the same
22 * encryption policy. This is needed as part of the enforcement that all files
23 * in an encrypted directory tree use the same encryption policy, as a
24 * protection against certain types of offline attacks. Note that this check is
25 * needed even when opening an *unencrypted* file, since it's forbidden to have
26 * an unencrypted file in an encrypted directory.
27 *
28 * Return: 0 on success, -ENOKEY if the key is missing, or another -errno code
29 */
30 int fscrypt_file_open(struct inode *inode, struct file *filp)
31 {
32 int err;
33 struct dentry *dentry, *dentry_parent;
34 struct inode *inode_parent;
35
36 err = fscrypt_require_key(inode);
37 if (err)
38 return err;
39
40 dentry = file_dentry(filp);
41
42 /*
43 * Getting a reference to the parent dentry is needed for the actual
44 * encryption policy comparison, but it's expensive on multi-core
45 * systems. Since this function runs on unencrypted files too, start
46 * with a lightweight RCU-mode check for the parent directory being
47 * unencrypted (in which case it's fine for the child to be either
48 * unencrypted, or encrypted with any policy). Only continue on to the
49 * full policy check if the parent directory is actually encrypted.
50 */
51 rcu_read_lock();
52 dentry_parent = READ_ONCE(dentry->d_parent);
53 inode_parent = d_inode_rcu(dentry_parent);
54 if (inode_parent != NULL && !IS_ENCRYPTED(inode_parent)) {
55 rcu_read_unlock();
56 return 0;
57 }
58 rcu_read_unlock();
59
60 dentry_parent = dget_parent(dentry);
61 if (!fscrypt_has_permitted_context(d_inode(dentry_parent), inode)) {
62 fscrypt_warn(inode,
63 "Inconsistent encryption context (parent directory: %lu)",
64 d_inode(dentry_parent)->i_ino);
65 err = -EPERM;
66 }
67 dput(dentry_parent);
68 return err;
69 }
70 EXPORT_SYMBOL_GPL(fscrypt_file_open);
71
72 int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
73 struct dentry *dentry)
74 {
75 if (fscrypt_is_nokey_name(dentry))
76 return -ENOKEY;
77 /*
78 * We don't need to separately check that the directory inode's key is
79 * available, as it's implied by the dentry not being a no-key name.
80 */
81
82 if (!fscrypt_has_permitted_context(dir, inode))
83 return -EXDEV;
84
85 return 0;
86 }
87 EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
88
89 int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
90 struct inode *new_dir, struct dentry *new_dentry,
91 unsigned int flags)
92 {
93 if (fscrypt_is_nokey_name(old_dentry) ||
94 fscrypt_is_nokey_name(new_dentry))
95 return -ENOKEY;
96 /*
97 * We don't need to separately check that the directory inodes' keys are
98 * available, as it's implied by the dentries not being no-key names.
99 */
100
101 if (old_dir != new_dir) {
102 if (IS_ENCRYPTED(new_dir) &&
103 !fscrypt_has_permitted_context(new_dir,
104 d_inode(old_dentry)))
105 return -EXDEV;
106
107 if ((flags & RENAME_EXCHANGE) &&
108 IS_ENCRYPTED(old_dir) &&
109 !fscrypt_has_permitted_context(old_dir,
110 d_inode(new_dentry)))
111 return -EXDEV;
112 }
113 return 0;
114 }
115 EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
116
117 int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
118 struct fscrypt_name *fname)
119 {
120 int err = fscrypt_setup_filename(dir, &dentry->d_name, 1, fname);
121
122 if (err && err != -ENOENT)
123 return err;
124
125 fscrypt_prepare_dentry(dentry, fname->is_nokey_name);
126
127 return err;
128 }
129 EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
130
131 /**
132 * fscrypt_prepare_lookup_partial() - prepare lookup without filename setup
133 * @dir: the encrypted directory being searched
134 * @dentry: the dentry being looked up in @dir
135 *
136 * This function should be used by the ->lookup and ->atomic_open methods of
137 * filesystems that handle filename encryption and no-key name encoding
138 * themselves and thus can't use fscrypt_prepare_lookup(). Like
139 * fscrypt_prepare_lookup(), this will try to set up the directory's encryption
140 * key and will set DCACHE_NOKEY_NAME on the dentry if the key is unavailable.
