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