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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 <linux/export.h>
9
10#include "fscrypt_private.h"
11
12/*
13 * For encrypted symlinks, the ciphertext length is stored at the beginning
14 * of the string in little-endian format.
15 */
16struct fscrypt_symlink_data {
17 __le16 len;
18 char encrypted_path[];
19} __packed;
20
21/**
22 * fscrypt_file_open() - prepare to open a possibly-encrypted regular file
23 * @inode: the inode being opened
24 * @filp: the struct file being set up
25 *
26 * Currently, an encrypted regular file can only be opened if its encryption key
27 * is available; access to the raw encrypted contents is not supported.
28 * Therefore, we first set up the inode's encryption key (if not already done)
29 * and return an error if it's unavailable.
30 *
31 * We also verify that if the parent directory (from the path via which the file
32 * is being opened) is encrypted, then the inode being opened uses the same
33 * encryption policy. This is needed as part of the enforcement that all files
34 * in an encrypted directory tree use the same encryption policy, as a
35 * protection against certain types of offline attacks. Note that this check is
36 * needed even when opening an *unencrypted* file, since it's forbidden to have
37 * an unencrypted file in an encrypted directory.
38 *
39 * Return: 0 on success, -ENOKEY if the key is missing, or another -errno code
40 */
41int fscrypt_file_open(struct inode *inode, struct file *filp)
42{
43 int err;
44 struct dentry *dentry, *dentry_parent;
45 struct inode *inode_parent;
46
47 err = fscrypt_require_key(inode);
48 if (err)
49 return err;
50
51 dentry = file_dentry(filp);
52
53 /*
54 * Getting a reference to the parent dentry is needed for the actual
55 * encryption policy comparison, but it's expensive on multi-core
56 * systems. Since this function runs on unencrypted files too, start
57 * with a lightweight RCU-mode check for the parent directory being
58 * unencrypted (in which case it's fine for the child to be either
59 * unencrypted, or encrypted with any policy). Only continue on to the
60 * full policy check if the parent directory is actually encrypted.
61 */
62 rcu_read_lock();
63 dentry_parent = READ_ONCE(dentry->d_parent);
64 inode_parent = d_inode_rcu(dentry_parent);
65 if (inode_parent != NULL && !IS_ENCRYPTED(inode_parent)) {
66 rcu_read_unlock();
67 return 0;
68 }
69 rcu_read_unlock();
70
71 dentry_parent = dget_parent(dentry);
72 if (!fscrypt_has_permitted_context(d_inode(dentry_parent), inode)) {
73 fscrypt_warn(inode,
74 "Inconsistent encryption context (parent directory: %llu)",
75 d_inode(dentry_parent)->i_ino);
76 err = -EPERM;
77 }
78 dput(dentry_parent);
79 return err;
80}
81EXPORT_SYMBOL_GPL(fscrypt_file_open);
82
83int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
84 struct dentry *dentry)
85{
86 if (fscrypt_is_nokey_name(dentry))
87 return -ENOKEY;
88 /*
89 * We don't need to separately check that the directory inode's key is
90 * available, as it's implied by the dentry not being a no-key name.
91 */
92
93 if (!fscrypt_has_permitted_context(dir, inode))
94 return -EXDEV;
95
96 return 0;
97}
98EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
99
100int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
101 struct inode *new_dir, struct dentry *new_dentry,
102 unsigned int flags)
103{
104 if (fscrypt_is_nokey_name(old_dentry) ||
105 fscrypt_is_nokey_name(new_dentry))
106 return -ENOKEY;
107 /*
108 * We don't need to separately check that the directory inodes' keys are
109 * available, as it's implied by the dentries not being no-key names.
110 */
111
112 if (old_dir != new_dir) {
113 if (IS_ENCRYPTED(new_dir) &&
114 !fscrypt_has_permitted_context(new_dir,
115 d_inode(old_dentry)))
116 return -EXDEV;
117
118 if ((flags & RENAME_EXCHANGE) &&
119 IS_ENCRYPTED(old_dir) &&
120 !fscrypt_has_permitted_context(old_dir,
121 d_inode(new_dentry)))
122 return -EXDEV;
123 }
124 return 0;
125}
126EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
127
128int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
129 struct fscrypt_name *fname)
130{
131 int err = fscrypt_setup_filename(dir, &dentry->d_name, 1, fname);
132
133 if (err && err != -ENOENT)
134 return err;
135
136 fscrypt_prepare_dentry(dentry, fname->is_nokey_name);
137
138 return err;
139}
140EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
141
142/**
143 * fscrypt_prepare_lookup_partial() - prepare lookup without filename setup
144 * @dir: the encrypted directory being searched
145 * @dentry: the dentry being looked up in @dir
146 *
147 * This function should be used by the ->lookup and ->atomic_open methods of
148 * filesystems that handle filename encryption and no-key name encoding
149 * themselves and thus can't use fscrypt_prepare_lookup(). Like
150 * fscrypt_prepare_lookup(), this will try to set up the directory's encryption
151 * key and will set DCACHE_NOKEY_NAME on the dentry if the key is unavailable.
