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

source code of linux/fs/crypto/hooks.c