141 * However, this function doesn't set up a struct fscrypt_name for the filename.
142 *
143 * Return: 0 on success; -errno on error. Note that the encryption key being
144 * unavailable is not considered an error. It is also not an error if
145 * the encryption policy is unsupported by this kernel; that is treated
146 * like the key being unavailable, so that files can still be deleted.
147 */
148 int fscrypt_prepare_lookup_partial(struct inode *dir, struct dentry *dentry)
149 {
150 int err = fscrypt_get_encryption_info(dir, true);
151 bool is_nokey_name = (!err && !fscrypt_has_encryption_key(dir));
152
153 fscrypt_prepare_dentry(dentry, is_nokey_name);
154
155 return err;
156 }
157 EXPORT_SYMBOL_GPL(fscrypt_prepare_lookup_partial);
158
159 int __fscrypt_prepare_readdir(struct inode *dir)
160 {
161 return fscrypt_get_encryption_info(dir, true);
162 }
163 EXPORT_SYMBOL_GPL(__fscrypt_prepare_readdir);
164
165 int __fscrypt_prepare_setattr(struct dentry *dentry, struct iattr *attr)
166 {
167 if (attr->ia_valid & ATTR_SIZE)
168 return fscrypt_require_key(d_inode(dentry));
169 return 0;
170 }
171 EXPORT_SYMBOL_GPL(__fscrypt_prepare_setattr);
172
173 /**
174 * fscrypt_prepare_setflags() - prepare to change flags with FS_IOC_SETFLAGS
175 * @inode: the inode on which flags are being changed
176 * @oldflags: the old flags
177 * @flags: the new flags
178 *
179 * The caller should be holding i_rwsem for write.
180 *
181 * Return: 0 on success; -errno if the flags change isn't allowed or if
182 * another error occurs.
183 */
184 int fscrypt_prepare_setflags(struct inode *inode,
185 unsigned int oldflags, unsigned int flags)
186 {
187 struct fscrypt_inode_info *ci;
188 struct fscrypt_master_key *mk;
189 int err;
190
191 /*
192 * When the CASEFOLD flag is set on an encrypted directory, we must
193 * derive the secret key needed for the dirhash. This is only possible
194 * if the directory uses a v2 encryption policy.
195 */
196 if (IS_ENCRYPTED(inode) && (flags & ~oldflags & FS_CASEFOLD_FL)) {
197 err = fscrypt_require_key(inode);
198 if (err)
199 return err;
200 ci = inode->i_crypt_info;
201 if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
202 return -EINVAL;
203 mk = ci->ci_master_key;
204 down_read(&mk->mk_sem);
205 if (mk->mk_present)
206 err = fscrypt_derive_dirhash_key(ci, mk);
207 else
208 err = -ENOKEY;
209 up_read(&mk->mk_sem);
210 return err;
211 }
212 return 0;
213 }
214
215 /**
216 * fscrypt_prepare_symlink() - prepare to create a possibly-encrypted symlink
217 * @dir: directory in which the symlink is being created
218 * @target: plaintext symlink target
219 * @len: length of @target excluding null terminator
220 * @max_len: space the filesystem has available to store the symlink target
221 * @disk_link: (out) the on-disk symlink target being prepared
222 *
223 * This function computes the size the symlink target will require on-disk,
224 * stores it in @disk_link->len, and validates it against @max_len. An
225 * encrypted symlink may be longer than the original.
226 *
227 * Additionally, @disk_link->name is set to @target if the symlink will be
228 * unencrypted, but left NULL if the symlink will be encrypted. For encrypted
229 * symlinks, the filesystem must call fscrypt_encrypt_symlink() to create the
230 * on-disk target later. (The reason for the two-step process is that some
231 * filesystems need to know the size of the symlink target before creating the
232 * inode, e.g. to determine whether it will be a "fast" or "slow" symlink.)
233 *
234 * Return: 0 on success, -ENAMETOOLONG if the symlink target is too long,
235 * -ENOKEY if the encryption key is missing, or another -errno code if a problem
236 * occurred while setting up the encryption key.