152 * However, this function doesn't set up a struct fscrypt_name for the filename.
153 *
154 * Return: 0 on success; -errno on error. Note that the encryption key being
155 * unavailable is not considered an error. It is also not an error if
156 * the encryption policy is unsupported by this kernel; that is treated
157 * like the key being unavailable, so that files can still be deleted.
158 */
159int fscrypt_prepare_lookup_partial(struct inode *dir, struct dentry *dentry)
160{
161 int err = fscrypt_get_encryption_info(dir, true);
162 bool is_nokey_name = (!err && !fscrypt_has_encryption_key(dir));
163
164 fscrypt_prepare_dentry(dentry, is_nokey_name);
165
166 return err;
167}
168EXPORT_SYMBOL_GPL(fscrypt_prepare_lookup_partial);
169
170int __fscrypt_prepare_readdir(struct inode *dir)
171{
172 return fscrypt_get_encryption_info(dir, true);
173}
174EXPORT_SYMBOL_GPL(__fscrypt_prepare_readdir);
175
176int __fscrypt_prepare_setattr(struct dentry *dentry, struct iattr *attr)
177{
178 if (attr->ia_valid & ATTR_SIZE)
179 return fscrypt_require_key(d_inode(dentry));
180 return 0;
181}
182EXPORT_SYMBOL_GPL(__fscrypt_prepare_setattr);
183
184/**
185 * fscrypt_prepare_setflags() - prepare to change flags with FS_IOC_SETFLAGS
186 * @inode: the inode on which flags are being changed
187 * @oldflags: the old flags
188 * @flags: the new flags
189 *
190 * The caller should be holding i_rwsem for write.
191 *
192 * Return: 0 on success; -errno if the flags change isn't allowed or if
193 * another error occurs.
194 */
195int fscrypt_prepare_setflags(struct inode *inode,
196 unsigned int oldflags, unsigned int flags)
197{
198 struct fscrypt_inode_info *ci;
199 struct fscrypt_master_key *mk;
200 int err;
201
202 /*
203 * When the CASEFOLD flag is set on an encrypted directory, we must
204 * derive the secret key needed for the dirhash. This is only possible
205 * if the directory uses a v2 encryption policy.
206 */
207 if (IS_ENCRYPTED(inode) && (flags & ~oldflags & FS_CASEFOLD_FL)) {
208 err = fscrypt_require_key(inode);
209 if (err)
210 return err;
211 ci = fscrypt_get_inode_info_raw(inode);
212 if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
213 return -EINVAL;
214 mk = ci->ci_master_key;
215 down_read(&mk->mk_sem);
216 if (mk->mk_present)
217 fscrypt_derive_dirhash_key(ci, mk);
218 else
219 err = -ENOKEY;
220 up_read(&mk->mk_sem);
221 return err;
222 }
223 return 0;
224}
225
226/**
227 * fscrypt_prepare_symlink() - prepare to create a possibly-encrypted symlink
228 * @dir: directory in which the symlink is being created
229 * @target: plaintext symlink target
230 * @len: length of @target excluding null terminator
231 * @max_len: space the filesystem has available to store the symlink target
232 * @disk_link: (out) the on-disk symlink target being prepared
233 *
234 * This function computes the size the symlink target will require on-disk,
235 * stores it in @disk_link->len, and validates it against @max_len. An
236 * encrypted symlink may be longer than the original.
237 *
238 * Additionally, @disk_link->name is set to @target if the symlink will be
239 * unencrypted, but left NULL if the symlink will be encrypted. For encrypted
240 * symlinks, the filesystem must call fscrypt_encrypt_symlink() to create the
241 * on-disk target later. (The reason for the two-step process is that some
242 * filesystems need to know the size of the symlink target before creating the
243 * inode, e.g. to determine whether it will be a "fast" or "slow" symlink.)
244 *
245 * Return: 0 on success, -ENAMETOOLONG if the symlink target is too long,
246 * -ENOKEY if the encryption key is missing, or another -errno code if a problem
247 * occurred while setting up the encryption key.
248 */
249int fscrypt_prepare_symlink(struct inode *dir, const char *target,
250 unsigned int len, unsigned int max_len,
251 struct fscrypt_str *disk_link)
252{
253 const union fscrypt_policy *policy;
254
255 /*
256 * To calculate the size of the encrypted symlink target we need to know
257 * the amount of NUL padding, which is determined by the flags set in
258 * the encryption policy which will be inherited from the directory.
259 */
260 policy = fscrypt_policy_to_inherit(dir);
261 if (policy == NULL) {
262 /* Not encrypted */
263 disk_link->name = (unsigned char *)target;
264 disk_link->len = len + 1;
265 if (disk_link->len > max_len)
266 return -ENAMETOOLONG;
267 return 0;
268 }
269 if (IS_ERR(policy))
270 return PTR_ERR(policy);
271
272 /*
273 * Calculate the size of the encrypted symlink and verify it won't
274 * exceed max_len. Note that for historical reasons, encrypted symlink
275 * targets are prefixed with the ciphertext length, despite this
276 * actually being redundant with i_size. This decreases by 2 bytes the
277 * longest symlink target we can accept.
278 *
279 * We could recover 1 byte by not counting a null terminator, but
280 * counting it (even though it is meaningless for ciphertext) is simpler
281 * for now since filesystems will assume it is there and subtract it.
282 */
283 if (!__fscrypt_fname_encrypted_size(policy, len,
284 max_len - sizeof(struct fscrypt_symlink_data) - 1,
285 &disk_link->len))
286 return -ENAMETOOLONG;
287 disk_link->len += sizeof(struct fscrypt_symlink_data) + 1;
288
289 disk_link->name = NULL;
290 return 0;
291}
292EXPORT_SYMBOL_GPL(fscrypt_prepare_symlink);
293
294int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
295 unsigned int len, struct fscrypt_str *disk_link)
296{
297 int err;
298 struct qstr iname = QSTR_INIT(target, len);
299 struct fscrypt_symlink_data *sd;
300 unsigned int ciphertext_len;
301
302 /*
303 * fscrypt_prepare_new_inode() should have already set up the new
304 * symlink inode's encryption key. We don't wait until now to do it,
305 * since we may be in a filesystem transaction now.
306 */
307 if (WARN_ON_ONCE(!fscrypt_has_encryption_key(inode)))
308 return -ENOKEY;
309
310 if (disk_link->name) {
311 /* filesystem-provided buffer */
312 sd = (struct fscrypt_symlink_data *)disk_link->name;
313 } else {
314 sd = kmalloc(disk_link->len, GFP_NOFS);
315 if (!sd)
316 return -ENOMEM;
317 }
318 ciphertext_len = disk_link->len - sizeof(*sd) - 1;
319 sd->len = cpu_to_le16(ciphertext_len);
320
321 err = fscrypt_fname_encrypt(inode, &iname, sd->encrypted_path,
322 ciphertext_len);
323 if (err)
324 goto err_free_sd;
325
326 /*
327 * Null-terminating the ciphertext doesn't make sense, but we still
328 * count the null terminator in the length, so we might as well
329 * initialize it just in case the filesystem writes it out.
330 */
331 sd->encrypted_path[ciphertext_len] = '\0';
332
333 /* Cache the plaintext symlink target for later use by get_link() */
334 err = -ENOMEM;
335 inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
336 if (!inode->i_link)
337 goto err_free_sd;
338
339 if (!disk_link->name)
340 disk_link->name = (unsigned char *)sd;
341 return 0;
342
343err_free_sd:
344 if (!disk_link->name)
345 kfree(sd);
346 return err;
347}
348EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
349
350/**
351 * fscrypt_get_symlink() - get the target of an encrypted symlink
352 * @inode: the symlink inode
353 * @caddr: the on-disk contents of the symlink
354 * @max_size: size of @caddr buffer
355 * @done: if successful, will be set up to free the returned target if needed
356 *
357 * If the symlink's encryption key is available, we decrypt its target.
358 * Otherwise, we encode its target for presentation.
359 *
360 * This may sleep, so the filesystem must have dropped out of RCU mode already.