237 */
238 int fscrypt_prepare_symlink(struct inode *dir, const char *target,
239 unsigned int len, unsigned int max_len,
240 struct fscrypt_str *disk_link)
241 {
242 const union fscrypt_policy *policy;
243
244 /*
245 * To calculate the size of the encrypted symlink target we need to know
246 * the amount of NUL padding, which is determined by the flags set in
247 * the encryption policy which will be inherited from the directory.
248 */
249 policy = fscrypt_policy_to_inherit(dir);
250 if (policy == NULL) {
251 /* Not encrypted */
252 disk_link->name = (unsigned char *)target;
253 disk_link->len = len + 1;
254 if (disk_link->len > max_len)
255 return -ENAMETOOLONG;
256 return 0;
257 }
258 if (IS_ERR(policy))
259 return PTR_ERR(policy);
260
261 /*
262 * Calculate the size of the encrypted symlink and verify it won't
263 * exceed max_len. Note that for historical reasons, encrypted symlink
264 * targets are prefixed with the ciphertext length, despite this
265 * actually being redundant with i_size. This decreases by 2 bytes the
266 * longest symlink target we can accept.
267 *
268 * We could recover 1 byte by not counting a null terminator, but
269 * counting it (even though it is meaningless for ciphertext) is simpler
270 * for now since filesystems will assume it is there and subtract it.
271 */
272 if (!__fscrypt_fname_encrypted_size(policy, len,
273 max_len - sizeof(struct fscrypt_symlink_data) - 1,
274 &disk_link->len))
275 return -ENAMETOOLONG;
276 disk_link->len += sizeof(struct fscrypt_symlink_data) + 1;
277
278 disk_link->name = NULL;
279 return 0;
280 }
281 EXPORT_SYMBOL_GPL(fscrypt_prepare_symlink);
282
283 int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
284 unsigned int len, struct fscrypt_str *disk_link)
285 {
286 int err;
287 struct qstr iname = QSTR_INIT(target, len);
288 struct fscrypt_symlink_data *sd;
289 unsigned int ciphertext_len;
290
291 /*
292 * fscrypt_prepare_new_inode() should have already set up the new
293 * symlink inode's encryption key. We don't wait until now to do it,
294 * since we may be in a filesystem transaction now.
295 */
296 if (WARN_ON_ONCE(!fscrypt_has_encryption_key(inode)))
297 return -ENOKEY;
298
299 if (disk_link->name) {
300 /* filesystem-provided buffer */
301 sd = (struct fscrypt_symlink_data *)disk_link->name;
302 } else {
303 sd = kmalloc(disk_link->len, GFP_NOFS);
304 if (!sd)
305 return -ENOMEM;
306 }
307 ciphertext_len = disk_link->len - sizeof(*sd) - 1;
308 sd->len = cpu_to_le16(ciphertext_len);
309
310 err = fscrypt_fname_encrypt(inode, &iname, sd->encrypted_path,
311 ciphertext_len);
312 if (err)
313 goto err_free_sd;
314
315 /*
316 * Null-terminating the ciphertext doesn't make sense, but we still
317 * count the null terminator in the length, so we might as well
318 * initialize it just in case the filesystem writes it out.
319 */
320 sd->encrypted_path[ciphertext_len] = '\0';
321
322 /* Cache the plaintext symlink target for later use by get_link() */
323 err = -ENOMEM;
324 inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
325 if (!inode->i_link)
326 goto err_free_sd;
327
328 if (!disk_link->name)
329 disk_link->name = (unsigned char *)sd;
330 return 0;
331
332 err_free_sd:
333 if (!disk_link->name)
334 kfree(sd);
335 return err;
336 }
337 EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
338
339 /**
340 * fscrypt_get_symlink() - get the target of an encrypted symlink
341 * @inode: the symlink inode
342 * @caddr: the on-disk contents of the symlink
343 * @max_size: size of @caddr buffer
344 * @done: if successful, will be set up to free the returned target if needed
345 *
346 * If the symlink's encryption key is available, we decrypt its target.
347 * Otherwise, we encode its target for presentation.
348 *
349 * This may sleep, so the filesystem must have dropped out of RCU mode already.