361 *
362 * Return: the presentable symlink target or an ERR_PTR()
363 */
364const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
365 unsigned int max_size,
366 struct delayed_call *done)
367{
368 const struct fscrypt_symlink_data *sd;
369 struct fscrypt_str cstr, pstr;
370 bool has_key;
371 int err;
372
373 /* This is for encrypted symlinks only */
374 if (WARN_ON_ONCE(!IS_ENCRYPTED(inode)))
375 return ERR_PTR(-EINVAL);
376
377 /* If the decrypted target is already cached, just return it. */
378 pstr.name = READ_ONCE(inode->i_link);
379 if (pstr.name)
380 return pstr.name;
381
382 /*
383 * Try to set up the symlink's encryption key, but we can continue
384 * regardless of whether the key is available or not.
385 */
386 err = fscrypt_get_encryption_info(inode, false);
387 if (err)
388 return ERR_PTR(err);
389 has_key = fscrypt_has_encryption_key(inode);
390
391 /*
392 * For historical reasons, encrypted symlink targets are prefixed with
393 * the ciphertext length, even though this is redundant with i_size.
394 */
395
396 if (max_size < sizeof(*sd) + 1)
397 return ERR_PTR(-EUCLEAN);
398 sd = caddr;
399 cstr.name = (unsigned char *)sd->encrypted_path;
400 cstr.len = le16_to_cpu(sd->len);
401
402 if (cstr.len == 0)
403 return ERR_PTR(-EUCLEAN);
404
405 if (cstr.len + sizeof(*sd) > max_size)
406 return ERR_PTR(-EUCLEAN);
407
408 err = fscrypt_fname_alloc_buffer(cstr.len, &pstr);
409 if (err)
410 return ERR_PTR(err);
411
412 err = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
413 if (err)
414 goto err_kfree;
415
416 err = -EUCLEAN;
417 if (pstr.name[0] == '\0')
418 goto err_kfree;
419
420 pstr.name[pstr.len] = '\0';
421
422 /*
423 * Cache decrypted symlink targets in i_link for later use. Don't cache
424 * symlink targets encoded without the key, since those become outdated
425 * once the key is added. This pairs with the READ_ONCE() above and in
426 * the VFS path lookup code.
427 */
428 if (!has_key ||
429 cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
430 set_delayed_call(done, kfree_link, pstr.name);
431
432 return pstr.name;
433
434err_kfree:
435 kfree(pstr.name);
436 return ERR_PTR(err);
437}
438EXPORT_SYMBOL_GPL(fscrypt_get_symlink);
439
440/**
441 * fscrypt_symlink_getattr() - set the correct st_size for encrypted symlinks
442 * @path: the path for the encrypted symlink being queried
443 * @stat: the struct being filled with the symlink's attributes
444 *
445 * Override st_size of encrypted symlinks to be the length of the decrypted
446 * symlink target (or the no-key encoded symlink target, if the key is
447 * unavailable) rather than the length of the encrypted symlink target. This is
448 * necessary for st_size to match the symlink target that userspace actually
449 * sees. POSIX requires this, and some userspace programs depend on it.
450 *
451 * This requires reading the symlink target from disk if needed, setting up the
452 * inode's encryption key if possible, and then decrypting or encoding the
453 * symlink target. This makes lstat() more heavyweight than is normally the
454 * case. However, decrypted symlink targets will be cached in ->i_link, so
455 * usually the symlink won't have to be read and decrypted again later if/when
456 * it is actually followed, readlink() is called, or lstat() is called again.
457 *
458 * Return: 0 on success, -errno on failure
459 */
460int fscrypt_symlink_getattr(const struct path *path, struct kstat *stat)
461{
462 struct dentry *dentry = path->dentry;
463 struct inode *inode = d_inode(dentry);
464 const char *link;
465 DEFINE_DELAYED_CALL(done);
466
467 /*
468 * To get the symlink target that userspace will see (whether it's the
469 * decrypted target or the no-key encoded target), we can just get it in
470 * the same way the VFS does during path resolution and readlink().
471 */
472 link = READ_ONCE(inode->i_link);
473 if (!link) {
474 link = inode->i_op->get_link(dentry, inode, &done);
475 if (IS_ERR(link))
476 return PTR_ERR(link);
477 }
478 stat->size = strlen(link);
479 do_delayed_call(&done);
480 return 0;
481}
482EXPORT_SYMBOL_GPL(fscrypt_symlink_getattr);