350 *
351 * Return: the presentable symlink target or an ERR_PTR()
352 */
353 const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
354 unsigned int max_size,
355 struct delayed_call *done)
356 {
357 const struct fscrypt_symlink_data *sd;
358 struct fscrypt_str cstr, pstr;
359 bool has_key;
360 int err;
361
362 /* This is for encrypted symlinks only */
363 if (WARN_ON_ONCE(!IS_ENCRYPTED(inode)))
364 return ERR_PTR(-EINVAL);
365
366 /* If the decrypted target is already cached, just return it. */
367 pstr.name = READ_ONCE(inode->i_link);
368 if (pstr.name)
369 return pstr.name;
370
371 /*
372 * Try to set up the symlink's encryption key, but we can continue
373 * regardless of whether the key is available or not.
374 */
375 err = fscrypt_get_encryption_info(inode, false);
376 if (err)
377 return ERR_PTR(err);
378 has_key = fscrypt_has_encryption_key(inode);
379
380 /*
381 * For historical reasons, encrypted symlink targets are prefixed with
382 * the ciphertext length, even though this is redundant with i_size.
383 */
384
385 if (max_size < sizeof(*sd) + 1)
386 return ERR_PTR(-EUCLEAN);
387 sd = caddr;
388 cstr.name = (unsigned char *)sd->encrypted_path;
389 cstr.len = le16_to_cpu(sd->len);
390
391 if (cstr.len == 0)
392 return ERR_PTR(-EUCLEAN);
393
394 if (cstr.len + sizeof(*sd) > max_size)
395 return ERR_PTR(-EUCLEAN);
396
397 err = fscrypt_fname_alloc_buffer(cstr.len, &pstr);
398 if (err)
399 return ERR_PTR(err);
400
401 err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
402 if (err)
403 goto err_kfree;
404
405 err = -EUCLEAN;
406 if (pstr.name[0] == '\0')
407 goto err_kfree;
408
409 pstr.name[pstr.len] = '\0';
410
411 /*
412 * Cache decrypted symlink targets in i_link for later use. Don't cache
413 * symlink targets encoded without the key, since those become outdated
414 * once the key is added. This pairs with the READ_ONCE() above and in
415 * the VFS path lookup code.
416 */
417 if (!has_key ||
418 cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
419 set_delayed_call(done, kfree_link, pstr.name);
420
421 return pstr.name;
422
423 err_kfree:
424 kfree(pstr.name);
425 return ERR_PTR(err);
426 }
427 EXPORT_SYMBOL_GPL(fscrypt_get_symlink);
428
429 /**
430 * fscrypt_symlink_getattr() - set the correct st_size for encrypted symlinks
431 * @path: the path for the encrypted symlink being queried
432 * @stat: the struct being filled with the symlink's attributes
433 *
434 * Override st_size of encrypted symlinks to be the length of the decrypted
435 * symlink target (or the no-key encoded symlink target, if the key is
436 * unavailable) rather than the length of the encrypted symlink target. This is
437 * necessary for st_size to match the symlink target that userspace actually
438 * sees. POSIX requires this, and some userspace programs depend on it.
439 *
440 * This requires reading the symlink target from disk if needed, setting up the
441 * inode's encryption key if possible, and then decrypting or encoding the
442 * symlink target. This makes lstat() more heavyweight than is normally the
443 * case. However, decrypted symlink targets will be cached in ->i_link, so
444 * usually the symlink won't have to be read and decrypted again later if/when
445 * it is actually followed, readlink() is called, or lstat() is called again.
446 *
447 * Return: 0 on success, -errno on failure
448 */
449 int fscrypt_symlink_getattr(const struct path *path, struct kstat *stat)
450 {
451 struct dentry *dentry = path->dentry;
452 struct inode *inode = d_inode(dentry);
453 const char *link;
454 DEFINE_DELAYED_CALL(done);
455
456 /*
457 * To get the symlink target that userspace will see (whether it's the
458 * decrypted target or the no-key encoded target), we can just get it in
459 * the same way the VFS does during path resolution and readlink().
460 */
461 link = READ_ONCE(inode->i_link);
462 if (!link) {
463 link = inode->i_op->get_link(dentry, inode, &done);
464 if (IS_ERR(link))
465 return PTR_ERR(link);
466 }
467 stat->size = strlen(link);
468 do_delayed_call(&done);
469 return 0;
470 }
471 EXPORT_SYMBOL_GPL(fscrypt_symlink_getattr);
472
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